Use of read-only variables in shell scripts [closed]

Answer 1

A classic use of read-only variables is with TMOUT. Setting this variable to a non-zero value will logout an interactive terminal session after TMOUT seconds of inactivity (i.e. no keyboard input). To defeat a smart user from overriding the setting, use readonly:

readonly TMOUT=60

Having done this, there is no way to do:

export TMOUT=0

Answer 2

It sounds like you might think that readonly does more than it really does. For one thing, readonly status is not exported into the environment or inherited by child processes:

$ declare -rx LOGS=hello
$ LOGS=goodbye
bash: LOGS: readonly variable
$ bash -c 'echo "$LOGS"'
hello
$ bash -c 'LOGS=goodbye; echo "$LOGS"'
goodbye
$ 

Answer 3

I don't think readonly variables in bash are useful. I can't think of any issue I've seen that could have been prevented by making a variable readonly. Such limitations goes against the dynamic nature of bash. There are other more common causes of issues (like mispellings or forgetting to declare variables as local) which are impossible to prevent anyway.

If you want to split things off try to only split "functions", not just chunks of code. It's easier to reuse a small thing if you know "source ~/myscript.sh" doesn't actually do anything.

Answer 4

Generally speaking, using read only variables (in any language) and modularizing your program (in any language) is a good thing.

Read only variables protect against a common source of bugs, and helps improve readability and maintainability. Knowing that you can rely on a value of a variable enables you to reason better about your program, and to make assumptions about that variable later on -- things you couldn't do if the variable were mutable.

Modularization improves maintainability and re-usability. More modules generally means more fine-grained units that can find re-use in different circumstances, shorter code that's easier to read, and if your modules are independent, less interactions among parts that could wreck a modification.