Tuesday, April 03, 2001 12:12 PM 
TechTalk #1: Setting Environment Variables


First of all, allow me start this very first TechTalk by extending
a warm welcome to all Macintosh users. As a long-time member of
the *BSD tribe, I wish to greet all Mac users about to embrace Mac
OS X (a BSD derivative): Welcome to a real operating system! Welcome
to a new world of unprecedented stability, powerful flexibility,
and networking compliance!

On the down side, it took me about 2 years to become a decent Unix
user. I can only hope Mac OS X manages to lower that learning curve
for all of you. :>


SETTING ENVIRONMENT VARIABLES

One common problem faced when running Java applications on a Unix
system is getting the CLASSPATH right. As I understand it, this is
even one of the root reasons InstallAnywhere was created.

So, how do you set/correct the CLASSPATH to get that spiffy Java
app to start on your new Mac OS X system? There are two ways,
depending on the breed of the shell you happen to be using:

1- the Bourne family of shells uses a regular variable assignment,
followed by "export" 
2- the C-shell family uses "setenv"

Note that both shells use "unset" to erase (forget) a variable, 
should you ever need to do that.


SHELL? WHAT SHELL?

But first, how do you know what shell is yours? "echo $SHELL" should
give you a clue. E.g. (the '%' sign is the shell's prompt):

	% echo $SHELL 
	/usr/local/bin/zsh

	% echo $SHELL 
	/opt/contrib/bin/bash

	% echo $SHELL 
	/bin/sh

These three examples, taken from 3 different Unix systems, list
"zsh", "bash" and "sh" respectively.


THE BOURNE FAMILY OF SHELLS

The Bourne family derives from the original Unix shell, simply
named "sh". Modern shells based on the original Bourne shell include
bash, zsh, ksh.

The Bourne shell family is of the opinion that an environment
variable is a variable first, a bit of environment next. So you
will assign a value to any variable, then "export" it into the
environment.

Assigning a value to a variable is deceptively easy:

	% abc=123 
	% mY_vAR_MixEdCasE="asdf asdf asdf"

Note the absence of spaces around the '=' sign. It wouldn't work
otherwise; shells are quite finicky about the quotes, spaces, case,
etc. If the value contains any spaces, it must be quoted. As it's
way too easy to forget, I advise you to always quote everything
with double quotes.

	% mY_vAR_MixEdCasE="asdf asdf asdf"

The value of a variable can be recalled by prepending a dollar sign
to the variable's name. E.g. :

	% my_var=word 
	% MY_ENV_VAR="this is a $my_var"

	% echo $MY_ENV_VAR 
	this is a word

Once your variable has been assigned a value, it can be exported
into the environment with "export".

	% export MY_ENV_VAR

That's all there is to it. The following example prepends
"/usr/local/java" to the CLASSPATH:

	% CLASSPATH="/usr/local/java:$CLASSPATH" 
	% export CLASSPATH

Next, the C shell.


THE C SHELL FAMILY

The C shell family has been popular with programmers frustrated by
the limitations of the original Bourne shell. The first C shell is
"csh", a newer incarnation is named "tcsh". Tcsh, along with bash,
is one of the most popular shells today.

The C shells use a specific command to set an environment variable,
"setenv". It does NOT use an equal sign. For example:

	% setenv MY_ENV_VAR "this is a random value" 
	% setenv CLASSPATH "$CLASSPATH:/usr/my_java_path"

Same warning about the quotes: stick to double-quotes, and case
matters.


HOW TO TEST WHAT YOU'VE JUST DONE

"Env" is your friend. Env is a standard Unix command to prints out
the content of the environment to your terminal window. E.g.:

	% env 
	PATH=/home/renaud/bin:/bin:/sbin:/usr/bin:/usr/sbin
	MAIL=/home/renaud/Maildir 
	USER=renaud 
	LOGNAME=renaud
	HOME=/home/renaud 
	SHELL=/usr/local/bin/zsh 
	TERM=vt100
	MY_ENV_VAR=this is a random value 
	PWD=/home/renaud
	OLDPWD=/home/renaud 
	VISUAL=vi 
	EDITOR=vi

Unfortunately, if your environment is large (it does tend to get
so pretty quickly), the list will scroll and you'll be left trying
to fetch your variable out of that list visually. I advise you to
make use of another staple of the Unix diet, "grep".

	% env | grep MY_ENV_VAR 
	MY_ENV_VAR=this is a random value

That's it!


THANKS

This TechTalk was suggested by Eric Shapiro. I hope you found it
as useful as Eric hoped it would be. Let me know if you have
comments/suggestions about this article! 


--Renaud Waldura <renaud@waldura.com>