How to make a Variable read-only (constant) in Bash
I'm writing a Bash script on a Linux system to automize some boring stuff that needs to be executed regularly. My need now is to create a read-only variable in it. How to make a read-only variable in a Bash script?
In Bash a variable can be made read-only with the following syntax:
readonly var[=value]
The square brackets around =value mean that the assignment of a value is not always necessary. For instance, if the variable had previously been created and assigned a value, and we want to make it read-only (and not change its current value), we will not need to use =value.
If the variable did not previously exist, and wemake it read-only, you may never assign a value to the variable. Take note that the value of a read-only variable cannot be changed. This is why read-only variables are referred to as constants:
$ var=constant
$ readonly var
$ unset var
var: is read only
$ var=new_value
var: is read only
A very similar approach is used also in another command line shell called Korn Shell (developed by IBM for AIX systems) in which we have an additional way to declare a variable as read-only (the above one is valid also in Korn Shell or ksh):
A variable can be made read-only by using either of the following syntaxes:
typeset -r var[=value]
Or, as we have already seen in the above example:
readonly var[=value]
The following is a Korn shell example:
$ ksh
$ typeset -r cvar=constant
$ unset cvar
ksh: cvar: is read only
$ cvar=new_value
ksh: cvar: is read only