Environmental Variables

There are many global variables that are used by the shell and other programs. You can see what has been set by typing env . You will see something similar to the following:

[emcnabb@grape emcnabb]$ env
HOSTNAME=grape
PVM_RSH=/usr/bin/rsh
TERM=xterm
SHELL=/bin/bash
HISTSIZE=1000

If you can see what has been set for a certain variable, for example HOME, by typing echo $HOME:

[emcnabb@grape emcnabb]$ echo $HOME
/users/admin/emcnabb

This is where my home directory is located.

You can also set custom variables:

[emcnabb@grape emcnabb]$ BOB=hello
[emcnabb@grape emcnabb]$ echo $BOB
hello

If you put export in front of the variable definition, all other terminals will recognize it:

[emcnabb@grape emcnabb]$ export BOB=hello
[emcnabb@grape emcnabb]$ echo $BOB
hello

Any variables you set from the command line will last only for the duration of that instance of the shell. If you want to have a variable set every time you start up a terminal/shell, add exactly what you typed to your .bashrc file.

PATH

The PATH variable is a list of directories that the shell looks in when a command is run. For example, the directory list command ls is located in /bin/ls. How does your shell know what command to run when you type ls, It looks in your PATH variable. Try the following:

[emcnabb@grape emcnabb]$ echo $PATH
/bin:/usr/bin:/usr/local/bin:/sbin:/usr/sbin:/usr/X11R6/bin:/usr/java/jdk1.3.1_01/bin:/users/ta/cs324ta/bin:.

As you can see, /bin is one of the directories that is searched. The PATH is read from left to right, so if there is an ls command located in /bin and /usr/bin, the /bin one will be executed. I have customized my PATH variable to include the /users/ta/cs324ta/bin directory so that I don't have to type "/users/ta/cs324ta/bin/myexecutable" every time I want to run the TA's example program, just "myexecutable".

You'll want a line in your .bash_profile that looks something like this:

export PATH="$PATH:/usr/java/jdk1.3.1_01/bin:/users/admin/cs324ta/bin:."

This will take the system wide PATH and then add on other directories that you want (for example /users/admin/cs324ta/bin).

CLASSPATH

The CLASSPATH is an environmental variable that tells the Java compiler where to look for class files to import or where to find class files to interpret. If you do medium to large Java programming projects you'll probably want to edit this variable. You can set this just like you do your PATH variable. Here is what my CLASSPATH variable is set to:

export CLASSPATH="/opt/jakarta/tomcat/common/lib/servlet.jar:/opt/sun-j2ee-1.3.1/lib/j2ee.jar:/home/emcnabb/postgresql.jar:."

aliases

You probably have some long commands that you type in quite often. In order to save time, you can create aliases. The basic structure is alias <myalias>='<command>'

For example, on my home computer I ssh into grape, but don't want to type in the whole command each time. I can do the following:

bash-2.05b$ alias grape='ssh emcnabb@grape.cs.byu.edu'
bash-2.05b$ grape
emcnabb@grape.cs.byu.edu's password:

If you want to have that alias set for all of your terminals, add it to your .bashrc file. The following are several other examples of what I have in my .bashrc:

alias ll='ls -l'
alias c='clear'
alias p='pine'
alias temp='cd $HOME/tmp' #Go to my tmp directory

Command Prompt

You can change the text that is displayed while the shell is waiting for input. This configuration is stored in the variable PS1 where PS1='value'.

The following are different values that can be set:

• \a an ASCII bell character (07)
• \d the date in "Weekday Month Date" format (e.g., "Tue May 26")
• \e an ASCII escape character (033)
• \h the hostname up to the first '.'
• \H the hostname
• \j the number of jobs currently managed by the shell
• \l the basename of the shell's terminal device name
• \n newline
• \r carriage return
• \s the name of the shell, the basename of $0 (the portion following the final slash)
• \t the current time in 24-hour HH:MM:SS format
• \T the current time in 12-hour HH:MM:SS format
• \@ the current time in 12-hour am/pm format
• \u the username of the current user
• \v the version of bash (e.g., 2.00)
• \V the release of bash, version + patchlevel (e.g., 2.00.0)
• \w the current working directory
• \W the basename of the current working directory
• \! the history number of this command
• \# the command number of this command
• \$ if the effective UID is 0, a #, otherwise a $
• \nnn the character corresponding to the octal number nnn
• \\ a backslash
• \[ begin a sequence of non-printing characters, which could be used to embed a terminal control sequence into the prompt
• \] end a sequence of non-printing characters

Here is an example of me playing with the PS1 variable. Notice how the command prompt changes as PS1 changes:

emcnabb@medusa Thu Apr 10 ~/SANDBOX/PHP $  PS1='\u@\h \d $ '
emcnabb@medusa Thu Apr 10 $  PS1='\u@\h $ '
emcnabb@medusa $  PS1='\u $ '
emcnabb $  PS1=' $ '
 $ PS1='\u@\h \d \w $ '
emcnabb@medusa Thu Apr 10 ~/SANDBOX/PHP $

The PS1 value that I personally use is:

PS1='[ \e[31m\u@\h:\e[34m\w \e[m] '