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About readonly Bash variables

Doing this:

$ readonly myvar=1

means that $myvar can’t have its value changed within the global scope.

$ myvar=2
-bash: myvar: readonly variable

You can use declare -r to enforce immutability within a local scope (eg within a function body).

See the Linux Shell Scripting Tutorial wiki for basic examples. See this Stack Overflow answer for more details on the differences between readonly and declare -r.