How to Restrict Variable and Function Scope in Bash Scripts

Variable scope#

Aim of this section: Understand how variables are scoped to avoid namespace pollution and associated bugs.

It can get really difficult to tell what is happening when there is a lot of global state. Here we’ll look at some ways to reduce the scope of variables to avoid namespace pollution.

Command scope#

A variable assigned in the prefix of a command is scoped to that command. For example, it is common practice to ensure consistent sorting by setting the $LC_COLLATE variable. Compare

$ printf '%s\n' a A | LC_COLLATE=en_US.utf8 sort

to

$ printf '%s\n' a A | LC_COLLATE=POSIX sort

LC_COLLATE is a special locale setting. “POSIX” is a special locale which is available on all modern *nix systems, which makes it ideal for portability.

Please note that the assignment becomes part of the context of the command, not the shell, so assigning one before a shell builtin does not work:

$ name=Smith echo "$name"

Function scope#

A variable can be scoped to a function:

        printf '> %s\n' "$line"