Fedora Linux

$ 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

4.12.1.2. Special variables

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

Name	Description	Notes
$$	Process ID of the shell	Since process IDs are unique (at any one point in time), this can be used to ensure unique filenames (e.g., /tmp/$$.txt will never conflict with the same filename used by another copy of the same script).
$0	Name of the script	Useful to generate error messages, and when one script is invoked through more than one name.
$1, $2, $3, ...	Arguments given on the script's command line	The shift command will eliminate $1 and then shift all of the parameters accordingly ($2 becomes $1, $3 becomes $2, and so forth).