Environment variables

by Oleg Sadov — last modified Aug 07, 2020 07:58 AM

The operating system supports a special kind of resource called environment variables. These variables are a NAME/VALUE pair. The name can start with a letter and be composed of letters, numbers, and underscores. Variable values have only one type -- character strings. Names and values are case sensitive. And, as we'll see, variables can be global and local, just like in regular programming languages.

To get the value of a variable on the Shell, precede the variable name with a dollar sign:

$ echo $USER
guest

If the variable is not set, an empty string is returned.

The assignment operator is used to set the value of a variable (in the case of Bourne-like shells):

$ TEST=123

or the built-in 'set' operator (in the case of C-like Shells):

$ set TEST=123

You can check your settings by calling the 'echo' command, which simply sends its own arguments to stdout.

$ echo $TEST
123

The 'set' command with no arguments lists the values of all variables set in the environment:

$ set
COLUMNS=197
CVS_RSH=ssh
DIRSTACK=()
....

Shell commands can be combined into command files called scripts, where the first line, in a special kind of comment, specifies the shell to execute the set. For example, let's create a file called test in a text editor or just by command 'cat' with the following content:

$ cat > test

And this "cat" has nothing to do with dogs or other animals, it's just short for "concatenation." This command simply writes the contents of the files given as arguments to standard output, with concatenating them all. If we don't set arguments, it just means reading from stdin and writing to stdout. By this way we redirect standard output to a file.

#!/bin/sh

echo TEST:
echo $TEST

This program will print the text message "TEST:" and the value of the TEST variable, if this one specified, to standard output. You can run it from the command line by passing it as a parameter to the command interpreter:

$ sh test
TEST:

We didn't see the value of the variable. But why? Because we started a new shell process to run the script. And we have not set this variable in the context of this new shell. Let's do it. Okay, let's expand our definition to the environment space of all processes started from our current shell:

$ export TEST=456
$ sh test
TEST:
456

And, as we can see, the value of the variable in our current SHELL has also changed:

$ echo $TEST
456

The export operation is the globalization of our variable. You can get the settings for such global exported variables for a session by calling the interpreter builtin command 'env', in the case of Bourne-like interpreters (sh, ksh, bash, zsh, pdksh ...), and 'printenv' in the case of the C-Shell style (csh, tcsh. ..):

$ env
HOSTNAME=myhost
TERM=xterm
SHELL=/bin/bash
...

And this is our first example of using the command pipeline -- we just look at only the TEST variable in the full set:

$ env | grep TEST
TEST=456

Variables can be local to a given process or global to a session. You can set local values for variables by preceding command calls:

$ TEST=789 sh -c 'echo $TEST'
789

But, as we can see, our global settings are the same:

$ echo $TEST
456
$ sh /tmp/test
TEST:
456

We can remove the setting of variables using the 'unset' command:

$ unset TEST
$ echo $TEST

As we can see, this variable has also been removed from the list of global environment variables:

$ sh /tmp/test
TEST:
$ env | grep TEST
$

And there is no "unexport" command -- just only "unset" command.

Finally, as with traditional programming languages, we can use shell scripts such as libraries that can be run from an interactive shell session or from another shell script to set variables for top-level processes. Let's try to write another script in which we simply set the TEST variable.

$ cat > /tmp/test_set
TEST=qwe
$ source /tmp/test_set
$ echo $TEST
qwe

But for our first script, this variable is still invisible:

$ sh /tmp/test 
TEST:
$

Why? Just because it is not exported. Let's export:

$ cat > /tmp/test_exp
export TEST=asd

And run our test script again:

$ sh /tmp/test 
TEST:
asd

As we can see, in our main shell session, the variable has changed too:

$ echo $TEST
asd