Constants

In Bash, constants are created by making a variable read-only. The readonly built-in marks each specified variable as unchangeable. The syntax is:

readonly OPTION VARIABLE(s)

The values of these variables can then no longer be changed by subsequent assignment. If the -f option is given, each variable refers to a shell function; see Chapter 11. If -a is specified, each variable refers to an array of variables. If no arguments are given, or if -p is supplied, a list of all read-only variables is displayed. Using the -p option, the output can be reused as input.

The return status is zero, unless an invalid option was specified, one of the variables or functions does not exist, or -f was supplied for a variable name instead of for a function name.

[bob in ~] readonly TUX=penguinpower

[bob in ~] TUX=Mickeysoft
bash: TUX: readonly variable