Slackware Linux Essentials
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8.3 The Bourne Again Shell -(bash)

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8.3.1 -Environment Variables

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A Linux system is a complex beast, and there's a lot to keep track of, a lot of little -details that come into play in your normal interactions with various programs (some of -which you might not even need to be aware of). Nobody wants to pass a bunch of options to -every program that gets run, telling it what kind of terminal is being used, the hostname -of the computer, how their prompt should look...

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So as a coping mechanism, users have what's called an environment. The environment -defines the conditions in which programs run, and some of this definition is variable; -the user can alter and play with it, as is only right in a Linux system. Pretty much any -shell will have environment variables (if not, it's probably not a very useable shell). -Here we will give an overview of the commands bash provides for manipulating its -environment variables.

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set by itself will show you all of the environment variables -that are currently defined, as well as their values. Like most bash built-ins, it can also do several other things (with -parameters); we'll leave it to the bash(1) man page to cover -that, though. Example 8-1 shows -an excerpt from a set command run on one of the author's -computers. Notice in this example the PATH variable that was -discussed earlier. Programs in any of those directories can be run simply by typing the -base filename.

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Example 8-1. Listing Environment Variables with set

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-% set
-PATH=/usr/local/lib/qt/bin:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:
-/usr/openwin/bin:/usr/games:.:/usr/local/ssh2/bin:/usr/local/ssh1/bin:
-/usr/share/texmf/bin:/usr/local/sbin:/usr/sbin:/home/logan/bin
-PIPESTATUS=([0]="0")
-PPID=4978
-PS1='\h:\w\$ '
-PS2='> '
-PS4='+ '
-PWD=/home/logan
-QTDIR=/usr/local/lib/qt
-REMOTEHOST=ninja.tdn
-SHELL=/bin/bash
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-% unset VARIABLE
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unset will remove any variables that you give it, wiping out -both the variable and its value; bash will forget that variable -ever existed. (Don't worry. Unless it's something you explicitly defined in that shell -session, it'll probably get redefined in any other session.)

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-% export VARIABLE=some_value
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Now, export is truly handy. Using it, you give the -environment variable VARIABLE the value “some_value”; if VARIABLE didn't exist, -it does now. If VARIABLE already had a value, well, it's gone. -That's not so good, if you're just trying to add a directory to your PATH. In that case, you probably want to do something like this:

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-% export PATH=$PATH:/some/new/directory
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Note the use of $PATH there: when you want bash to interpret a variable (replace it with its value), tack a -$ onto the beginning of the variable's name. For instance, echo $PATH will echo the value of PATH, in my -case:

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-% echo $PATH
-/usr/local/lib/qt/bin:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:
-/usr/openwin/bin:/usr/games:.:/usr/local/ssh2/bin:/usr/local/ssh1/bin:
-/usr/share/texmf/bin:/usr/local/sbin:/usr/sbin:/home/logan/bin
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8.3.2 Tab -Completion

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(Here comes something cool again.)

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A commandline interface means lots of typing.
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Typing is work.
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Nobody likes work.
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From 3 and 2, we can determine that (4) nobody likes typing. Fortunately, bash saves us from (5) (nobody likes a commandline interface).

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How does bash accomplish this wonderful feat, you ask? In -addition to the wildcard expansion we discussed before, bash -features tab completion.

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Tab completion works something like this: You're typing the name of a file. Maybe it's -in your PATH, maybe you're typing it out explicitly. All you have -to do is type enough of the filename to uniquely identify it. Then hit the tab key. bash will figure out what you want and finish typing it for you!

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Example time. /usr/src contains two subdirectories: /usr/src/linux and /usr/src/sendmail. I -want to see what's in /usr/src/linux. So I just type ls /usr/src/l, hit the TAB key, and bash gives me ls /usr/src/linux.

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Now, suppose there are two directories /usr/src/linux and -/usr/src/linux-old; If I type /usr/src/l and hit TAB, bash will fill in as much as it can, and I'll get /usr/src/linux. I can stop there, or I can hit TAB again, and bash will show a list of -directories that match what I've typed so far.

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Hence, less typing (and hence, people can like commandline interfaces). I told you it -was cool.

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