A local variable is a variable that is present within the current instance of the shell. It is not available to programs that are started by the shell. The variables that you looked at previously have all been local variables.
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When a shell is running, three main types of variables are present:
An environment variable is a variable that is available to any child process of the shell. Some programs need environment variables in order to function correctly. Usually a shell script defines only those environment variables that are needed by the programs that it runs.
A shell variable is a special variable that is set by the shell and is required by the shell in order to function correctly. Some of these variables are environment variables whereas others are local variables.
Table 7.1 gives a summary of the different types of variables discussed in this section. This table compares local variables set by the user, environment variables set by the user, and shell variables set by the shell.
| Attribute | Local | Environment | Shell |
|---|---|---|---|
| Accessible by | No | Yes | Yes |
| child processes | |||
| Set by users | Yes | Yes | No |
| Set by the shell | No | No | Yes |
| User modifiable | Yes | Yes | No |
| Required by the shell | No | No | Yes |
You place variables in the environment by exporting them. Exporting can be done as follows:
export name
This command marks the variable with the specified name for export. This is the only form supported by sh, thus it is the most commonly encountered form. The standard shell idiom for exporting environment variables is
name=value ; export name
An example of this is
PATH=/sbin:/bin ; export PATH
Here you set the value of the variable PATH and then export it. Usually the assignment statement of an environment variable and the corresponding export statement are written on one line to clarify that the variable is an environment variable. This helps the next programmer who has to maintain the script quickly grasp the use of certain variables.
You can also use the export command to export more than one variable to the environment. For example,
export PATH HOME UID
exports the variables PATH, HOME, and UID to the environment.
Exporting Variables in ksh and bash
A second form for exporting variables is supported by ksh and bash:
export name=value
In this form, the variable specified by name is assigned the given value. Then that variable is marked for export. In this form, you can write the previous example as
export PATH=/sbin:/bin
In bash and ksh, any combination of name or name=value pairs can be given to the export command. For example, the command
export FMHOME=/usr/frame CLEARHOME=/usr/atria PATH
assigns the given values to the variables FMHOME and CLEARHOME and then exports the variables FMHOME, CLEARHOME, and PATH.
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