Chris Tyler - Fedora Linux

SSH_AUTH_SOCK=/tmp/ssh-dNhrfX2621/agent.2621

TERM=xterm

UID=503

USER=hank

USERNAME=hank

WINDOWID=58721388

XAUTHORITY=/home/hank/.Xauthority

_=

qt_prefix=/usr/lib/qt-3.3

Many of these variables contain settings for particular programs. Some of the common variables used by many programs are shown in Table 4-16.

Table 4-16. Key environment variables

To set a shell variable, type the variable name, an equal sign, and the value you wish to assign (all values are treated as text):

$ A= red

Once a variable has been assigned a value, you can use it in commands, preceded by a dollar sign:

$ ls -l red

ls: red: No such file or directory

$ touch $A

$ ls -l red

-rw-r--r-- 1 hank hank 0 Jul 18 15:26 red

The echo command can be used to view the value of a variable:

$ echo $A

red

To destroy a variable, use the unset command:

$ echo $A

red

$ unset A

$ echo $A

$

Finally, to make a variable accessible to processes started by the current process, use the export command:

$ unset A

$ TEST=blue

$ echo $TEST # variable is known to the shell

blue

$ bash # start a child shell

[hank@beige foo]$ echo $TEST # variable is not known to child

[hank@beige foo]$ exit # exit back to parent shell

exit

$ export TEST # export the variable

$ echo $TEST # value is still known to the shell

blue

$ bash # start a new child shell

[hank@beige foo]$ echo $TEST # exported value is known to the child

blue

The PATH value is stored in an environment variable of the same name. Its value can be viewed like any other environment variable:

$ echo $PATH

/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin

To add a directory to the existing directories, use $PATH on the righthand side of an assignment to insert the current value of the variable into the new value:

$ PATH=$PATH: /home/hank/bin

$ echo $PATH

/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/home/hank/bin

You don't need to export PATH in this case because it has already been exported; assigning a new value does not changes its exported status.

Assuming that the topten script is saved in /home/hank/bin , you can now execute it by just typing its name:

$ topten

-rw-r--r-- 1 root root 807103 Jul 12 21:18 termcap

-rw-r--r-- 1 root root 499861 Jul 17 08:08 prelink.cache

-rw-r--r-- 1 root root 362031 Feb 23 08:09 services

-rw-r--r-- 1 root root 97966 Jul 15 11:19 ld.so.cache

-rw-r--r-- 1 root root 92794 Jul 12 12:46 Muttrc

-rw-r--r-- 1 root root 83607 Mar 23 07:23 readahead.files

-rw-r--r-- 1 root root 73946 Jul 13 02:23 sensors.conf

-rw-r--r-- 1 root root 45083 Jul 12 18:33 php.ini

-rw-r--r-- 1 root root 30460 Jul 13 20:36 jwhois.conf

-rw-r--r-- 1 root root 26137 Mar 23 07:23 readahead.early.files

Within a script, you can prompt the user using the echo command, and then use the read command to read a line from the user and place it in an environment variable:

echo "Please enter your name:"

read NAME

echo "Hello $NAME!"

Or you can collect the standard output of a command and assign it to a variable using the $( ) symbols:

$ NOW=$(date)

$ echo $NOW

Tue Jul 18 22:25:48 EDT 2006

There are several special parameters , or special variables , that bash sets automatically; Table 4-17 contains a list of the most important ones.

Table 4-17. Important special variables