What are Environment Variables?

A very basic way to think of Environment Variables is like so... Think of the shell as any other program that you were capable of writing, compiling and running. Your program maintains information about it's current state, and well a shell is no different. Since the shell's main job is to act as a liaison between the kernel and the user, it maintains information about the computing environment.

Certain applications and commands may communicate with the shell and reference the environment variables that it maintains. For example it seems that "frm" and "nfrm" seem not to work it $MAIL is not defined.

How Do I See My Environment Variables?

Most all UNIX systems provide a command that will allow you to see all of these variables that the shell is maintaining. On must systems this is accomplished by using the "env" command.

Here is an example of the "env" command issued on one of the linix.gl servers using the tcsh shell...

HOME=/afs/umbc.edu/users/d/h/dhood2/home
USER=dhood2
LOGNAME=dhood2
PATH=.:/afs/umbc.edu/users/d/h/dhood2/home/bin:/afs/umbc.edu/users/d/h/dhood2/home/bin/sgi:/usr/local/bin:/usr/bsd:/bin:/usr/bin:/usr/sbin:/usr/bin/X11:/usr/local/X11:/etc:/usr/etc:/usr/k5/bin:/usr/afsws/bin:/bin:/usr/java/bin:/usr/afsws/bin:/usr/X11R6/bin
MAIL=/afs/umbc.edu/users/d/h/dhood2/Mail/inbox
SHELL=/bin/tcsh
SSH_CLIENT=XX.XX.XX.XX 33887 22   (sorry - I'm hiding my IP address)
SSH_TTY=/dev/pts/0
TERM=xterm
DISPLAY=linux2.gl.umbc.edu:10.0
KRB5CCNAME=FILE:/tmp/krb5cc_l29893
HOSTTYPE=i386-linux
VENDOR=intel
OSTYPE=linux
MACHTYPE=i386
SHLVL=1
PWD=/afs/umbc.edu/users/d/h/dhood2/home
GROUP=general
HOST=linux2.gl.umbc.edu
REMOTEHOST=XX.md.comcast.net   (sorry - I'm hiding my hostname)
HOSTNAME=linux2.gl.umbc.edu
LS_COLORS=no=00:fi=00:di=01;34:ln=01;36:pi=40;33:so=01;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=01;32:*.cmd=01;32:*.exe=01;32:*.com=01;32:*.btm=01;32:*.bat=01;32:*.sh=01;32:*.csh=01;32:*.tar=01;31:*.tgz=01;31:*.arj=01;31:*.taz=01;31:*.lzh=01;31:*.zip=01;31:*.z=01;31:*.Z=01;31:*.gz=01;31:*.bz2=01;31:*.bz=01;31:*.tz=01;31:*.rpm=01;31:*.cpio=01;31:*.jpg=01;35:*.gif=01;35:*.bmp=01;35:*.xbm=01;35:*.xpm=01;35:*.png=01;35:*.tif=01;35:
LESSOPEN=|/usr/bin/lesspipe.sh %s
TZ=EST5EDT
MANPATH=/usr/X11R6/man:/usr/k5/man:/usr/local/man:/usr/man:/usr/share/man
EDITOR=/usr/local/bin/pico

Some things to note...

• HOME - is your home directory
• USER and LOGNAME - is your login ID
• HOSTNAME - the name of the host that you are logged into
• PWD - the current working directory
• MAIL - where your mail is being written to

PATH

The PATH is nothing more than a list of directories in which to look for executable commands. Note that if the same command lives in more than one of these places the first one in the path listing is the one that is used. To complicate this matter even more, the versions that are in these different directories may be different, as it the case with gcc (the C compiler) at the time of this writing.