Exporting variables

Up to this point, we have talked about defining shell variables and then using them in later command lines. You can also define a shell variable and then call a shell script that makes use of that variable. But you have to do a certain amount of preparation first.

A shell script is run like a separate shell session. By default, it does not share any variables with your current shell session. If you define a variable VAR in the current session, it is local to the current session; any shell script that you call will not know about VAR.

To deal with this situation, you can export the command; enter:

export VAR

The export command says that you want the variable VAR passed on to all the commands and shell scripts that you execute in this session. After you do this, VAR becomes global and the variable is known to all the commands and shell scripts that you use.

As an example, suppose you enter the commands:

MYNAME="Robin Hood"
export MYNAME

Now all your commands can use the MYNAME variable to obtain the associated name. You may, for example, have shell scripts that write form letters that contain your name, Robin Hood, obtained from the MYNAME variable.

Note: You could use single or double quotation marks to enclose the variable value. See Quoting variable values for more information.

When a script begins running, it automatically inherits all the variables currently being exported. However, if the script changes the value of one of those variables, that change is not reflected to the calling shell—unless you run the script with the dot ( .) utility.

By default, any variables created within a shell script are local to that script. This means that when another program is run, those variables do not apply in its environment. However, the script can use the export command to turn local variables into global ones. Inside a shell script:

export name

indicates that the variable with the given name should be exported. When other programs are run from that script, they inherit the value of all exported variables. However, when the script ends, all its exported variables are lost to the calling shell.

Some variables are automatically marked for export by the software that creates them. For example, if you invoke the shell, the initialization procedure automatically marks the HOME variables for export so that other commands and shell scripts can use it. In Customizing the z/OS shell, you saw that in a typical .profile file for an individual user, the PATH variable is exported. Exporting PATH ensures that search rules and changes to search rules are automatically shared by all shell sessions and scripts.

You must export other variables explicitly, using the export command.