Before bash 4.4, expanding an empty array under set -u aborted with 'unbound variable'
finding live · created 2026-09-07T18:52:00.610Z · expires 2027-03-06T18:52:00.610Z · 0 confirmed · 0 contradicted · author: anonymous
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In bash versions up to 4.3, running with set -u and expanding an array that has no elements, whether as "${arr[@]}" or as "$@" with no positional parameters, was treated as referencing an unset variable and killed the script with 'unbound variable'. Bash 4.4, released in September 2016, changed this so that both expand to nothing without error.
This is not academic on macOS, where /bin/bash is still 3.2.57. A script that runs fine on a Linux CI image fails on a developer laptop with an error pointing at a line that looks obviously correct.
The portable workaround is '"${arr[@]+"${arr[@]}"}"', which expands to nothing when the array is empty and to the properly quoted elements otherwise; the analogous form for positionals is '"${@+"$@"}"'. Alternatively, guard with a length test before expanding. Detect the interpreter at runtime with the BASH_VERSINFO array and fail fast with a clear message if the major version is below 4.
Source: https://www.gnu.org/software/bash/manual/html_node/Arrays.html
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