Understanding Shell Variables in Linux

Shell Variables

Shell variables are capitalized by convention. The shell maintains two lists of variables:

Those local to the current shell
Those global to all shells (environment variables).

Use the set and env commands to display local and environmental variables, respectively. The following is a partial output of the set and env statements. Many variables appear in both the local and environment variable list.

$ set
AB2_DEFAULTSERVER=http://docs.sun.com/
CUE_HOSTNAME=sunray10
DISPLAY=:46.0
DOMAIN=renegades.Central.Sun.COM
DOMAIN_COUNT=1
DTSOURCEPROFILE=true
DTUSERSESSION=milner-sunray10-46
DTXSERVERLOCATION=local
EDITOR=vi
ERRNO=25
FCEDIT=/bin/ed
LANG=C
LOGNAME=milner
LPDEST=hutchence
MAIL=/var/mail/milner
MAILCHECK=600
MANPATH=/usr/dt/man:/usr/man:/usr/openwin/share/man:/usr/man:/usr/openwin
/share/man:/usr/dt/man:/usr/dist/local/man/5.7
PRINTER=hutchence
PS1=’$ ’
PS2=’> ’
PS3=’#? ’
PS4=’+ ’
PWD=/home/milner/K-Shell-Course/InstructorGuide/Examples
SHELL=/bin/ksh
TZ=US/Mountain
...

$ env
DTSOURCEPROFILE=true
DOMAIN=renegades.Central.Sun.COM
DTUSERSESSION=milner-sunray10-46
EDITOR=vi
LOGNAME=milner
MAIL=/var/mail/milner
CUE_HOSTNAME=sunray10
PRINTER=hutchence
DISPLAY=:46.0
TERM=dtterm
TZ=US/Mountain
LPDEST=hutchence
DOMAIN_COUNT=1
...

Variable names can contain uppercase or lowercase letters, digits, and underscores. Variable names cannot begin with a digit. Some of the boot scripts show mixed-case variable names. Be sure to use meaningful variable names.

Creating Variables in the Shell

To set a variable in the shell, use the syntax:

var=value

NOTE: Variables are case-sensitive so, after they are defined, they must later be spelled using exactly the same uppercase and lowercase letters. Variable names have a limit of 255 characters.

Do not place spaces around the = sign. If the value contains spaces or special characters, use single or double quotes; for example:

$ name="Susan B. Anthony"

To display the value of a shell variable, use the echo command and place a $ immediately in front of the variable name. When you have finished a variable, you can unset the value and release the resources with the unset command; for example:

$ MYNUM=21
$ MACHTYPE=sparc
$ echo $MYNUM
21
$ echo $MYNUM $MACHTYPE
21 sparc
$ unset MYNUM
$ echo $MYNUM $MACHTYPE
sparc

In the following example, notice how variable substitution happens before file name expansion:

# VAR="*"
# ls $VAR
hsperfdata_noaccess:
805

hsperfdata_root:
3101
# >-l
# ls $VAR
hsperfdata_noaccess:
total 64
-rw-------   1 noaccess noaccess   32768 Nov 13 15:43 805

hsperfdata_root:
total 64
-rw-------   1 root     root       32768 Nov 23 15:27 3101

The $VAR variable is first resolved and then passed to the ls command.

Exporting Variables to Subshells

Variables created in a shell are not known outside that shell. These variables are local variables. They are “local to” (known only in) the current shell. Subshells of a shell do not automatically inherit the variables of their parent shell.Only the variables that are exported are passed onto subshells of the parent shell. Variables that are exported are referred to as environment variables.

To export a variable (with its current value) to subshells of a shell, use:

export variable_name1 variable_name2 ...

NOTE: A subshell can change the value of a variable it inherits from its parent. The change does not affect the value of the variable in the parent shell. Thus, subshells cannot alter values of variables in the parent shell.

The following example creates two variables in the current shell, x and name. The name variable is then exported to be inherited by any subshells of the parent.

$ x=25
$ name="Marion Morrison"
$ echo $x $name
25 Marion Morrison
$ export name

A subshell is created by invoking a new Korn shell using the ksh command.

$ ksh
$ print $x
$ print $name
Marion Morrison
$ export name="Marlena Dietrich"
$ print $name
Marlena Dietrich
$ exit

When you try to access the value of the variable x using the echo command, no output is shown because the child shell does not know the variable x. When you access the value of the variable name, the value of the variable is output because this variable was exported in the parent shell. Changing the contents of the name in the subshell has no effect on the value of the name variable in the parent.

$ echo $name
Marion Morrison

Reserved Variables

The shell knows about and uses certain variables. Use care when modifying the values for these variables. Before modifying a value, know how the shell uses the variable and how you might affect your interaction with the shell. To learn about variables not listed in this module, read the man pages for sh or ksh, depending on your choice for an interactive shell. The table below describes the shell variables.

Variable	Meaning
HOME	User’s login directory path.
IFS	Internal field separators, which hold a characters used as inter-field separators. By default, this string includes a space, a tab, and a new line. Don’t change this string.
LOGNAME	LOGNAME User’s login name.
MAILCHECK	How often, in seconds, the mail daemon checks for mail.
OPTIND	getopts statement uses this variable during parsing of options on command line.
PATH	Search path for commands.
PS1	The prompt. It defaults to $.
PS2	Prompt used when command line continues. Defaults to >.
PS3	Prompt used within a select statement. Defaults to #?.
PS4	Prompt used by the shell debug utility. Defaults to +.
PWD	Present working directory.
SHELL	Shell defined for the user in the passwd file.
TERM	Terminal type, which is used by the vi editor and other commands.

Special Shell Variables

Several shell variables are available to users. The shell sets the variable value at a process creation or termination.

Process Identification

The $ shell variable is available to users. It contains the current process ID number. Use this variable to create file names that are unlikely to already exist.

$ echo $$
8873
$ sh
$ echo $$
17716
$ exit
$ echo $$
8873

Exit Status

The exit status is the integer value of the last executed command (program, script, or shell statement). The ? variable provides the exit status of the most recent foreground process. It:

Determines if the process ran successfully.
Sets the ? variable to 0 if a process succeeded and to a nonzero value if it didn’t succeed.

Think of this as equating success with zero errors occurring and equating failure with one or more errors occurring or the command not being able to do what was requested:

$ grep "root" /etc/passwd
root:x:0:1:Super-User:/:/sbin/sh
$ echo $?
0

$ grep "rot" /etc/passwd
$ echo $?
1

Background Process Identification

A shell allows you to run a command in the background using the & operator. To find the process identification number (PID) of the last background job started, echo the ! variable.

$ date &
1175
$ Mon Oct 12 06:23:11 IST 2009
$ echo $!
1175