We'll go over a POSIX compliant solution using parameter expansion as well as a Bash 4.2+ variable existence check.

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Nick Janetakis

Covers how to distinguish between an unset variable and a variable explicitly set to an empty string in Bash/POSIX shell scripts. Two approaches are presented: a POSIX-compliant method using parameter expansion (`${var+x}`) that works even with `set -o nounset`, and a Bash 4.2+ built-in check using the `-v` flag. Both approaches are shown with if/elif and case statement variants, along with example outputs for all three states: unset, empty, and non-empty.

Differentiate Between an Empty or Unset String Variable with Bash — Nick Janetakis