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[Seminar - Unix Shells, Environments]

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[Douglas Kline - Harvard University]

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[This is CS50. - CS50.TV]

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>> Today's topic is the Unix shell.

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I'm Douglas Kline, expert, or at least reasonably competent user, of the shell.

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A shell is the interface for the user to the computer's operating system.

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The name is misleading as, unlike an animal's shell,

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which is hard and protective, the computer shell allows for communication.

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So porous membrane would probably be a better metaphor.

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>> The original shell for Unix is the Bourne shell.

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Bourne is spelled B-O-U-R-N-E.

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Bourne was one of the original authors of Unix,

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and so the shell is named after him.

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The name of that shell as a command is just simply sh.

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That's the command you can execute.

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The shell starts at login.

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When you log in to the computer, the shell just starts running for you,

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and that's what takes your commands.

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It can start at other times also.

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If you bring up a window with no other indication, it will start a shell for you.

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That's how it is that you can go to a window and start typing commands

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and so forth there even though you didn't log in to that window.

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In addition, if you do a remote login,

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then it will start a shell on the remote computer.

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And it's possible to run commands without an interactive shell.

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That can mean within your current operation,

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and it can also mean a remote operation.

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You could send a command to another computer,

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which includes starting up a shell there.

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In fact, it has to include starting up a shell there

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even if that isn't your final purpose.

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When something starts up like this, it doesn't necessarily start a new shell.

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If you bring up a new window, it's possible to tell it to bring up an editor 

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or some other command.

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In that case, the editor will start from scratch.

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When the editor ends, the window ends.

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This is a little unusual but it can be done.

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In those cases, it won't be a shell.

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So it's not necessarily the case that a window or some such application will bring up a shell.

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>> Shell parses commands.

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Parsing means identifying the various elements and classifying them.

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Within a command, the complete string that you type,

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there will be 1 or more single commands to be executed.

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Other elements can be arguments.

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There can also be special characters which affect the execution of a command.

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They can send the output somewhere other than the screen

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if the command would ordinarily send it to the screen.

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It can redirect input; it can do other things also.

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There are various other symbols, characters, and so forth.

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Parsing involves detecting and interpreting those things.

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>> Now if there are no more questions, which is rather likely since there are no more people,

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we will go on to my next page here.

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>> I said earlier that the Bourne shell is the initial shell.

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There are others.

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One is the C-shell. The command is csh.

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The name C-shell is just a play on words.

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This shell was introduced with Berkeley Unix in the mid-1970s.

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Berkeley Unix was a seminal event in the development of Unix.

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It was a huge revolution and included the introduction of this shell.

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The reason for that play on words, C-shell,

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is that the C-shell has some characteristics in it which resemble the C language,

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which the Bourne shell does not have--

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or it did not have at that time.

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There's also the TC-shell.

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This is a superset of the C-shell.

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It has additional features, many of which are useful for interactive use,

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such as recalling commands in the history mechanism,

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which I'll describe somewhat later--

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in a simple manner, modeled after an editor.

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It also has bindings which allow you to bind a short key string to a longer command.

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We're not going to be getting into that today.

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It has some features that are useful for programming.

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However, the C-shell is not often used for shell programming.

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Shell programs, if you didn't already know, 

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are programs that consist of shell characteristics.

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You could run these as programs.

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You write a bunch of shell commands into a file and execute the file.

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You don't need to compile it. This is an interpretive language.

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The phrase C-shell is now ambiguous since it might refer only to the original C-shell, csh,

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or to all C-shells, including tcsh. It's a little ambiguous.

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>> A later shell is the Korn shell, ksh, named after the programmer, Korn.

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This shell attempted to incorporate into 1 shell

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the advantages of the C-shell for interactive use

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and the Bourne shell for programming.

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It has been used as an interactive shell by some people--a minority.

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Later though, there was another introduction, the Bash shell, B-A-S-H,

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again a play on words, Bourne-again shell.

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It's an extension of the Bourne shell. Korn shell is also. Both of them are.

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It has the same objectives of the Korn shell of amalgamating the C-shell's

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and Bourne shell's advantages in 1 shell.

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Many of the enhancements of the Korn shell are also included in Bash.

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Bash, however, has more and is therefore preferable.

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The Bourne-again shell and the Korn shell are called Bourne-type shells

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because they include the Bourne shell's characteristics,

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which are incompatible in some respects with C-shells.

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There are other shells besides those, some intended for restricted use,

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maybe limited to some commands, maybe specialized purposes, not often used.

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>> Okay. Next item here.

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The Bash shell has become associated with various forms of Linux.

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I'm not sure if that's true of every form. 

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There are many forms out there and I haven't used them all,

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but in those that I have used it has become associated with it.

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So far as I know, there is nothing about Bash 

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which makes it any more compatible with Linux 

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than any other combination of shell and operating system.

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I think this probably just reflects the inclinations of the programmers.

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That it has become associated with Linux is another reason to prefer Bash to ksh

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since things are likely to be written in it and it's likely to spread.

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I'll give you other reasons for that later on.

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Bourne shell scripts should run under the Korn shell or Bash.

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If you write something for the Bourne shell,

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you can probably execute it under ksh or bash.

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Korn shell scripts will probably run under Bash, but I can't guarantee that.

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Later on here, C-shell scripts should run under the TC-shell.

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The C-shell was actually never extensively used for scripting

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since the Bourne shell and later the Bourne-type shells were preferable for that purpose.

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So that really isn't all that important.

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There are quite a lot of Bourne shell scripts which were written long ago,

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before the Korn shell or the Bourne-again shell were introduced.

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Those are still in use, part of the operating systems,

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and so you will find them if you look into the operating system

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or some old programming packages.

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>> Bash is to some extent becoming a kind of lingua franca for operating systems.

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It's already been extended to Windows and to VMS.

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VMS, in case you don't know, is a proprietary operating system 

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of Digital Equipment Corporation which is still in use, largely behind the scenes.

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And if it's going to be running on several different operating systems,

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likely the people tend to shift for it.

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But this development is relatively recent.

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It's just beginning, so I can't predict if this will turn out to really be that kind of lingua franca.

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Also, because file pathnames and libraries differ 

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between these different operating systems,

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you might not be able to write a Bash script on one operating system

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and then run it on another one.

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You should be able to move it between different Unix, Linux 

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Mac OS operating systems but not necessarily to Windows or VMS.

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You might have to change file pathname descriptions,

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and some libraries might be different, 

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which may affect the way that some commands work 

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or how they process arguments and the like.

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In addition to that, another caution here is that there is no guarantee

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that all the different shells I've mentioned--Bourne shell, C-shell,

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TC-shell, Korn shell, Bourne-again shell--will be available under any Unix

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or Linux or Mac OS computer.

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They simply might not be there.

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That's one of the cautions here.

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It's an unfortunate limitation here since you'd like things to work everywhere,

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but unfortunately, you can't rely on that.

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>> Okay. Next one here.

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Let's say that you want to write a shell script,

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a program consisting of shell commands.

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You write your commands, put them in a file, and execute the file.

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What if you want to include arguments?

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In the case of shell operations, arguments are called parameters or positional parameters

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and they'll be called by a dollar sign and numeral, $1, $2.

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So if the script has this name, my first argument might be argument 1

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and my second might be argument 2,

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and inside my script if I want to refer to these things--

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let's erase this since I'm not really going to run it--

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inside my script I might have $1 to refer to arg1,

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$2, which will come out that way, arg2.

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So those symbols are available to refer to arguments,

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and those apply to all of the shells.

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In addition, there are other characters.

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$* refers to the entire argument list, all of them.

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$# refers to the number of arguments.

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Again, this applies to all the shells.

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Those symbols, * and #, can be used with those meanings in other places also.

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We won't be getting into that.

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>> Shell specifier line. What's that for?

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Let's say you've written a script and it's for a particular shell and you want to run it.

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How do you know what shell your operating system will use to run your script?

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At one point you could assume that it would run it in the Bourne shell

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if you didn't say otherwise, 

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but people aren't writing scripts in the Bourne shell that much anymore

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and you can't even rely on that anymore.

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So here we have a shell specifier line right here.

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That specifies Bash.

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Note that it specifies it in the pathname, /bin/bash.

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If a computer has the Bash shell but not in the bin directory, /bin, this won't work.

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That's another qualifier, another caution here.

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The pound sign is the comment line character.

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That applies to all shells.

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The particular case here, #! at the beginning of a script, is a special case.

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That specifies the shell in which to run the script.

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As I was saying, it might not be the same place/bin.

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In addition, there's another thing here.

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If you just use the pound sign with no exclamation point and pathname,

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that should indicate a C-shell.

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However, I don't recommend doing that because I'm not able to guarantee

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that that will always work.

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If you want a C-shell, it would be better to say so.

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Then there's something rather confusing here.

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If you use a shell specifier line such as /bin/bash

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and that shell is not available there, 

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there's no such thing as /bin/bash on that particular computer,

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either because it doesn't have Bash or because it's in a different location,

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you'll get an error telling you that the script you ran doesn't exist.

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And of course your script exists, so that error message is confusing.

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The reason that the operating system gives you that error

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or, more accurately, that your interactive shell in which you are running this gives that error,

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is that it reports the command you used, which is the name of the script.

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That command effectively called the shell by the name of the script.

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That's where you get that confusing error message.

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Another way to call shell script 

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is by specifying the shell on the command line, as here.

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This is a command. This says run Bash and then run my script in Bash.

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That will take precedence over a specifier line,

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and this has the feature of allowing you to provide for varying pathnames.

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If you just give a command, the operating system will look for that command

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in various places.

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If it's available, it should find it.

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The computer will find Bash wherever it's located and run it,

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so you don't need then to be concerned about where it finds it.

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There are potentially other concerns here,

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as if there's more than 1 version of Bash, which is possible although unlikely.

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So that's another way to deal with these things.

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Specifier lines can call any shell.

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They can also call things other than shells.

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Examples I have here are sed, which is the stream editor;

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awk, which is a pattern processing language; 

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and perl, a very highly developed scripting language.

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If you put a specifier line indicating one of those programs at the beginning,

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it will go directly into that program rather than starting a shell.

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Those programs have limits to their abilities.

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Perl is very capable. Sed is an editor. It can do things beyond simply editing.

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But it can be difficult to program that.

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In addition, passing arguments and stuff to script is either impossible or confusing.

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So in those cases, with awk or sed, it's, at least in my experience,

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preferable to write a shell script and call awk or sed from the shell script

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rather than calling awk or sed as the script specifier line.

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Perl is a highly diversified language, as I said.

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You can't run interactive commands in perl,

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which means that you can't test parts of scripts that you're developing

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by running them interactively.

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However, it's an extremely capable language

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and has developed into a very widely used tool.

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That's just a little bit of a parenthetical remark about the specifier lines.

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>> In all or most forms of Linux--again, I can't be certain that's all--

240
00:15:50,610 --> 00:15:57,900
and in Mac OS, if you type csh you get tcsh,

241
00:15:57,900 --> 00:16:00,570
and if you type sh you get bash.

242
00:16:00,570 --> 00:16:05,020
They were trying there to give you the more advanced versions of these shells,

243
00:16:05,020 --> 00:16:07,940
but this can be confusing.

244
00:16:07,940 --> 00:16:16,720
If you write a script using tcsh or Bash features while calling csh or sh

245
00:16:16,720 --> 00:16:22,230
and then try to run it on a computer which doesn't have tcsh or Bash,

246
00:16:22,230 --> 00:16:25,050
you might get some errors if there are commands in there 

247
00:16:25,050 --> 00:16:27,970
which those shells don't recognize.

248
00:16:27,970 --> 00:16:34,120
In addition, you may have called up your shell on your local computer

249
00:16:34,120 --> 00:16:37,700
calling it as sh or csh and then getting the more advanced shells.

250
00:16:37,700 --> 00:16:41,440
You may not even think of the fact that you're using the more advanced shell.

251
00:16:41,440 --> 00:16:45,670
So this is a potential pitfall.

252
00:16:45,670 --> 00:16:50,290
How is it established that if you type sh you get Bash,

253
00:16:50,290 --> 00:16:55,580
if you type csh you get tsch?

254
00:16:55,580 --> 00:16:59,940
There are things in these computers called links

255
00:16:59,940 --> 00:17:06,460
which can connect to file names to refer to the same thing.

256
00:17:06,460 --> 00:17:12,180
It can either be 2 names for the same file or a file whose purpose is to refer to another file.

257
00:17:12,180 --> 00:17:17,550
They're called hard and symbolic links. We won't be going into that anymore today.

258
00:17:17,550 --> 00:17:21,619
There may also be separate files--1 file sh, 1 file Bash--

259
00:17:21,619 --> 00:17:23,880
but they both run Bash.

260
00:17:23,880 --> 00:17:29,350
Then there's another qualifier here.

261
00:17:29,350 --> 00:17:42,640
If you're calling one of these shells by one name,

262
00:17:42,640 --> 00:17:46,640
you might think you'd get the same functionality as calling it by another name.

263
00:17:46,640 --> 00:17:49,700
Well, that actually isn't necessarily true.

264
00:17:49,700 --> 00:17:55,020
These commands can examine the name by which they were called

265
00:17:55,020 --> 00:18:00,020
and they can, on the basis of that name, behave differently.

266
00:18:00,020 --> 00:18:02,740
There may be issues of trying to conform to a standard.

267
00:18:02,740 --> 00:18:06,060
Some of you may have heard of the POSIX standard or another,

268
00:18:06,060 --> 00:18:08,730
maybe other features.

269
00:18:08,730 --> 00:18:14,520
This can be selected sometimes by command line arguments

270
00:18:14,520 --> 00:18:17,310
or by setting shell variables.

271
00:18:17,310 --> 00:18:22,170
Calling it as sh or bash may actually lead to a different execution

272
00:18:22,170 --> 00:18:25,300
even if it's the same file that you're executing.

273
00:18:25,300 --> 00:18:31,800
Another thing to consider is that even if another computer has tcsh or Bash,

274
00:18:31,800 --> 00:18:35,310
if they aren't linked as they are on your local computer

275
00:18:35,310 --> 00:18:37,990
if you have a Linux or Mac OS local computer,

276
00:18:37,990 --> 00:18:45,630
then again you'll get the shell that you call sh or csh, not the one that you might prefer.

277
00:18:50,430 --> 00:19:01,130
The current Bourne shell has enhancements lesser than those in Bash

278
00:19:01,130 --> 00:19:06,100
but past those in the original Bourne shell.

279
00:19:06,100 --> 00:19:09,690
As a result of that, even the current Bourne shell, sh,

280
00:19:09,690 --> 00:19:14,560
even when it's not Bash, resembles the C language more than the C-shell does.

281
00:19:14,560 --> 00:19:20,460
That wasn't true when the C-shell was first created, but it has developed that way.

282
00:19:20,460 --> 00:19:26,560
You might notice here that all these shell names except for the Bourne shell

283
00:19:26,560 --> 00:19:30,640
have something to indicate which shell they are--csh, bash--

284
00:19:30,640 --> 00:19:32,550
but the Bourne shell is just sh.

285
00:19:32,550 --> 00:19:34,910
Why? That was the original shell.

286
00:19:34,910 --> 00:19:37,770
It was THE shell then, not A shell,

287
00:19:37,770 --> 00:19:41,090
and since it was THE shell, there was no reason to distinguish it from another shell.

288
00:19:41,090 --> 00:19:45,030
So that's why it has that name and still does.

289
00:19:50,630 --> 00:19:58,990
>> This top here is a line from a password database for an account I have there

290
00:19:58,990 --> 00:20:01,680
on another computer.

291
00:20:01,680 --> 00:20:08,300
I'm going to try to get that name so you can see that part at the end, the shell.

292
00:20:09,720 --> 00:20:15,450
The password database holds the login characteristics for all the users.

293
00:20:15,450 --> 00:20:20,330
At the beginning is the username, which you can see the last 2 letters of mine now.

294
00:20:20,330 --> 00:20:23,970
The fields here are separated by colons.

295
00:20:23,970 --> 00:20:28,210
The last field, as you can see, is bin/tcsh, the shell.

296
00:20:28,210 --> 00:20:30,230
That's the shell specifier.

297
00:20:30,230 --> 00:20:33,240
There's something interesting here.

298
00:20:33,240 --> 00:20:36,950
When Unix was first developed, there was only 1 shell,

299
00:20:36,950 --> 00:20:38,350
so there was no choice there.

300
00:20:38,350 --> 00:20:45,570
So why did they allow a field in the password database to specify a shell?

301
00:20:45,570 --> 00:20:47,920
I don't know, but it's fortunate that they did.

302
00:20:47,920 --> 00:20:52,030
It's rather difficult to make changes in the password database format

303
00:20:52,030 --> 00:20:54,420
because many programs refer to its format

304
00:20:54,420 --> 00:20:57,720
and would have to be rewritten.

305
00:20:57,720 --> 00:21:04,130
It's a felicitous or fortuitous development that they included that field.

306
00:21:04,130 --> 00:21:12,780
That kind of a password file line is used on all Unix and Linux computers so far as I know.

307
00:21:12,780 --> 00:21:14,650
The Mac has its own system. 

308
00:21:14,650 --> 00:21:17,810
It actually has a password file with the lines in that format,

309
00:21:17,810 --> 00:21:21,060
but that isn't where the user characteristics are defined.

310
00:21:21,060 --> 00:21:24,200
Another parenthetical remark there.

311
00:21:36,470 --> 00:21:46,020
>> If you're calling a shell, you can call it as a sub-shell of your existing shells.

312
00:21:46,020 --> 00:21:50,480
So if I go here, let's get rid of these things.

313
00:21:50,480 --> 00:21:53,350
Here I am in the C-shell.

314
00:21:56,830 --> 00:22:01,200
That variable, which accurately identifies my shell,

315
00:22:01,200 --> 00:22:04,300
actually isn't always a reliable way of determining what shell you're running,

316
00:22:04,300 --> 00:22:06,220
but in this case it is.

317
00:22:06,220 --> 00:22:08,040
What if I just type--

318
00:22:09,970 --> 00:22:12,470
Now I'm in Bash.

319
00:22:12,470 --> 00:22:19,540
Some things are going to be the same. ls tells me my commands.

320
00:22:19,540 --> 00:22:24,500
If I do a suspend back to my C-shell, ls, same. Right?

321
00:22:24,500 --> 00:22:28,890
fg, foreground, back to my Bash shell.

322
00:22:28,890 --> 00:22:38,290
pwd, current directory, back to the C-shell.

323
00:22:38,290 --> 00:22:43,180
pwd, different directory--actually not a different directory in this case. 

324
00:22:43,180 --> 00:22:45,110
It's the same directory.

325
00:22:45,110 --> 00:22:50,000
Let's say I want to call a command here: where ls.

326
00:22:50,000 --> 00:22:52,140
What does that do?

327
00:22:52,140 --> 00:22:53,670
It tells me where the ls command,

328
00:22:53,670 --> 00:22:56,670
the one that gives me a directory listing, is located in ls.

329
00:22:56,670 --> 00:23:01,460
Let's go back to Bash shell. Let's try the same thing.

330
00:23:01,460 --> 00:23:05,830
Hmm, interesting there, where: command not found.

331
00:23:05,830 --> 00:23:07,400
Why is that?

332
00:23:07,400 --> 00:23:11,570
The where command is built in to the C-shell.

333
00:23:11,570 --> 00:23:15,630
This isn't a command that has to be read in to memory from somewhere else and executed.

334
00:23:15,630 --> 00:23:20,310
The C-shell runs it by transferring execution to part of its own code

335
00:23:20,310 --> 00:23:22,790
and it's not in the Bash shell.

336
00:23:22,790 --> 00:23:25,710
So Bash, not having such a built-in command, looks for it, doesn't find it, 

337
00:23:25,710 --> 00:23:27,720
and we get an error.

338
00:23:27,720 --> 00:23:32,290
So there we have a Bash shell running under a C-shell, and we call that a sub-shell.

339
00:23:32,290 --> 00:23:38,480
And just in case you're curious, Bash shell has its own way of locating commands.

340
00:23:38,480 --> 00:23:42,590
hashed refers to the fact that it can be executed more rapidly,

341
00:23:42,590 --> 00:23:44,960
being found more rapidly.

342
00:23:44,960 --> 00:23:48,610
That's one of the enhancements built in to some of these shells.

343
00:23:50,220 --> 00:23:54,200
>> Bourne-type shells are preferred for programming.

344
00:23:54,200 --> 00:23:57,300
They have control structures like loops, conditional statements,

345
00:23:57,300 --> 00:24:00,240
the sort of commands that you might use in programming languages like C

346
00:24:00,240 --> 00:24:04,190
or whatever language. Maybe you're programming in Java or whatever.

347
00:24:04,190 --> 00:24:06,460
Shells have those too.

348
00:24:06,460 --> 00:24:11,790
The Bourne-type shells, particularly Bash, have more

349
00:24:11,790 --> 00:24:15,730
and they are designed with greater flexibility.

350
00:24:15,730 --> 00:24:20,700
The Bash shell has arrays. The original Bourne shell doesn't.

351
00:24:20,700 --> 00:24:26,130
So that can be considerably advantageous for programming.

352
00:24:26,130 --> 00:24:29,810
The C-shell actually does have arrays but doesn't have a lot of these other features.

353
00:24:29,810 --> 00:24:33,450
The Bourne-type shells will execute faster

354
00:24:33,450 --> 00:24:36,520
if they don't have the features intended for interactive use.

355
00:24:36,520 --> 00:24:39,340
You load things down for one purpose; this loads them down for another purpose.

356
00:24:39,340 --> 00:24:41,520
There's that trade-off there.

357
00:24:41,520 --> 00:24:44,510
Those features which are intended for interactive use 

358
00:24:44,510 --> 00:24:46,920
really are of little or no use for scripting.

359
00:24:46,920 --> 00:24:52,160
It's possible to use an interactive sub-shell just like the one I started there

360
00:24:52,160 --> 00:24:57,780
to test out commands which you intend to use in a script.

361
00:24:57,780 --> 00:25:01,180
That's what you can't do with perl. You can do it with the shells.

362
00:25:01,180 --> 00:25:04,850
Even the structures like for loops and so forth can be run interactively.

363
00:25:04,850 --> 00:25:07,000
They are occasionally useful to run interactively,

364
00:25:07,000 --> 00:25:10,180
but more likely you're using them to develop a script.

365
00:25:15,690 --> 00:25:17,400
>> Aliases.

366
00:25:17,400 --> 00:25:21,630
This is going to be about the C-shell.

367
00:25:23,270 --> 00:25:27,570
History mechanism where you get back to earlier commands

368
00:25:27,570 --> 00:25:30,340
or parts of them that you've already run.

369
00:25:30,340 --> 00:25:33,680
Again, about the C-shell, the Bourne shell and the Korn shell have these things,

370
00:25:33,680 --> 00:25:35,620
but I'm not going to get into them.

371
00:25:35,620 --> 00:25:40,340
So here are some useful aliases that I have.

372
00:25:43,100 --> 00:25:44,880
Instead of typing ls--it's a common command--

373
00:25:44,880 --> 00:25:47,620
just type l and save yourself 1 character.

374
00:25:47,620 --> 00:25:50,600
ls with various options, all those work.

375
00:25:50,600 --> 00:25:54,460
Note that those definitions have quotes around them.

376
00:25:54,460 --> 00:25:57,520
In these cases, the quotes aren't necessary.

377
00:25:57,520 --> 00:26:00,100
If you can define those aliases without the quotes, it would still work.

378
00:26:00,100 --> 00:26:02,910
They are recommended.

379
00:26:02,910 --> 00:26:04,900
There are situations in which you can't use the quote

380
00:26:04,900 --> 00:26:08,050
because you want something to happen which the quote would prevent.

381
00:26:08,050 --> 00:26:11,210
Sometimes you can quote part of the definition but not all of it.

382
00:26:11,210 --> 00:26:17,010
It's also generally recommended to use single quotes rather than double quotes.

383
00:26:17,010 --> 00:26:19,750
Double quotes have effects on variable definitions,

384
00:26:19,750 --> 00:26:22,950
particularly causing them to be evaluated rather than stopping it.

385
00:26:22,950 --> 00:26:25,910
Why would we want to stop the evaluation?

386
00:26:25,910 --> 00:26:28,710
And how do quotes do that for us?

387
00:26:28,710 --> 00:26:32,600
>> Here is a command which you might find interesting.

388
00:26:32,600 --> 00:26:35,470
'ls g*'

389
00:26:35,470 --> 00:26:37,640
g*, as you probably know, is a wildcard expression

390
00:26:37,640 --> 00:26:40,290
for all the file names beginning with g.

391
00:26:40,290 --> 00:26:46,410
If I just write in a command ls g*, I'll get a list of all those names in my current directory.

392
00:26:46,410 --> 00:26:50,870
If I define that alias as it is here with the quotes,

393
00:26:50,870 --> 00:26:56,990
it will run that command in your current directory where you're running it.

394
00:26:56,990 --> 00:27:01,250
But if you run the alias definition without the quotes,

395
00:27:01,250 --> 00:27:09,620
it will evaluate the wildcard g* when it runs this defining command.

396
00:27:09,620 --> 00:27:14,400
So the definition of the alias will be ls followed by the list of files in the directory

397
00:27:14,400 --> 00:27:16,310
in which the alias command is executed,

398
00:27:16,310 --> 00:27:19,180
regardless of where you actually intend to run the command.

399
00:27:19,180 --> 00:27:26,360
This isn't of much use, and the single quotes prevent the evaluation of the asterisk.

400
00:27:26,360 --> 00:27:30,780
So you just get the definition being ls g*.

401
00:27:30,780 --> 00:27:35,510
Then when you run the alias, lgs, it then puts that out.

402
00:27:35,510 --> 00:27:40,490
Now there are no quotes, and it will evaluate the asterisk when you run the alias command.

403
00:27:40,490 --> 00:27:43,900
So that's one thing.

404
00:27:43,900 --> 00:27:46,590
Double quotes would have that same effect here,

405
00:27:46,590 --> 00:27:50,580
but there are other cases in which double quotes wouldn't work so well.

406
00:27:50,580 --> 00:27:52,450
>> Here is another one.

407
00:27:52,450 --> 00:27:54,270
You might know the grep command.

408
00:27:54,270 --> 00:28:02,110
The grep command can be used to scan a file for lines which have certain strings.

409
00:28:02,110 --> 00:28:10,350
So let's go over here and I'll exit from my Bourne shell.

410
00:28:23,570 --> 00:28:25,450
Okay. Here's a file.

411
00:28:25,450 --> 00:28:31,490
Let's say it's grep abc strings. There it is.

412
00:28:31,490 --> 00:28:37,930
If I do grep zddd, I get nothing. Okay.

413
00:28:37,930 --> 00:28:40,960
So it finds a string, it reports; it doesn't find, it doesn't report it.

414
00:28:40,960 --> 00:28:44,930
It outputs any line which has that string on it.

415
00:28:44,930 --> 00:28:49,080
There are all sorts of options here which you can find in the documentation.

416
00:28:49,080 --> 00:28:52,160
Here's one way to do it.

417
00:28:52,160 --> 00:29:03,290
What about this one, alias grabc 'grep abc'?

418
00:29:03,290 --> 00:29:09,000
That's going to include 1 argument when the alias is defined.

419
00:29:09,000 --> 00:29:26,300
So if I do that here, now if I do grabc,

420
00:29:26,300 --> 00:29:30,620
now the alias includes more than the simple command. It also has the argument.

421
00:29:30,620 --> 00:29:32,190
So far that works.

422
00:29:32,190 --> 00:29:38,590
I have another command here, this one, so those are different strings in there

423
00:29:38,590 --> 00:29:46,790
and show that this doesn't find anything there since it doesn't match.

424
00:29:46,790 --> 00:29:56,180
>> What if I want to include in the alias definition the file that I'm going to search

425
00:29:56,180 --> 00:30:02,970
and I want to give as an argument to the alias the string that I'm looking for?

426
00:30:02,970 --> 00:30:08,040
I might want to say abc as the argument to my alias,

427
00:30:08,040 --> 00:30:10,870
but the alias already determined the file.

428
00:30:10,870 --> 00:30:15,710
And that's where this expression comes in.

429
00:30:20,430 --> 00:30:25,270
Notice here we have grep just like before.

430
00:30:25,270 --> 00:30:28,130
We have the file here, strings.

431
00:30:28,130 --> 00:30:35,610
\!^, kind of an odd expression, I suppose, if you haven't seen this before.

432
00:30:35,610 --> 00:30:39,920
Exclamation point is part of the C-shell history mechanism.

433
00:30:39,920 --> 00:30:45,220
It can recall earlier commands, it can recall arguments to those commands and so forth.

434
00:30:46,760 --> 00:31:01,570
The history mechanism is used as part of aliasing.

435
00:31:01,570 --> 00:31:07,390
If you specify a line after the exclamation point, it will refer to that line in the history list,

436
00:31:07,390 --> 00:31:11,910
which we won't be getting into now since it's a whole other topic.

437
00:31:11,910 --> 00:31:16,280
It is possible to specify part of a line.

438
00:31:16,280 --> 00:31:22,950
So !3:2 would be the second argument of command number 3.

439
00:31:22,950 --> 00:31:30,430
The caret here in this expression stands for the first argument.

440
00:31:30,430 --> 00:31:34,410
If you don't give it an indication of which command you're referring to,

441
00:31:34,410 --> 00:31:37,300
it refers to the immediately previous command,

442
00:31:37,300 --> 00:31:41,990
and the caret is a symbol for the first argument.

443
00:31:41,990 --> 00:31:46,820
Because it's the caret and not the number, you don't need to use the colon,

444
00:31:46,820 --> 00:31:52,660
so !^ means the first argument to the previous command.

445
00:31:52,660 --> 00:31:55,020
A little mixed up here.

446
00:31:55,020 --> 00:31:58,450
In this case, when you use this as an alias definition,

447
00:31:58,450 --> 00:32:04,650
the history reference refers back to the commands in which the alias is used.

448
00:32:04,650 --> 00:32:08,470
So this is going back 1 command as a history operation,

449
00:32:08,470 --> 00:32:11,810
but as an alias operation it refers to the command in which you would type,

450
00:32:11,810 --> 00:32:14,780
say, grstrings_file.

451
00:32:17,440 --> 00:32:20,240
We have the quotes here in it. What's the backslash for?

452
00:32:20,240 --> 00:32:30,810
In this case, as elsewhere, we don't want to execute the history mechanism

453
00:32:30,810 --> 00:32:33,680
while defining the alias.

454
00:32:33,680 --> 00:32:37,900
If we didn't have the backslash there, the shell would pull in the first argument

455
00:32:37,900 --> 00:32:41,870
of the command right before it ran this alias command, which we don't want.

456
00:32:41,870 --> 00:32:47,520
We want this to be built in to the alias command to call in an argument later.

457
00:32:47,520 --> 00:32:53,550
Single quotes don't escape an exclamation point, the history reference.

458
00:32:53,550 --> 00:32:57,450
Maybe you know the expression escape means to change the meaning of something.

459
00:32:57,450 --> 00:33:00,260
In this case, it means to stop something from having a special meaning.

460
00:33:00,260 --> 00:33:03,030
Exclamation point's special meaning is history.

461
00:33:03,030 --> 00:33:05,790
Escape and it doesn't have that meaning.

462
00:33:05,790 --> 00:33:08,080
Quotes don't do that; backslash does.

463
00:33:08,080 --> 00:33:11,900
So we're actually using 2 levels of escaping here.

464
00:33:23,500 --> 00:33:29,620
I'm going to move this command into the other window without typing it

465
00:33:29,620 --> 00:33:35,210
by using these editing operations, which you may find useful.

466
00:33:40,620 --> 00:33:42,460
Something else here I'll show you.

467
00:33:42,460 --> 00:33:46,730
If you just type alias with no arguments, it tells you all your arguments.

468
00:33:46,730 --> 00:33:48,640
This is a bunch of aliases I already had here 

469
00:33:48,640 --> 00:33:53,400
besides those that I have been using here today.

470
00:33:53,400 --> 00:34:00,220
But if I just type with the name of an alias, it tells me what it means.

471
00:34:00,220 --> 00:34:03,390
Notice that the quotes are gone and the backslash is gone.

472
00:34:03,390 --> 00:34:08,620
This string here is the result of that alias definition,

473
00:34:08,620 --> 00:34:12,199
and now it has just !^ in it.

474
00:34:12,199 --> 00:34:19,150
This is going to look in the file strings for anything.

475
00:34:19,150 --> 00:34:34,900
So if I do grstrings_file strings, I didn't give it anything to look for there,

476
00:34:34,900 --> 00:34:37,429
but it's looking in strings.

477
00:34:37,429 --> 00:34:42,330
It didn't find the word strings in the file strings, but it does find abc.

478
00:34:42,330 --> 00:34:46,770
And it doesn't find that.

479
00:34:46,770 --> 00:34:52,330
So here we are giving an argument that hits into the definition of the alias,

480
00:34:52,330 --> 00:34:55,530
that is inserted into it.

481
00:34:55,530 --> 00:34:58,540
It's where this expression comes from.

482
00:34:58,540 --> 00:35:00,240
You can use more than 1.

483
00:35:00,240 --> 00:35:03,170
The caret is a symbol for the first argument.

484
00:35:03,170 --> 00:35:07,510
If you wanted to use a second argument, you would then say :2.

485
00:35:07,510 --> 00:35:11,250
There's no special symbol for the second argument.

486
00:35:11,250 --> 00:35:14,790
And because you're using a numeral, you would have to use the colon.

487
00:35:14,790 --> 00:35:17,220
There is, however, another choice here.

488
00:35:17,220 --> 00:35:21,220
The dollar sign stands for the last argument.

489
00:35:21,220 --> 00:35:23,320
And because this is a symbol, you can omit the colon.

490
00:35:23,320 --> 00:35:25,870
So it would be the last argument in the list.

491
00:35:25,870 --> 00:35:27,900
And there's also that one.

492
00:35:27,900 --> 00:35:31,380
Asterisk means all of, so this is the complete argument list,

493
00:35:31,380 --> 00:35:35,150
and again, you can omit the colon because it's not a numeral.

494
00:35:36,970 --> 00:35:39,950
I hope you're all observing all this.

495
00:35:39,950 --> 00:35:54,100
>> The history mechanism can go back to earlier lines in the history list.

496
00:35:54,100 --> 00:36:01,370
You could do this in an alias definition.

497
00:36:01,370 --> 00:36:02,950
I've never seen this done.

498
00:36:02,950 --> 00:36:05,840
It would have the effect of pulling out earlier commands from the history list

499
00:36:05,840 --> 00:36:08,130
when you execute the alias, which could be different commands

500
00:36:08,130 --> 00:36:11,240
depending on when and where you execute it.

501
00:36:11,240 --> 00:36:14,020
Conceivably you might want to pull out such a reference

502
00:36:14,020 --> 00:36:15,900
just to know what an earlier command was.

503
00:36:15,900 --> 00:36:17,280
I've never seen this happen. 

504
00:36:17,280 --> 00:36:19,970
I suppose somebody might want to, but this is very unlikely.

505
00:36:19,970 --> 00:36:26,480
There is another thing here.

506
00:36:26,480 --> 00:36:33,060
If you use that history-type reference,

507
00:36:33,060 --> 00:36:38,190
then only the arguments to which there is such a reference are used.

508
00:36:38,190 --> 00:36:42,180
If you have an alias definition which doesn't use a history-type reference,

509
00:36:42,180 --> 00:36:44,060
if it just becomes the beginning of the command

510
00:36:44,060 --> 00:36:46,520
and you have further arguments, then anything you type after that

511
00:36:46,520 --> 00:36:48,450
will be added to the command.

512
00:36:48,450 --> 00:36:52,040
In this case, the example I just gave there, we used the first argument; 

513
00:36:52,040 --> 00:36:54,610
we didn't use any others.

514
00:36:54,610 --> 00:36:57,960
If other arguments had been given on the command line, they would not be used.

515
00:36:57,960 --> 00:37:04,630
So if you use the history reference at all, then you must use it to get any argument.

516
00:37:04,630 --> 00:37:11,310
>> There's another thing here I just want to mention, partly parenthetically,

517
00:37:11,310 --> 00:37:15,250
namely that this history mechanism with the exclamation point

518
00:37:15,250 --> 00:37:18,010
goes back to the original C-shell.

519
00:37:18,010 --> 00:37:27,060
The tcsh introduced history operations

520
00:37:27,060 --> 00:37:30,910
which use the sorts of commands and strings from the editors,

521
00:37:30,910 --> 00:37:33,650
either Emacs or vi.

522
00:37:33,650 --> 00:37:36,430
My personal opinion is Emacs is much easier to use for this purpose

523
00:37:36,430 --> 00:37:39,390
even if you use vi for your regular editing.

524
00:37:39,390 --> 00:37:43,900
There are various Emacs commands which are now adapted for history.

525
00:37:43,900 --> 00:37:46,410
Control P gets the previous line in the history list.

526
00:37:46,410 --> 00:37:48,840
Another Control P will get you the one before that.

527
00:37:48,840 --> 00:37:50,540
The up arrow does the same thing.

528
00:37:50,540 --> 00:37:54,190
Control N gets the next command if you've already scrolled back some ways.

529
00:37:54,190 --> 00:37:55,880
Down arrow does that too. 

530
00:37:55,880 --> 00:38:00,480
You can move left to right with the arrows and various other things.

531
00:38:00,480 --> 00:38:02,390
This can make use of the history mechanism 

532
00:38:02,390 --> 00:38:05,070
much easier than using the exclamation point syntax,

533
00:38:05,070 --> 00:38:07,930
but you wouldn't use that in an alias definition.

534
00:38:17,780 --> 00:38:20,020
We'll go over that some other time.

535
00:38:24,300 --> 00:38:25,810
>> Variables.

536
00:38:26,880 --> 00:38:29,510
You know what variables are in programming languages.

537
00:38:29,510 --> 00:38:31,680
The shells have them also.

538
00:38:31,680 --> 00:38:37,350
The C-shell uses the command set to assign variables,

539
00:38:37,350 --> 00:38:41,360
so that sets the variable a to the value of b--

540
00:38:41,360 --> 00:38:46,390
as I said, a useless definition but an illustration of how this is used.

541
00:38:48,790 --> 00:38:52,410
The set command will create a variable if it doesn't already exist.

542
00:38:55,270 --> 00:39:02,490
The positional parameters for shell scripts can be considered variables,

543
00:39:02,490 --> 00:39:10,750
but the use of them and the rules for them are somewhat different.

544
00:39:10,750 --> 00:39:14,320
You can't assign a value to $1 in the course of a script.

545
00:39:14,320 --> 00:39:18,340
You would have to define a new variable for that purpose if some of you wanted to.

546
00:39:23,000 --> 00:39:28,470
Type set with no arguments and you get a list of all the currently defined variables.

547
00:39:28,470 --> 00:39:34,220
And let's get over to my other shell here and see what we get if we do that.

548
00:39:34,220 --> 00:39:37,110
Quite a long list there, right?

549
00:39:37,110 --> 00:39:40,990
Scroll up a little bit. Look at all that.

550
00:39:40,990 --> 00:39:44,330
Some of these things are defined automatically by the shell.

551
00:39:44,330 --> 00:39:49,320
The shell creates the variable and gives it a value.

552
00:39:49,320 --> 00:39:52,730
Some of them are defined by the shell but then redefined by the user

553
00:39:52,730 --> 00:39:54,820
according to his preferences.

554
00:39:54,820 --> 00:39:59,110
And some of them are created by the user depending on what he's doing that day.

555
00:39:59,110 --> 00:40:01,880
That's just set with no arguments.

556
00:40:06,920 --> 00:40:10,050
There's an odd feature here of this thing.

557
00:40:10,050 --> 00:40:17,980
There have to be either no spaces between the equals sign and the variable name

558
00:40:17,980 --> 00:40:23,700
and the value or spaces on both sides of the equals sign,

559
00:40:23,700 --> 00:40:28,940
as in this one.

560
00:40:35,620 --> 00:40:41,340
This won't work, and this actually is a valid command

561
00:40:41,340 --> 00:40:43,390
but it won't do what you intend.

562
00:40:43,390 --> 00:40:50,070
That command will work because if you just say set and a variable name

563
00:40:50,070 --> 00:40:54,890
with no equals sign or set and a variable name with an equals sign and no value,

564
00:40:54,890 --> 00:40:57,770
it will set the variable to a null value.

565
00:40:57,770 --> 00:41:00,120
So set a= is a valid command.

566
00:41:00,120 --> 00:41:04,370
The set command can define more than 1 variable on the same line.

567
00:41:04,370 --> 00:41:11,240
So this command here has the effect of defining both a and b to null values.

568
00:41:11,240 --> 00:41:13,470
Probably not what you want.

569
00:41:13,470 --> 00:41:17,940
This one here, mentioned earlier, will lead to an error

570
00:41:17,940 --> 00:41:21,270
because =b is not a valid expression.

571
00:41:21,270 --> 00:41:23,680
A variable name can't begin with the equals sign.

572
00:41:26,760 --> 00:41:29,080
And there are these further things here.

573
00:41:29,080 --> 00:41:36,820
The colons were used to select arguments from history lines,

574
00:41:36,820 --> 00:41:41,210
and they can be used--and I didn't go into before--to modify those things.

575
00:41:41,210 --> 00:41:44,480
They can also be used to modify shell variables.

576
00:41:44,480 --> 00:41:49,050
This one here, $a, has a value.

577
00:41:49,050 --> 00:41:55,040
:r will take off an extension.

578
00:41:55,040 --> 00:41:57,200
An extension will be anything following a dot,

579
00:41:57,200 --> 00:41:59,200
a dot and anything following it at the end of a file,

580
00:41:59,200 --> 00:42:03,230
only at the end of the list after the last slash.

581
00:42:03,230 --> 00:42:05,480
So I have it here.

582
00:42:05,480 --> 00:42:10,730
a is that. It will drop the .o.

583
00:42:10,730 --> 00:42:16,510
If there's no extension, only the pathnames after the last slash, it will have no effect.

584
00:42:16,510 --> 00:42:27,480
a:h, that variable expression, will take off the last element of a directory list,

585
00:42:27,480 --> 00:42:29,660
again, only after the last slash.

586
00:42:29,660 --> 00:42:33,160
So /a/b/c becomes /a/b,

587
00:42:33,160 --> 00:42:38,870
but this one is changed because the element after the list is null.

588
00:42:38,870 --> 00:42:43,070
Here there is something which also I want to emphasize.

589
00:42:43,070 --> 00:42:46,770
These qualifiers don't search for the existence of these files.

590
00:42:46,770 --> 00:42:48,910
They just look for strings.

591
00:42:48,910 --> 00:42:54,520
These are intended to manipulate file names, pathnames,

592
00:42:54,520 --> 00:42:57,520
but they can be used on any string even if it's not a file name.

593
00:42:57,520 --> 00:42:58,920
And they don't look for the existence,

594
00:42:58,920 --> 00:43:03,550
so if there's no such file, /a/b/c, this will still work.

595
00:43:03,550 --> 00:43:06,930
Whether it's of any use is another question, but it will still work.

596
00:43:06,930 --> 00:43:12,850
Variables are different in the Bourne shells. We'll get to that later.

597
00:43:12,850 --> 00:43:18,240
Dollar sign can be escaped just like the exclamation point and the asterisk.

598
00:43:18,240 --> 00:43:21,760
Dollar sign can be escaped with a backslash or the single quotes.

599
00:43:21,760 --> 00:43:24,790
Double quotes have the odd effect in all shells 

600
00:43:24,790 --> 00:43:28,690
of forcing the evaluation of a dollar sign variable expression.

601
00:43:28,690 --> 00:43:31,960
So if it's being escaped one way, the double quotes can have the effect

602
00:43:31,960 --> 00:43:34,380
of causing it to be evaluated anyway.

603
00:43:34,380 --> 00:43:37,090
This is a little confusing.

604
00:43:37,090 --> 00:43:43,740
If there are multiple levels of escaping, such as single quotes inside double quotes

605
00:43:43,740 --> 00:43:46,770
or double quotes inside single quotes, you should test to see what will happen

606
00:43:46,770 --> 00:43:49,520
to a variable if you're using one.

607
00:43:49,520 --> 00:43:53,410
Those 2 situations--double inside of single, single inside of double--

608
00:43:53,410 --> 00:43:55,980
don't necessarily give you the same result.

609
00:44:02,520 --> 00:44:05,600
Environment variables, bound C-shell variables.

610
00:44:05,600 --> 00:44:08,340
Environment variables are also variables in the C-shell,

611
00:44:08,340 --> 00:44:11,250
and they are also variables in other shells too.

612
00:44:11,250 --> 00:44:15,230
In the C-shell, they are distinct sets.

613
00:44:15,230 --> 00:44:18,130
The things I was saying before are about shell variables.

614
00:44:18,130 --> 00:44:21,300
Environment variables are a distinct set of variables

615
00:44:21,300 --> 00:44:28,650
with the exception of several variables which we call bound variables,

616
00:44:28,650 --> 00:44:30,640
which are very important and we'll get into those later.

617
00:44:30,640 --> 00:44:34,950
Environment variables are automatically passed on 

618
00:44:34,950 --> 00:44:41,800
to shells or commands that are run from your shell.

619
00:44:41,800 --> 00:44:46,220
The other things aren't. The shell variables, the aliases aren't. Environment variables are.

620
00:44:46,220 --> 00:44:48,630
That's why we call them environment variables,

621
00:44:48,630 --> 00:44:55,030
the idea being that the environment extends past just your current shell.

622
00:44:55,030 --> 00:45:00,510
They can be used to define things for commands.

623
00:45:00,510 --> 00:45:05,470
Here is an example. PRINTER, LPDEST.

624
00:45:05,470 --> 00:45:12,270
Both of those variables can define a printer that a command will use to print things.

625
00:45:12,270 --> 00:45:16,500
If you have multiple printers around, you might want to put the one you like.

626
00:45:16,500 --> 00:45:21,320
The reason we have 2 variables is that different sets of commands were written

627
00:45:21,320 --> 00:45:23,870
using these different variables.

628
00:45:23,870 --> 00:45:25,910
You might give them different values.

629
00:45:25,910 --> 00:45:28,860
Most likely you'll give them both the same value.

630
00:45:28,860 --> 00:45:35,840
Those things work because the commands that do printing

631
00:45:35,840 --> 00:45:40,740
were programmed to examine the values of these variables.

632
00:45:42,200 --> 00:45:46,150
If a program were not written that way, if it were written to do something else,

633
00:45:46,150 --> 00:45:48,280
the variable would be irrelevant.

634
00:45:48,280 --> 00:45:52,530
So the operating system isn't looking for these variables 

635
00:45:52,530 --> 00:45:55,210
every time you refer to a printer.

636
00:45:55,210 --> 00:45:59,090
A command that does printing is looking for these variables if it is programmed that way.

637
00:46:11,030 --> 00:46:15,240
These variables are often defined in your initialization files

638
00:46:15,240 --> 00:46:19,440
but not necessarily.

639
00:46:19,440 --> 00:46:21,050
You can define them on the command line.

640
00:46:21,050 --> 00:46:24,090
They may be defined in a command.

641
00:46:24,090 --> 00:46:28,740
A command that runs something might have its own selection of variables--

642
00:46:28,740 --> 00:46:32,390
variables that are unique to a particular software package, for example.

643
00:46:32,390 --> 00:46:36,740
They will be defined when you run that package.

644
00:46:39,690 --> 00:46:42,680
How are these variables passed to a sub-shell?

645
00:46:42,680 --> 00:46:48,210
When a sub-shell is written, it doesn't write into that area.

646
00:46:48,210 --> 00:46:53,260
The area of the sub-shell that is devoted to environment variables

647
00:46:53,260 --> 00:46:56,450
isn't written by the sub-shell; it's written by copying.

648
00:46:56,450 --> 00:47:00,530
When you run an ordinary command, such as these commands to print or whatever,

649
00:47:00,530 --> 00:47:03,840
they start off by creating a new shell.

650
00:47:03,840 --> 00:47:06,190
The shell creates a shell and then overwrites part of it 

651
00:47:06,190 --> 00:47:08,800
with the command that you're running, which is a little confusing,

652
00:47:08,800 --> 00:47:10,740
but that's how these commands get the environment variables

653
00:47:10,740 --> 00:47:14,890
that they then refer to later on.

654
00:47:21,920 --> 00:47:28,010
The command here for defining the variable setenv.

655
00:47:28,010 --> 00:47:36,470
That's how you define it. It's 3 elements: setenv, variable, value.

656
00:47:36,470 --> 00:47:44,710
If you just do setenv with no arguments, what do you get?

657
00:47:47,220 --> 00:47:48,810
A list of all of those variables.

658
00:47:48,810 --> 00:47:53,190
Again, it's a nice long list and in this case, as in the others,

659
00:47:53,190 --> 00:47:57,320
these variables are defined largely by my login operation by the shell itself

660
00:47:57,320 --> 00:47:59,740
rather than by anything I did.

661
00:47:59,740 --> 00:48:03,580
There's another command here, printenv.

662
00:48:07,520 --> 00:48:10,340
That also prints out the environment.

663
00:48:10,340 --> 00:48:15,240
Notice this last thing here, EDITOR=vi.

664
00:48:15,240 --> 00:48:21,120
That says that if I'm using something that calls an editor

665
00:48:21,120 --> 00:48:25,530
and I don't specify an editor and it allows me the choice, it may give me vi.

666
00:48:25,530 --> 00:48:37,280
What if I do printenv EDITOR? It tells me what it is.

667
00:48:37,280 --> 00:48:41,340
Right before that, there was a variable, LESS.

668
00:48:41,340 --> 00:48:46,040
These are your defaults options when I run the LESS command,

669
00:48:46,040 --> 00:48:49,360
which displays files.

670
00:48:49,360 --> 00:48:55,910
So if I do that, printenv can take 1 argument or 0 arguments,

671
00:48:55,910 --> 00:48:58,070
not more than 1.

672
00:49:01,800 --> 00:49:05,690
There are other commands also, but we're not going to get into all that today.

673
00:49:05,690 --> 00:49:11,010
Remember there were the modifiers for the shell variables like :h,

674
00:49:11,010 --> 00:49:14,350
which will drop the last element of a pathname,

675
00:49:14,350 --> 00:49:17,950
or :r, which will drop an extension.

676
00:49:17,950 --> 00:49:23,110
Those now apply to the environment variables too. They didn't used to.

677
00:49:23,110 --> 00:49:24,960
It used to be they couldn't be modified. Now they can be.

678
00:49:24,960 --> 00:49:29,190
It's one of the advances with the developments of the shells over the years.

679
00:49:29,190 --> 00:49:35,620
I was saying that the shells as part of the environments

680
00:49:35,620 --> 00:49:43,040
and shell variables in the C-shell are, with some exceptions, distinct sets.

681
00:49:43,040 --> 00:49:46,790
You can establish an environment variable and a shell variable with the same name.

682
00:49:46,790 --> 00:49:49,220
They will be different variables; they can have different values.

683
00:49:49,220 --> 00:49:53,090
Changing the value of one won't change the value of the other.

684
00:49:53,090 --> 00:49:58,070
These variables are all evaluated with the dollar sign--$a, $whatever.

685
00:49:58,070 --> 00:50:02,340
So what if you have this? Do you know which one you get?

686
00:50:02,340 --> 00:50:04,520
In my tests I got the shell variable,

687
00:50:04,520 --> 00:50:07,240
but this isn't documented and you can't rely on that.

688
00:50:07,240 --> 00:50:10,270
So I ask you, is creating shell and environment variables 

689
00:50:10,270 --> 00:50:13,490
with the same names a good idea? No. Okay.

690
00:50:13,490 --> 00:50:17,460
What are those major exceptions in which the environment and shell variables

691
00:50:17,460 --> 00:50:19,860
are linked to each other?

692
00:50:19,860 --> 00:50:27,470
There are these 4.

693
00:50:32,030 --> 00:50:35,510
Capital letter TERM environment variable,

694
00:50:35,510 --> 00:50:41,540
shell variable term in small letters, type of terminal emulation.

695
00:50:41,540 --> 00:50:47,430
I'm just going to go over here and I'm going to do echo, a useful command here,

696
00:50:47,430 --> 00:50:52,560
$TERM $term. And there.

697
00:50:52,560 --> 00:51:00,570
xterm is a terminal type for windows displayed in the x Window System.

698
00:51:00,570 --> 00:51:04,330
xterm-color is a variation of that that allows different colors.

699
00:51:04,330 --> 00:51:06,580
Why do we define these? What is this good for?

700
00:51:06,580 --> 00:51:09,740
Commands that rearrange the screen like the editor

701
00:51:09,740 --> 00:51:13,680
send particular sequences, called escape sequences,

702
00:51:13,680 --> 00:51:18,160
to a terminal or a window to rearrange it and so forth.

703
00:51:18,160 --> 00:51:20,990
Those sequences are different for different types of terminals.

704
00:51:20,990 --> 00:51:23,100
This tells it which ones to use.

705
00:51:23,100 --> 00:51:25,900
Sometimes there are issues there.

706
00:51:25,900 --> 00:51:28,600
You might want to change that.

707
00:51:28,600 --> 00:51:30,780
If things aren't working, sometimes the terminal type is set wrong,

708
00:51:30,780 --> 00:51:36,440
you may be able to fix it by redefining the term variable.

709
00:51:36,440 --> 00:51:43,420
In these cases, changing one variable, the environment variable or the shell variable,

710
00:51:43,420 --> 00:51:45,970
should change the other one.

711
00:51:45,970 --> 00:51:50,970
I've discovered through experience that changing TERM in capital letters

712
00:51:50,970 --> 00:51:54,060
doesn't always change shell variable term in small letters.

713
00:51:54,060 --> 00:51:55,550
This is a bug.

714
00:51:55,550 --> 00:51:59,400
I don't know if that's always true. Most of the time it isn't true, but it can be.

715
00:51:59,400 --> 00:52:02,490
So if you make a change, just check that out.

716
00:52:02,490 --> 00:52:05,830
It's not often that you need to change that value, but once in a while you do.

717
00:52:05,830 --> 00:52:08,260
Environment variable USER.

718
00:52:08,260 --> 00:52:12,070
Again, environment variable in capital letters, shell variable in small letters.

719
00:52:12,070 --> 00:52:13,710
This is your username.

720
00:52:13,710 --> 00:52:16,730
It's only under very exceptional circumstances 

721
00:52:16,730 --> 00:52:18,420
that you would want to change that.

722
00:52:18,420 --> 00:52:22,350
If your username is someone else, it can throw all sorts of things off.

723
00:52:22,350 --> 00:52:26,040
Home directory, user's home directory.

724
00:52:26,040 --> 00:52:28,060
Again, you wouldn't want to change that.

725
00:52:28,060 --> 00:52:32,260
Notice in all of these cases and the one that we're about to cover, the path variable,

726
00:52:32,260 --> 00:52:37,070
environment variable is in capital letters and the bound shell variable is in small letters.

727
00:52:37,070 --> 00:52:39,240
If you change one, you should change the other.

728
00:52:39,240 --> 00:52:45,960
This kind of binding cannot be established as you can't bind 2 variables,

729
00:52:45,960 --> 00:52:50,570
other than these 4, and the binding in these variables can't be undone, 

730
00:52:50,570 --> 00:52:52,090
you can't separate them.

731
00:52:52,090 --> 00:52:55,820
So these 4 pairs of variables are bound.

732
00:52:55,820 --> 00:52:59,020
They always will be. None others will be.

733
00:52:59,020 --> 00:53:05,720
In addition, it would be possible to create variables with the same names

734
00:53:05,720 --> 00:53:07,780
of the opposite types.

735
00:53:07,780 --> 00:53:11,600
You could make a shell variable term in small letters

736
00:53:11,600 --> 00:53:14,990
or an environment variable TERM in capital letters.

737
00:53:14,990 --> 00:53:19,040
Those variables would be independent of these paired variables

738
00:53:19,040 --> 00:53:20,780
and they would be independent of each other.

739
00:53:20,780 --> 00:53:23,780
I can't imagine why you would do that unless you want to confuse people.

740
00:53:24,600 --> 00:53:29,730
This one here, path variable, this is a really important one.

741
00:53:29,730 --> 00:53:35,550
Another thing here is that there can be cases

742
00:53:35,550 --> 00:53:40,430
of variables with similar paired names which aren't bound to each other.

743
00:53:40,430 --> 00:53:45,000
There can be variables, SHELL and shell, in capital and small letters.

744
00:53:45,000 --> 00:53:48,300
Based on that name, you don't know if that variable is a shell variable

745
00:53:48,300 --> 00:53:51,580
or an environment variable, and they're not bound to each other.

746
00:53:51,580 --> 00:53:55,300
So that kind of paired names doesn't imply bound variables.

747
00:53:55,300 --> 00:53:58,830
The path variable, which I was showing before,

748
00:53:58,830 --> 00:54:01,880
is a list of pathnames in which the shell looks for commands.

749
00:54:01,880 --> 00:54:12,320
Let's get over to this window here and we'll do echo $PATH, capital letters--

750
00:54:12,320 --> 00:54:20,230
environment variable--echo $path, small letters--shell variable.

751
00:54:20,230 --> 00:54:24,980
Notice that the list of directories is the same. These are bound.

752
00:54:24,980 --> 00:54:26,590
Change one, you change the other.

753
00:54:26,590 --> 00:54:32,970
In the environment variable the elements are separated by colons. Notice that.

754
00:54:32,970 --> 00:54:35,130
The shell variables are separated by spaces.

755
00:54:35,130 --> 00:54:38,760
This environment variable is a single string.

756
00:54:38,760 --> 00:54:41,480
The shell variable is an array.

757
00:54:41,480 --> 00:54:43,490
The Bourne shell didn't have arrays.

758
00:54:43,490 --> 00:54:46,600
Bash does, but this is already a fixed part of the shell.

759
00:54:46,600 --> 00:54:48,660
This is a single string and not an array.

760
00:54:48,660 --> 00:54:50,420
The C-shell always had arrays.

761
00:54:50,420 --> 00:54:52,630
The arrays are much easier to work with.

762
00:54:52,630 --> 00:54:54,400
You can refer to parts of it.

763
00:54:54,400 --> 00:55:02,350
So echo $path[1] and I get /usr/bin, the first element.

764
00:55:02,350 --> 00:55:09,950
Again, remember dollar sign stands for the last element of the history list.

765
00:55:09,950 --> 00:55:16,850
What happens there? It tried to find dollar sign as a variable symbol.

766
00:55:16,850 --> 00:55:20,850
I escape it. Oops. It wouldn't take that either.

767
00:55:20,850 --> 00:55:23,690
Some of these things don't work so well.

768
00:55:23,690 --> 00:55:28,140
Maybe we'll just leave that out.

769
00:55:28,140 --> 00:55:36,980
Asterisk refers to the whole thing, but that's what you get if you don't specify an element.

770
00:55:36,980 --> 00:55:46,170
Another way that array variables can be manipulated,

771
00:55:46,170 --> 00:55:49,500
number of elements there, 7 elements.

772
00:55:49,500 --> 00:55:53,410
Here we put the pound sign before the variable name.

773
00:55:53,410 --> 00:55:58,280
Here's another one. Put a question mark there.

774
00:55:58,280 --> 00:56:03,170
That is a logical value. That indicates that the variable exists.

775
00:56:03,170 --> 00:56:05,160
It's another way of working with variables.

776
00:56:05,160 --> 00:56:06,660
That, by the way, doesn't have to be an array variable.

777
00:56:06,660 --> 00:56:08,210
That could be any variable.

778
00:56:08,210 --> 00:56:11,840
And if I do, there's no such variable and I get a 0.

779
00:56:11,840 --> 00:56:14,990
Another little thing there about variable evaluations.

780
00:56:23,670 --> 00:56:32,950
Back to this one here, if for some reason you wanted to work with this

781
00:56:32,950 --> 00:56:37,990
rather than working with the array, the shell variable,

782
00:56:37,990 --> 00:56:41,470
there are commands that can separate these things based on the colon.

783
00:56:41,470 --> 00:56:44,080
In fact, if you're going to be doing this in the Bash shell possibly,

784
00:56:44,080 --> 00:56:47,110
some kind of a script, that would be probably how you would do it.

785
00:56:47,110 --> 00:56:50,350
But in the C-shell it's much easier to use the array.

786
00:56:50,350 --> 00:56:58,250
In the Bourne shell, variables are assigned by a single expression like this,

787
00:56:58,250 --> 00:57:01,760
like the way you might assign a variable in a programming language,

788
00:57:01,760 --> 00:57:05,110
and here there must be no spaces.

789
00:57:05,110 --> 00:57:09,110
It's necessary that it be just 1 string.

790
00:57:09,110 --> 00:57:14,980
In the Bourne-type shells, all variables are shell variables.

791
00:57:14,980 --> 00:57:19,250
Environment variables are a subset of the shell variables.

792
00:57:19,250 --> 00:57:24,060
They are distinguished from the non-environment variables by exporting.

793
00:57:24,060 --> 00:57:28,860
The command to do that is export, like export PRINTER.

794
00:57:28,860 --> 00:57:34,930
If we were to define such a variable,

795
00:57:34,930 --> 00:57:38,480
if we wanted a printing command to find it, it would have to be an environment variable,

796
00:57:38,480 --> 00:57:40,730
and that's how we make it one.

797
00:57:40,730 --> 00:57:42,090
Here there's something kind of confusing.

798
00:57:42,090 --> 00:57:50,430
This expression, export to the environment, derives from this Bourne shell concept,

799
00:57:50,430 --> 00:57:54,520
and yet that expression is used in descriptions of the C-shell,

800
00:57:54,520 --> 00:57:57,920
where there is no such command as export.

801
00:57:57,920 --> 00:58:06,200
If you just say export by itself, you get a list of exported--

802
00:58:06,200 --> 00:58:10,620
So if I just do export here, no such thing.

803
00:58:13,620 --> 00:58:15,200
Okay, there we go.

804
00:58:15,200 --> 00:58:17,010
These things, by the way, are also defined by the shell.

805
00:58:17,010 --> 00:58:19,400
I didn't define any of these by myself.

806
00:58:19,400 --> 00:58:23,550
The shell does all sorts of things by itself.

807
00:58:23,550 --> 00:58:26,650
It should do things automatically.

808
00:58:30,240 --> 00:58:36,880
In Bash or Korn shell, you can run a command like this,

809
00:58:36,880 --> 00:58:42,000
which will both give a variable a value and export it in 1 command.

810
00:58:42,000 --> 00:58:46,150
In the Bourne shell they have to be separate commands like export a.

811
00:58:46,150 --> 00:58:48,410
Here is another aspect that's confusing.

812
00:58:48,410 --> 00:58:52,220
The set command in the C-shell defines variables 

813
00:58:52,220 --> 00:58:55,550
and with no arguments tells you what the variables' values are.

814
00:58:55,550 --> 00:59:01,140
In the Bash shell, the set command with no arguments does the same thing,

815
00:59:01,140 --> 00:59:03,580
but with arguments it does something quite different.

816
00:59:03,580 --> 00:59:06,200
So these are the various arguments here.

817
00:59:06,200 --> 00:59:10,460
Some of these are environment variables, some of them are shell variables.

818
00:59:10,460 --> 00:59:13,200
All of them are shell variables really. Some of those are environment variables.

819
00:59:15,690 --> 00:59:23,920
The set command with arguments can be used to operate

820
00:59:23,920 --> 00:59:28,220
on the positional parameters to a script,

821
00:59:28,220 --> 00:59:33,910
which is a way of getting them all at once.

822
00:59:33,910 --> 00:59:36,150
We can't really go into that today.

823
00:59:36,150 --> 00:59:39,580
It can also be used to change shell behavior. 

824
00:59:39,580 --> 00:59:46,700
Particularly in Bash there are variables which will determine how the shell behaves.

825
00:59:46,700 --> 00:59:51,310
Then also just this one command that you might see, this command.

826
00:59:51,310 --> 00:59:59,050
Typeset followed by variables and variable types is used in the Korn and Bash shells.

827
00:59:59,050 --> 01:00:04,970
It's not mandatory but it can be used to restrict the values of variables,

828
01:00:04,970 --> 01:00:08,400
which can be useful to prevent errors, and it's fairly common.

829
01:00:08,400 --> 01:00:11,640
So I'm just mentioning that in case you see it somewhere.

830
01:00:17,290 --> 01:00:19,160
The where command.

831
01:00:19,160 --> 01:00:22,490
Remember I mentioned earlier the where command in the C-shell,

832
01:00:22,490 --> 01:00:28,750
which can tell you the location of a command pathname.

833
01:00:28,750 --> 01:00:32,580
Here is command substitution.

834
01:00:32,580 --> 01:00:41,900
You should find on your keyboard somewhere a character that looks like this.

835
01:00:41,900 --> 01:00:44,910
The location on the keyboard is going to vary.

836
01:00:44,910 --> 01:00:47,050
We've called it backquote. It's about the size of a quote.

837
01:00:47,050 --> 01:00:48,720
It goes from upper left to lower right.

838
01:00:48,720 --> 01:00:52,690
Here on my Mac keyboard it's in the upper left-hand corner.

839
01:00:52,690 --> 01:00:58,150
That character can be used to execute a command within a command.

840
01:00:58,150 --> 01:01:03,400
If you have an expression inside backquotes,

841
01:01:03,400 --> 01:01:07,080
that expression is a command, it's run.

842
01:01:07,080 --> 01:01:09,010
The output of that command 

843
01:01:09,010 --> 01:01:11,980
is then substituted for the whole backquote expression

844
01:01:11,980 --> 01:01:16,110
inside a longer command which then runs with that output

845
01:01:16,110 --> 01:01:22,010
as part of its string of arguments and so forth.

846
01:01:22,010 --> 01:01:28,640
Here is a command which uses that.

847
01:01:28,640 --> 01:01:32,340
Let's demonstrate the operation here.

848
01:01:44,980 --> 01:01:49,090
Let's go up here, take out the backquotes.

849
01:01:49,090 --> 01:01:54,410
Control A gets me to the beginning of the line with the Emacs editing syntax.

850
01:01:54,410 --> 01:02:00,380
So far the pathnames is what where does,

851
01:02:00,380 --> 01:02:05,040
but when I do it like this, it then plugs in that list of pathnames

852
01:02:05,040 --> 01:02:08,750
in place of this whole backquote expression and runs ls -l on them.

853
01:02:08,750 --> 01:02:11,120
Kind of convenient, huh?

854
01:02:11,120 --> 01:02:14,860
So that's one neat thing. That's how backquotes work.

855
01:02:14,860 --> 01:02:17,560
Now let's go down a little further.

856
01:02:17,560 --> 01:02:22,050
These are aliases. I actually use these.

857
01:02:22,050 --> 01:02:26,410
I'll try to get this in with 1 editing operation.

858
01:02:34,900 --> 01:02:36,900
Okay. 

859
01:02:36,900 --> 01:02:39,630
Now let's see how those definitions came out.

860
01:02:39,630 --> 01:02:44,930
alias lwh telling me how it's defined.

861
01:02:44,930 --> 01:02:51,210
Notice it's just this, but the outer quotes have been taken off

862
01:02:51,210 --> 01:02:53,750
and the exclamation point is taken off.

863
01:02:53,750 --> 01:02:58,940
!*, complete list of all the arguments.

864
01:02:58,940 --> 01:03:03,580
In an alias definition it will apply back to where I use this.

865
01:03:03,580 --> 01:03:10,620
lwh ksh bash. Okay.

866
01:03:10,620 --> 01:03:13,960
See how that works? It saves me some typing.

867
01:03:13,960 --> 01:03:16,440
Let's go up a little bit just to mention something else here.

868
01:03:19,150 --> 01:03:23,120
Notice here these different shells. I should have mentioned this before.

869
01:03:23,120 --> 01:03:36,060
The csh has a 2 over here and so does /bin/tcsh.

870
01:03:36,060 --> 01:03:39,870
We could establish by other means that those are actually the same file.

871
01:03:39,870 --> 01:03:43,150
Remember I was saying if you type sh you get bash.

872
01:03:43,150 --> 01:03:47,390
Type this and you get this.

873
01:03:47,390 --> 01:03:51,730
But those aren't linked. Those have single ones there.

874
01:03:51,730 --> 01:03:54,910
And this isn't the kind of file which can call another one.

875
01:03:54,910 --> 01:03:59,460
So those are separate files; the C-shell ones are the same file.

876
01:03:59,460 --> 01:04:03,640
Back down here, the other one here, this alias,

877
01:04:03,640 --> 01:04:09,090
note that's running this command, file.

878
01:04:09,090 --> 01:04:13,810
That alias runs that. File tells you the type of a file.

879
01:04:13,810 --> 01:04:20,330
So fwh ksh bash. Okay.

880
01:04:20,330 --> 01:04:23,230
That's the output of the file command.

881
01:04:23,230 --> 01:04:24,630
I don't know if you know what this means here, 

882
01:04:24,630 --> 01:04:26,750
Mach-O universal binary with 2 architectures.

883
01:04:26,750 --> 01:04:30,470
There are 2 possible processor types in Mac,

884
01:04:30,470 --> 01:04:34,780
and some programs were written to be able to run with both,

885
01:04:34,780 --> 01:04:37,950
and the file command can determine that, so that's what this means.

886
01:04:37,950 --> 01:04:40,660
Both of these files were written that way.

887
01:04:40,660 --> 01:04:43,760
So we see how the alias works, we see how the backquote works,

888
01:04:43,760 --> 01:04:48,640
we see how the actual file ls or file works.

889
01:04:52,050 --> 01:04:57,000
This might not work. Try "where where" and "lwh where". Okay, let's try that.

890
01:04:57,000 --> 01:05:01,040
where where.

891
01:05:01,040 --> 01:05:03,500
where is a shell built-in.

892
01:05:03,500 --> 01:05:06,970
Remember earlier we showed that Bash didn't have where.

893
01:05:06,970 --> 01:05:10,080
If you type where in the Bash shell, you get an error message.

894
01:05:10,080 --> 01:05:12,540
It's just part of the shell rather than being a separate command.

895
01:05:12,540 --> 01:05:20,000
What happens if I type lwh looking for where? See what happens there.

896
01:05:20,000 --> 01:05:22,850
Ran where where, got this output, and then tried to run ls

897
01:05:22,850 --> 01:05:25,600
as l on where is a shell built-in.

898
01:05:25,600 --> 01:05:28,790
where is there, but the other ones don't exist.

899
01:05:28,790 --> 01:05:32,090
None of these exist, actually.

900
01:05:32,090 --> 01:05:35,560
So that doesn't always work, and it also illustrates how some things

901
01:05:35,560 --> 01:05:39,580
don't do quite what you might have thought.

902
01:05:40,930 --> 01:05:43,010
Let's go down a little further here.

903
01:05:44,890 --> 01:05:54,760
This here is in Bash. That is also command substitution like the backquote.

904
01:05:54,760 --> 01:06:05,280
But unlike backquote, it uses this variable style.

905
01:06:05,280 --> 01:06:09,860
There are a number of expressions which begin with a dollar sign,

906
01:06:09,860 --> 01:06:16,070
and while these aren't variables, they borrowed the use of the dollar sign

907
01:06:16,070 --> 01:06:19,570
to indicate an expression of some kind.

908
01:06:19,570 --> 01:06:23,550
That can be surrounded by parentheses or brackets or double parentheses,

909
01:06:23,550 --> 01:06:26,320
which has a different purpose.

910
01:06:26,320 --> 01:06:29,500
Single parentheses here are a command substitution just like the backquotes.

911
01:06:29,500 --> 01:06:32,720
Double parentheses is actually an arithmetic operation.

912
01:06:32,720 --> 01:06:35,380
There are other syntaxes, other operations.

913
01:06:35,380 --> 01:06:41,520
Backquote syntax is available in Bash.

914
01:06:41,520 --> 01:06:46,780
However, this one is preferable. It's much easier to read and it allows nesting.

915
01:06:46,780 --> 01:06:51,300
You can have inside $(command) another command,

916
01:06:51,300 --> 01:06:54,590
something like--

917
01:07:14,560 --> 01:07:18,210
I get a list there.

918
01:07:18,210 --> 01:07:21,670
That would work if I had the backquote also.

919
01:07:32,050 --> 01:07:38,470
What if I want to do something like--

920
01:08:03,390 --> 01:08:06,430
You probably wouldn't actually use this command,

921
01:08:06,430 --> 01:08:14,160
but this internal command substitution echoes the names of all files beginning with a,

922
01:08:14,160 --> 01:08:18,229
then this one runs ls -l on those files,

923
01:08:18,229 --> 01:08:20,500
and then this one just echoes the output.

924
01:08:21,729 --> 01:08:24,479
You probably wouldn't do this; you'd just do the echo or ls,

925
01:08:24,479 --> 01:08:29,450
but this illustrates how the nesting of commands works.

926
01:08:29,450 --> 01:08:34,380
So just another feature here.

927
01:08:34,380 --> 01:08:37,450
 I mentioned this earlier, that when you have where in the C-shell,

928
01:08:37,450 --> 01:08:42,770
type works in the Bourne-type shells for locating commands.

929
01:08:48,939 --> 01:08:52,270
Built-in commands, just what I was saying there.

930
01:08:52,270 --> 01:08:54,640
Commands are part of the shell, like where.

931
01:08:54,640 --> 01:08:59,880
When the shell executes a command like ls, it locates it through the path,

932
01:08:59,880 --> 01:09:03,029
finds it in some directory somewhere,

933
01:09:03,029 --> 01:09:05,800
reads that into memory, creates a new shell,

934
01:09:05,800 --> 01:09:08,960
reads the command ls or whatever into the shell

935
01:09:08,960 --> 01:09:11,450
where the environment variables are already located,

936
01:09:11,450 --> 01:09:14,000
and then it transfers execution to it.

937
01:09:14,000 --> 01:09:18,319
Built-in command, the code for that command is inside the shell,

938
01:09:18,319 --> 01:09:21,460
so the shell just starts executing part of its own code.

939
01:09:21,460 --> 01:09:24,569
where is such a command. It actually gets faster.

940
01:09:24,569 --> 01:09:28,380
It doesn't have to read anything in memory; it's already in memory.

941
01:09:28,380 --> 01:09:32,460
Built-in commands always take precedence over commands with the same name.

942
01:09:32,460 --> 01:09:36,050
Commands that are in directories in the path may have the same name,

943
01:09:36,050 --> 01:09:39,090
commands in different directories, files in different directories.

944
01:09:39,090 --> 01:09:41,740
The one that occurs earlier in the path is the one you'll get.

945
01:09:41,740 --> 01:09:43,770
If there is a built-in command, you always get it.

946
01:09:43,770 --> 01:09:47,890
There's no way to give it a lower precedence than a command in the path.

947
01:09:47,890 --> 01:09:54,140
If you want to get that path command, you can type the full pathname.

948
01:09:54,140 --> 01:09:55,850
If there were a command where in the path somewhere,

949
01:09:55,850 --> 01:09:58,440
you could type /bin/where and you'd get it.

950
01:09:58,440 --> 01:10:01,800
If you don't want to type the whole pathname, you could define an alias.

951
01:10:01,800 --> 01:10:06,310
In fact, if you gave the alias the same name as the built-in command, it would work

952
01:10:06,310 --> 01:10:08,790
because the alias definition is evaluated 

953
01:10:08,790 --> 01:10:13,220
before the shell determines that it's a built-in command which should be executed.

954
01:10:18,810 --> 01:10:23,440
Then this gets a little more complicated with some commands here.

955
01:10:23,440 --> 01:10:29,880
The case of some commands are actually built-in commands and in the path.

956
01:10:29,880 --> 01:10:34,140
One of them is echo, the command I just used a little while ago in those examples.

957
01:10:34,140 --> 01:10:37,410
Echo is a command in the path and it's in every shell.

958
01:10:37,410 --> 01:10:40,580
They don't necessarily all behave the same way.

959
01:10:40,580 --> 01:10:42,970
It was originally a command only in the path.

960
01:10:42,970 --> 01:10:45,280
It was built in to the shells later.

961
01:10:45,280 --> 01:10:48,080
Because there are options which depend on the environment

962
01:10:48,080 --> 01:10:52,970
and the command line options, the built-in commands

963
01:10:52,970 --> 01:10:57,030
were written to function the same as the command that had been in the path,

964
01:10:57,030 --> 01:10:59,670
it's unlikely they would have been written that way 

965
01:10:59,670 --> 01:11:01,720
if the command hadn't already been written for the path.

966
01:11:01,720 --> 01:11:06,180
So this has side effects. Its history has effects here.

967
01:11:06,180 --> 01:11:08,380
There are options there.

968
01:11:14,280 --> 01:11:23,060
There's also an option defined by a variable in the tcsh called echo_style.

969
01:11:23,060 --> 01:11:27,700
That's one of these variables that can change the way that echo works.

970
01:11:27,700 --> 01:11:30,910
There are other cases in which you can assign a variable

971
01:11:30,910 --> 01:11:36,290
that changes the way that the shell operation, including a built-in command, works.

972
01:11:36,290 --> 01:11:38,130
It wouldn't affect anything else 

973
01:11:38,130 --> 01:11:40,640
since other commands don't have access to the shell variables, 

974
01:11:40,640 --> 01:11:42,090
only the environment variables.

975
01:11:42,090 --> 01:11:45,360
But shell operations can read the shell variables.

976
01:11:45,360 --> 01:11:50,710
That won't work for csh. That's only tcsh. That's one of the enhancements.

977
01:11:58,540 --> 01:12:04,620
Parsing has sequences when it evaluates metacharacters,

978
01:12:04,620 --> 01:12:08,140
when it evaluates variables, aliases, history references.

979
01:12:08,140 --> 01:12:11,830
There's a particular sequence for these things.

980
01:12:11,830 --> 01:12:13,730
If it does things in a particular sequence

981
01:12:13,730 --> 01:12:16,080
and gets to something that's an expression of a sort

982
01:12:16,080 --> 01:12:20,650
which has already been evaluated, it won't evaluate it again.

983
01:12:20,650 --> 01:12:24,520
If it gets it, then it will just pass on the characters.

984
01:12:24,520 --> 01:12:29,920
So if evaluation of some expressions like command substitution

985
01:12:29,920 --> 01:12:36,850
or variable or whatever gives rise to an expression 

986
01:12:36,850 --> 01:12:39,240
which you would want to be evaluated, 

987
01:12:39,240 --> 01:12:42,510
that will work only if that evaluation occurs later in the sequence.

988
01:12:42,510 --> 01:12:45,010
I hope I'm being clear there.

989
01:12:45,010 --> 01:12:50,460
That parsing sequence, an operation in the C-shell,

990
01:12:50,460 --> 01:12:56,490
isn't the same for built-in commands as it is for non-built-in commands.

991
01:12:56,490 --> 01:12:58,890
I'm not sure about Bash there.

992
01:12:58,890 --> 01:13:02,450
For example, if a shell variable produced a history reference,

993
01:13:02,450 --> 01:13:04,230
it probably would not go back in the history.

994
01:13:04,230 --> 01:13:06,010
It would just get the exclamation point.

995
01:13:06,010 --> 01:13:08,840
In fact, we can just try that out right now.

996
01:13:09,720 --> 01:13:18,240
set a= and we'll have to put this in there.

997
01:13:30,690 --> 01:13:34,580
Oh, wait. Sorry. I did this in the Bash. I wanted to do it here.

998
01:13:53,470 --> 01:13:56,080
See, so it didn't evaluate that history reference

999
01:13:56,080 --> 01:14:00,520
because it was already past the point of evaluating history expressions

1000
01:14:00,520 --> 01:14:02,720
when it evaluated the variable.

1001
01:14:02,720 --> 01:14:05,550
So that's 1 effect of parsing.

1002
01:14:05,550 --> 01:14:08,760
And again, built-in commands aren't done the same way.

1003
01:14:08,760 --> 01:14:11,230
All right. Let's go to the next one here.

1004
01:14:11,230 --> 01:14:16,060
This is intended to be 1 line, but it's making it easier to read.

1005
01:14:19,130 --> 01:14:21,530
What does that do?

1006
01:14:21,530 --> 01:14:28,640
You may recall that we can evaluate asterisks as filename wildcards,

1007
01:14:28,640 --> 01:14:33,890
and there are other filename wildcards like the question mark and bracket expressions.

1008
01:14:33,890 --> 01:14:39,000
That kind of evaluation is called globbing.

1009
01:14:39,000 --> 01:14:46,290
set noglob at the beginning of this command says don't do that.

1010
01:14:46,290 --> 01:14:53,370
unset noglob says go back to doing that.

1011
01:14:53,370 --> 01:14:56,440
Note that set glob would not have that effect.

1012
01:14:56,440 --> 01:15:00,800
In ordinary language, set glob or unset noglob would seem to be equivalent,

1013
01:15:00,800 --> 01:15:03,290
but here it isn't. It's unset noglob.

1014
01:15:05,120 --> 01:15:07,910
Now tset. tset stood for terminal set.

1015
01:15:07,910 --> 01:15:11,840
It's not used that often now, but before windowing systems became available

1016
01:15:11,840 --> 01:15:15,760
and you had a single terminal, you might have to determine the type.

1017
01:15:15,760 --> 01:15:18,700
And if something was coming over an Ethernet or from the network,

1018
01:15:18,700 --> 01:15:21,120
you might want to say it's a vt100.

1019
01:15:21,120 --> 01:15:26,630
VT100 is kind of a standard in the terminal business. It comes from the DEC terminal.

1020
01:15:26,630 --> 01:15:35,270
If you just do dialup--notice that? This goes back a ways, huh?

1021
01:15:35,270 --> 01:15:39,520
So if we just do tset over here,

1022
01:15:39,520 --> 01:15:45,250
if I just do tset, it's resetting my terminal, but you didn't see anything.

1023
01:15:45,250 --> 01:15:47,340
It didn't really change anything.

1024
01:15:47,340 --> 01:15:48,620
-s

1025
01:15:49,900 --> 01:15:51,480
Okay.

1026
01:15:51,480 --> 01:15:53,350
setenv TERM xterm-color.

1027
01:15:53,350 --> 01:15:57,080
We already know that the term was set that way, so that didn't change.

1028
01:15:57,080 --> 01:15:58,860
That's the way we'd want to do it.

1029
01:15:58,860 --> 01:16:07,080
But notice that this command, tset -s, just output these commands. It didn't run them.

1030
01:16:07,080 --> 01:16:09,770
It didn't run these commands; it output them.

1031
01:16:09,770 --> 01:16:13,650
So this is intended to produce commands which will then be run.

1032
01:16:13,650 --> 01:16:16,360
You remember the command in that file I just showed you had a Q in it.

1033
01:16:16,360 --> 01:16:18,910
So let's do that.

1034
01:16:18,910 --> 01:16:23,750
The Q suppresses some output, but that doesn't matter here, as you can see.

1035
01:16:23,750 --> 01:16:27,980
I'm just doing that to show you that it didn't matter.

1036
01:16:27,980 --> 01:16:31,870
This is in backquote syntax.

1037
01:16:31,870 --> 01:16:35,340
Note the backquote here, backquote here.

1038
01:16:35,340 --> 01:16:37,680
I'm omitting these things here.

1039
01:16:37,680 --> 01:16:39,570
These are cases of telling it what to do

1040
01:16:39,570 --> 01:16:42,050
in the case of particular types of terminals--

1041
01:16:42,050 --> 01:16:45,400
Ethernet, network, dialup, what have you.

1042
01:16:45,400 --> 01:16:48,050
It doesn't matter here because we're not actually doing any of these things.

1043
01:16:48,050 --> 01:16:49,720
I'm just illustrating the command.

1044
01:16:49,720 --> 01:16:55,170
If I do this with the backquote, what am I going to get?

1045
01:16:55,170 --> 01:17:00,210
Also notice here that this included the set noglob and the unset noglob,

1046
01:17:00,210 --> 01:17:02,630
so those are now redundant in the definition.

1047
01:17:02,630 --> 01:17:05,380
That wasn't always true, but now they're included in this command.

1048
01:17:05,380 --> 01:17:08,890
But let's see what happens if I do that

1049
01:17:08,890 --> 01:17:12,570
and go to the beginning of the line with Control A and I do that.

1050
01:17:14,380 --> 01:17:18,040
Okay, set: Command not found. That's kind of odd, isn't it?

1051
01:17:18,040 --> 01:17:20,570
set is a well-known command. It's part of the shell.

1052
01:17:20,570 --> 01:17:24,040
set: Command not found? Why is that?

1053
01:17:24,040 --> 01:17:26,790
Hmm. Well, let's think about this.

1054
01:17:26,790 --> 01:17:31,100
It's running a backquote command substitution,

1055
01:17:31,100 --> 01:17:37,430
and that occurs at a certain part of the sequence of parsing the command.

1056
01:17:37,430 --> 01:17:40,360
set is a built-in command.

1057
01:17:40,360 --> 01:17:43,900
So by the time it does that command substitution,

1058
01:17:43,900 --> 01:17:48,280
it's already gotten past the point of identifying built-in commands.

1059
01:17:48,280 --> 01:17:51,900
So it treats set as if it were a command in the path.

1060
01:17:51,900 --> 01:17:55,440
Needless to say, it doesn't find it and you get an error.

1061
01:17:55,440 --> 01:17:59,300
Well. There's an example of parsing sequence.

1062
01:17:59,300 --> 01:18:01,460
And what do we do about that?

1063
01:18:01,460 --> 01:18:04,800
Notice this very interesting command here, eval.

1064
01:18:04,800 --> 01:18:06,530
I wonder what that does.

1065
01:18:06,530 --> 01:18:08,760
If you look at the manual--and let's just do that

1066
01:18:08,760 --> 01:18:12,000
to show how confusing these manuals are--

1067
01:18:12,000 --> 01:18:19,400
man tcsh, confused manual, finding things here is not easy either.

1068
01:18:19,400 --> 01:18:31,850
Here we go, eval arg, so we can have 1 or more arguments

1069
01:18:31,850 --> 01:18:34,090
and there's a list of things there.

1070
01:18:34,090 --> 01:18:37,730
Treats the arguments as inputs to the shell

1071
01:18:37,730 --> 01:18:43,600
and executes the resulting commands in the context of the current shell.

1072
01:18:43,600 --> 01:18:46,900
This is usually used to execute commands generated as the result of command

1073
01:18:46,900 --> 01:18:51,310
or variable substitution because parsing occurs before these substitutions.

1074
01:18:51,310 --> 01:18:52,580
Very good. 

1075
01:18:52,580 --> 01:18:54,740
And here they even refer to the tset command for a sample use 

1076
01:18:54,740 --> 01:18:57,700
like the one I just showed you.

1077
01:18:57,700 --> 01:19:00,440
Now I have to get the window back to a useful place.

1078
01:19:03,150 --> 01:19:07,800
Let's get over here and we'll see that eval is used just before that.

1079
01:19:07,800 --> 01:19:14,010
So let's see what happens if we put--here we go up with the arrows to that command

1080
01:19:14,010 --> 01:19:20,940
and Control A to the beginning, eval.

1081
01:19:20,940 --> 01:19:22,850
Okay, so it works.

1082
01:19:22,850 --> 01:19:26,440
When you do eval, it takes what comes after it and makes it a command.

1083
01:19:26,440 --> 01:19:29,460
This enables you to essentially parse it twice.

1084
01:19:29,460 --> 01:19:33,710
The section here runs this command inside the backquotes,

1085
01:19:33,710 --> 01:19:36,210
gets the output.

1086
01:19:36,210 --> 01:19:42,850
Output is supposed to be run as those commands here like these

1087
01:19:42,850 --> 01:19:45,890
at this one and this one.

1088
01:19:45,890 --> 01:19:50,100
So those commands are now here in this sequence,

1089
01:19:50,100 --> 01:19:58,950
but these are built-in commands and it can't get them right away.

1090
01:19:58,950 --> 01:20:06,440
So we go to eval, eval picks that up, starts the whole thing all over again, and it works.

1091
01:20:06,440 --> 01:20:18,460
An example both of backquoting, eval, parsing, consequences of parsing,

1092
01:20:18,460 --> 01:20:21,910
and a command which is probably of very little use to you nowadays.

1093
01:20:21,910 --> 01:20:25,540
Okay. All right, umask.

1094
01:20:25,540 --> 01:20:32,160
Let's look at this command here, umask 022. I wonder what that does.

1095
01:20:32,160 --> 01:20:38,420
Let's just type umask with nothing after it. 22. Okay.

1096
01:20:38,420 --> 01:20:44,350
022 and do it again.

1097
01:20:44,350 --> 01:20:48,580
As you might have guessed, umask with no arguments tells you the current mask;

1098
01:20:48,580 --> 01:20:51,760
umask with arguments makes it that, but that was the one I already had.

1099
01:20:51,760 --> 01:20:53,800
What does 022 mean?

1100
01:21:01,650 --> 01:21:07,080
These are here the protections for a file.

1101
01:21:07,080 --> 01:21:11,440
They determine who is allowed to read or write or execute the file.

1102
01:21:11,440 --> 01:21:16,560
Protections are also called permissions.

1103
01:21:16,560 --> 01:21:21,390
The r stands for read, the w for write, 

1104
01:21:21,390 --> 01:21:25,500
and the x, which isn't present there, stands for execute.

1105
01:21:25,500 --> 01:21:27,260
There are 3 categories there.

1106
01:21:27,260 --> 01:21:33,540
The last 3 elements are in the category of user. Those apply to me, the user.

1107
01:21:33,540 --> 01:21:36,870
These 3 here apply to the group.

1108
01:21:36,870 --> 01:21:41,590
The file belongs to 1 group, user may belong to several groups,

1109
01:21:41,590 --> 01:21:47,150
but if the user is in the group to which this file belongs,

1110
01:21:47,150 --> 01:21:51,090
then these protections will apply to him if he's not the user.

1111
01:21:51,090 --> 01:21:54,230
And this one is everyone else.

1112
01:21:55,540 --> 01:21:57,690
These categories are mutually exclusive.

1113
01:21:57,690 --> 01:21:59,750
The user protections apply to him,

1114
01:21:59,750 --> 01:22:03,780
the group protections apply to members of the group other than the user,

1115
01:22:03,780 --> 01:22:08,110
and the other protections only apply to people other than the user and the group members.

1116
01:22:08,110 --> 01:22:12,320
If there's an r or a w or an x, it means that protection is granted.

1117
01:22:12,320 --> 01:22:13,950
If there's a hyphen, it means it isn't.

1118
01:22:13,950 --> 01:22:16,690
There actually are other things that can be put in here besides these,

1119
01:22:16,690 --> 01:22:18,350
which I won't get into now.

1120
01:22:18,350 --> 01:22:24,450
The umask defines a default for files that you create.

1121
01:22:24,450 --> 01:22:28,580
And as a mask, basically it says the bits that you don't set.

1122
01:22:28,580 --> 01:22:30,450
How has this become bits?

1123
01:22:30,450 --> 01:22:33,240
If you think of each of these as an octal number,

1124
01:22:33,240 --> 01:22:42,120
this is the 1s bit, this is the 2s, this is the 4s.

1125
01:22:42,120 --> 01:22:45,840
So 0 through 7 

1126
01:22:45,840 --> 01:22:51,770
will describe what combination of r's, w's, and x's you have for these 3

1127
01:22:51,770 --> 01:22:53,710
and then a similar number for these and then for these.

1128
01:22:53,710 --> 01:23:12,030
So 022 means 0 for other, 2 for the group, 2 for the user.

1129
01:23:12,030 --> 01:23:15,870
But this is a mask. The mask is what you don't have.

1130
01:23:19,380 --> 01:23:20,610
I'm sorry. I just gave you things in the wrong order.

1131
01:23:20,610 --> 01:23:25,620
It's the first 3. These 3 are the user, these 3 are the group, these 3 are the other.

1132
01:23:25,620 --> 01:23:27,970
Sorry I gave you these in the wrong order.

1133
01:23:27,970 --> 01:23:31,910
The 0, which is the first of those, doesn't display the value,

1134
01:23:31,910 --> 01:23:35,430
but if a number isn't there, it's a 0.

1135
01:23:35,430 --> 01:23:38,370
That means all 3 of these would be allowed.

1136
01:23:38,370 --> 01:23:41,550
Notice that in this particular one the x isn't allowed.

1137
01:23:41,550 --> 01:23:44,090
The reason is that the shell is capable of determining 

1138
01:23:44,090 --> 01:23:46,260
whether a file should be executed or not.

1139
01:23:46,260 --> 01:23:49,800
Since this is not an executable file, it didn't set the x.

1140
01:23:49,800 --> 01:23:54,000
The 2 means that write permission, the second category here,

1141
01:23:54,000 --> 01:23:56,500
the one in the middle, is denied.

1142
01:23:56,500 --> 01:23:58,500
So again, these are the things that it denied.

1143
01:23:58,500 --> 01:24:02,080
Well, x is allowed but it's not here because it's not executable

1144
01:24:02,080 --> 01:24:04,260
and similarly for the others.

1145
01:24:04,260 --> 01:24:08,880
So that's a common umask.

1146
01:24:08,880 --> 01:24:14,630
Another common one is 700--give yourself everything and no one else anything.

1147
01:24:14,630 --> 01:24:17,040
And there are other possibilities.

1148
01:24:21,340 --> 01:24:27,110
I'll go back to that. Using the history I can search back for that, lwh to there.

1149
01:24:27,110 --> 01:24:30,210
Okay. So here, these are the shells.

1150
01:24:30,210 --> 01:24:36,020
Bash, the owner who is system account, can do everything.

1151
01:24:36,020 --> 01:24:41,210
Group and everyone else can do read or execute but not write.

1152
01:24:41,210 --> 01:24:44,570
This one doesn't even allow the owner to write to it.

1153
01:24:44,570 --> 01:24:46,460
If the owner wanted to write to it, the system account, 

1154
01:24:46,460 --> 01:24:48,020
he would have to change the protection first.

1155
01:24:48,020 --> 01:24:53,940
But again, the umask sets the default by masking it,

1156
01:24:53,940 --> 01:24:57,160
by indicating the bits that will not be set.

1157
01:24:57,160 --> 01:25:04,380
This is typically in one of your initialization files, which is the .cshrc for the C-shell

1158
01:25:04,380 --> 01:25:07,500
or the .profile for the Bourne-type shells.

1159
01:25:07,500 --> 01:25:12,520
It can be elsewhere also if there are other initialization files on the system.

1160
01:25:12,520 --> 01:25:14,610
Anyway, that's umask.

1161
01:25:14,610 --> 01:25:18,180
There's something kind of odd here,

1162
01:25:18,180 --> 01:25:22,800
and that is, why is there a single command for this?

1163
01:25:22,800 --> 01:25:28,690
If I were writing this, I would make it a variable, umask = some value.

1164
01:25:28,690 --> 01:25:31,100
Why is there a whole command just for this purpose?

1165
01:25:31,100 --> 01:25:34,560
The reason is this just goes back to the origins of Unix.

1166
01:25:34,560 --> 01:25:41,050
Unix was just some programming project at Bell Labs in the early 1970s.

1167
01:25:41,050 --> 01:25:42,610
People just got together to program. 

1168
01:25:42,610 --> 01:25:45,290
They never intended it to become a worldwide operating system.

1169
01:25:45,290 --> 01:25:47,250
Different people wrote different parts without thinking very much 

1170
01:25:47,250 --> 01:25:49,790
of how they were going to be used--rather sketchy.

1171
01:25:49,790 --> 01:25:53,290
And it came together like that, and it's still like that in some respects.

1172
01:25:53,290 --> 01:25:57,930
So that reflects the history, and there are still these inconsistencies and odd elements of it.

1173
01:25:57,930 --> 01:26:00,750
Okay. Next one here.

1174
01:26:08,170 --> 01:26:11,000
As I wrote earlier, the C-shell isn't really used very much for programming,

1175
01:26:11,000 --> 01:26:12,420
although it can be.

1176
01:26:12,420 --> 01:26:15,080
It executes more slowly, again the trade-off between interactive use,

1177
01:26:15,080 --> 01:26:17,820
which has more processing involved than speed, 

1178
01:26:17,820 --> 01:26:20,710
which can do without the processing.

1179
01:26:20,710 --> 01:26:28,320
The extra features added to the Bourne shell by the Korn and Bourne-again shells

1180
01:26:28,320 --> 01:26:32,120
don't seem to slow them down, and I don't know why that is.

1181
01:26:32,120 --> 01:26:36,310
It might just be better programming, but I'm not in a position to know.

1182
01:26:36,310 --> 01:26:40,420
Speed here actually isn't such a big deal, although it is mentioned.

1183
01:26:40,420 --> 01:26:43,690
The reason is that shell scripts actually get fairly fast.

1184
01:26:43,690 --> 01:26:46,450
If there is a lot of commands like in a calculational program,

1185
01:26:46,450 --> 01:26:49,110
you probably wouldn't do it in a shell script.

1186
01:26:49,110 --> 01:26:51,450
The operations there are fairly simple and straightforward.

1187
01:26:51,450 --> 01:26:53,960
The ones that I've experienced that are too slow

1188
01:26:53,960 --> 01:26:57,110
involve repeated applications of slow commands.

1189
01:26:57,110 --> 01:27:00,480
Earlier I mentioned the stream editor sed. That command is slow.

1190
01:27:00,480 --> 01:27:03,760
If you execute sed many times, you'll get a slow script, but it's not the shell that's slow.

1191
01:27:03,760 --> 01:27:07,920
Running it in the Bourne shell won't be much faster than running it in the C-shell,

1192
01:27:07,920 --> 01:27:10,070
although there's maybe some advantages there.

1193
01:27:10,070 --> 01:27:12,760
The additional programming capabilities, on the other hand,

1194
01:27:12,760 --> 01:27:17,920
are significant reasons why you would use the Bourne-type shells.

1195
01:27:17,920 --> 01:27:21,390
C-shell has odd features to it--

1196
01:27:21,390 --> 01:27:25,250
the fact that you don't know if a variable is a shell variable or an environment variable.

1197
01:27:25,250 --> 01:27:27,440
It can be very confusing.

1198
01:27:27,440 --> 01:27:32,170
It's not so easy to write 

1199
01:27:32,170 --> 01:27:35,930
just based on your experience of programming in other languages.

1200
01:27:35,930 --> 01:27:41,350
I think you may find the Bourne-type shells more consistent with your experience.

1201
01:27:43,730 --> 01:27:49,270
Some scripts, though, can be thousands of lines in length.

1202
01:27:49,270 --> 01:27:52,450
Those that I've seen are used for patching operating systems.

1203
01:27:52,450 --> 01:27:55,450
Those can execute very slowly, but you don't run those very often.

1204
01:27:55,450 --> 01:27:57,180
It's only when you're doing patching, 

1205
01:27:57,180 --> 01:27:59,450
and it's only the system manager who does those things,

1206
01:27:59,450 --> 01:28:01,840
so it's not really much of an issue.

1207
01:28:01,840 --> 01:28:06,980
Those that are hundreds of lines long actually execute fairly quickly.

1208
01:28:06,980 --> 01:28:10,540
Mentioning this here, what are those enhancements?

1209
01:28:10,540 --> 01:28:13,170
I've already mentioned a few of them--arrays, calculations, 

1210
01:28:13,170 --> 01:28:20,540
the $( ) expression for calculations in the Bash shell,

1211
01:28:20,540 --> 01:28:23,050
the other kind of command substitution.

1212
01:28:23,050 --> 01:28:25,360
There are different kinds of testing commands

1213
01:28:25,360 --> 01:28:29,350
by which you can do conditional tests on the existence of a file or other things.

1214
01:28:29,350 --> 01:28:34,790
Last here, this command here.

1215
01:28:34,790 --> 01:28:38,480
What does this do, and why would anybody use it?

1216
01:28:51,170 --> 01:28:52,990
printenv variablename.

1217
01:28:52,990 --> 01:28:56,130
We know what printenv does. It tells us the value of a variable.

1218
01:28:56,130 --> 01:29:00,850
And printenv variablename won't tell us very much because there's no such variable.

1219
01:29:03,550 --> 01:29:05,120
Blank.

1220
01:29:05,120 --> 01:29:08,440
But let's give it something meaningful.

1221
01:29:13,420 --> 01:29:16,800
That's not there either. Okay. I guess I never defined that.

1222
01:29:16,800 --> 01:29:18,020
Let's just check my environment.

1223
01:29:18,020 --> 01:29:20,900
This is another command by which you can inspect your environment.

1224
01:29:20,900 --> 01:29:24,470
There is good old EDITOR, the one we saw before.

1225
01:29:42,360 --> 01:29:44,120
What does that do?

1226
01:29:44,120 --> 01:29:48,050
Here we have a backquote expression.

1227
01:29:48,050 --> 01:29:50,370
Remember this is the C-shell.

1228
01:29:50,370 --> 01:29:54,850
So printenv EDITOR will give us a value of EDITOR. It's vi.

1229
01:29:54,850 --> 01:29:59,790
And then it will set that value to variable a, the set command.

1230
01:29:59,790 --> 01:30:02,860
So now if I do echo $a, I get vi.

1231
01:30:02,860 --> 01:30:05,850
That doesn't seem terribly useful.

1232
01:30:05,850 --> 01:30:08,080
However, it actually does have a purpose.

1233
01:30:08,080 --> 01:30:12,260
Since we don't know whether a variable is a shell variable or an environment variable

1234
01:30:12,260 --> 01:30:16,280
by using the dollar sign evaluation syntax, we can use printenv

1235
01:30:16,280 --> 01:30:19,460
to make sure that it's an environment variable.

1236
01:30:19,460 --> 01:30:22,550
So if there were a shell variable editor, this wouldn't have gotten it.

1237
01:30:22,550 --> 01:30:25,640
This works only with the environment variable.

1238
01:30:25,640 --> 01:30:28,370
If there were a shell variable and I wanted its value,

1239
01:30:28,370 --> 01:30:29,980
I'd have to find some other way to do it.

1240
01:30:29,980 --> 01:30:33,530
One way to do that would be by doing set and piping.

1241
01:30:33,530 --> 01:30:36,130
This is one of the metacharacters, special characters.

1242
01:30:36,130 --> 01:30:38,370
It sends the output of set to something else.

1243
01:30:38,370 --> 01:30:40,650
Let's see what we might find there.

1244
01:30:40,650 --> 01:30:49,340
Nothing. Okay. Let's just see what's in there all together.

1245
01:30:49,340 --> 01:30:53,580
It was echo_style, the one I mentioned before. Okay, let's do that.

1246
01:31:02,460 --> 01:31:06,230
Remember I mentioned before, echo_style 

1247
01:31:06,230 --> 01:31:08,410
determines the way the echo command will run.

1248
01:31:08,410 --> 01:31:10,940
bsd stands for Berkeley Standard Distribution.

1249
01:31:10,940 --> 01:31:13,200
This is the Berkeley Unix from the 1970s.

1250
01:31:13,200 --> 01:31:16,630
That's one of the ways that echo can run.

1251
01:31:16,630 --> 01:31:22,310
Setting echo_style to that value in the TC-shell will cause echo to behave that way.

1252
01:31:22,310 --> 01:31:27,670
So set does that, but set only gets shell variables.

1253
01:31:27,670 --> 01:31:35,430
It wouldn't find EDITOR, which is not a shell variable.

1254
01:31:36,870 --> 01:31:38,050
Nothing.

1255
01:31:38,050 --> 01:31:39,660
So that's one way of distinguishing them.

1256
01:31:39,660 --> 01:31:42,000
But the fact that you have to go through some strange command like that

1257
01:31:42,000 --> 01:31:45,500
to distinguish between shell variables or environment variables

1258
01:31:45,500 --> 01:31:49,970
shows the kind of impractical nature of the C-shell for some purposes.

1259
01:31:52,290 --> 01:31:57,960
And now, last and maybe least, this is the man pages.

1260
01:31:57,960 --> 01:32:03,190
Those of who you may know, the man is the command short for manual.

1261
01:32:03,190 --> 01:32:08,610
The man pages for the shells are hard to read. They're very long.

1262
01:32:08,610 --> 01:32:14,060
They're organized in a way that may make it difficult to find what you're looking for.

1263
01:32:14,060 --> 01:32:15,980
So if you're looking for something with a purpose,

1264
01:32:15,980 --> 01:32:20,050
you may not know if that purpose is a shell variable or something else,

1265
01:32:20,050 --> 01:32:21,630
so you may not know where to look for it.

1266
01:32:21,630 --> 01:32:25,030
You can look for various strings, but the strings are often repeated.

1267
01:32:25,030 --> 01:32:27,640
So it's generally hard to read.

1268
01:32:27,640 --> 01:32:33,810
We just looked at the TC-shell man page a little before to find the eval command.

1269
01:32:33,810 --> 01:32:36,610
Some things go faster.

1270
01:32:36,610 --> 01:32:38,860
One approach is to search for a string.

1271
01:32:38,860 --> 01:32:40,360
You can use the pager.

1272
01:32:40,360 --> 01:32:49,080
Pager has the slash to look for a command or a string inside a pager operation.

1273
01:32:49,080 --> 01:32:52,830
Man by default will use pagers, either be MORE or LESS.

1274
01:32:52,830 --> 01:32:56,560
I don't know if you're familiar with those, but those can show files bit by bit.

1275
01:32:56,560 --> 01:33:00,550
I've been using LESS to display these particular files we've got here.

1276
01:33:00,550 --> 01:33:03,300
You can search inside there.

1277
01:33:03,300 --> 01:33:04,880
You can try using different search strings.

1278
01:33:04,880 --> 01:33:08,420
Also man pages in different operating systems may not be the same.

1279
01:33:08,420 --> 01:33:11,130
They can be separate pages for csh and tcsh.

1280
01:33:11,130 --> 01:33:14,500
They're aren't on the Mac, but they might be if those are separate commands.

1281
01:33:14,500 --> 01:33:19,000
If sh doesn't really call Bash, there probably would be a separate man page.

1282
01:33:19,000 --> 01:33:25,820
Some systems have separate man pages just for the C-shell built-in commands.

1283
01:33:25,820 --> 01:33:30,250
Sometimes if you want to read a description of a built-in command

1284
01:33:30,250 --> 01:33:35,350
that's also in the path, like echo, you need to read the man page on that command on echo

1285
01:33:35,350 --> 01:33:37,610
to determine how it will work as a built-in command

1286
01:33:37,610 --> 01:33:39,760
even if you're not calling the built-in command.

1287
01:33:41,630 --> 01:33:46,090
That's a drawback of the operating system in general, not only for the shells,

1288
01:33:46,090 --> 01:33:50,710
although for the shells in particular the man pages are quite long,

1289
01:33:50,710 --> 01:33:56,180
partly because they've added useful features to them, which may be a positive.

1290
01:33:56,180 --> 01:34:00,290
Okay. Are there any questions? Any topics you want to bring up?

1291
01:34:00,290 --> 01:34:03,390
Anything relevant here?

1292
01:34:04,540 --> 01:34:07,100
Well, it's been very nice talking to you all.

1293
01:34:07,100 --> 01:34:09,690
I hope you got something out of this seminar

1294
01:34:09,690 --> 01:34:13,080
that will be useful for you in your future endeavors.

1295
01:34:17,330 --> 01:34:19,000
[CS50.TV]