You can change the value of a null or unset variable to its default in a script using the := modifier:

When the shell expands the expression ${name:=default}, it sets the value of name to the expanded value of default. If a script contains a line such as the following and LIT is unset or null at the time this line is executed, the shell assigns LIT the value /home/max/literature:

Shell scripts frequently start with the : (null) builtin followed on the same line by the := expansion modifier to set any variables that might be null or unset. The : builtin evaluates each token in the remainder of the command line but does not execute any commands.

Use the following syntax to set a default for a null or unset variable in a shell script (a SPACE follows the first colon). Without the leading colon (:), the shell would evaluate and attempt to execute the “command” that results from the evaluation.

When a script needs a directory for temporary files and uses the value of TEMPDIR for the name of this directory, the following line assigns to TEMPDIR the value /tmp if TEMPDIR is null: