UNIX and Linux shells provide an abundance of useful built-in information that can be referenced in globally available variables. In order to see the information provided in a shell, the set command can be run as demonstrated below.
Here’s a partial output of the set command:
$ set
BASH=/bin/bash
BASH_VERSINFO=([0]="2" [1]="05b" [2]="0" [3]="1" [4]="release" [5]="i386-redhat-linux-gnu")
BASH_VERSION='2.05b.0(1)-release'
GROUPS=()
G_BROKEN_FILENAMES=1
HISTFILE=/home/tclark/.bash_history
HISTFILESIZE=1000
HISTSIZE=1000
HOME=/home/tclark
HOSTNAME=appsvr.mytec.com
OSTYPE=linux-gnu
PATH=/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/tclark/bin
...
PS1='[\u@\h \W]\$ '
PS2='> '
PS4='+ '
PWD=/home/tclark
SHELL=/bin/bash
SHLVL=1
SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass
SSH_CLIENT='206.107.231.178 1379 22'
SSH_CONNECTION='206.107.231.178 1379 192.168.15.105 22'
SSH_TTY=/dev/pts/0
SUPPORTED=en_US.UTF-8:en_US:en
TERM=vt100
UID=503
USER=tclark
_=clear
The contents of a shell variable can be displayed by using the echo command and prefacing the variable name with a dollar sign as demonstrated below. Shell variables are referenced using all capital letters.
$ echo $TERM
vt100
$ echo $USER
tclark
$ echo $HOSTNAME ... $LOGNAME
appsvr.mytec.com ... tclark
There are also some special built-in variables that can be useful when creating shell scripts. Some of them are listed in the table below.
| Built-in Variable | Description |
| $# | The total number of arguments passed to a shell script on the command line. |
| $* | All arguments passed to the shell script. |
| $0 | The command (script) invoked on the command line. |
| $1 – $9 | The first through ninth arguments passed to the shell script from the command line. |
These variables are provided by the shell and the names should not be used for other variables.
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