SC2048 "${array[@]:-}" pitfall with empty arrays and set -u

[PeterPitterling]

here is a pitfall with empty arrays and set -u and recommendation from SC2048 "${array[@]}" or "${array[@]:-}" .. the behavior changes because with quotes it will run at least 1 time now.

For bugs

• Rule Id : SC2048
• My shellcheck version (shellcheck --version or "online"): online
• [x] The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2048)
• [x] I tried on https://www.shellcheck.net/ and verified that this is still a problem on the latest commit

#!/usr/bin/env bash
set -u

declare -a array=()

for item in ${array[@]}; do
 printf "throws error - unbound variable \n\n"
done

for item in "${array[@]}"; do
 printf "throws error - unbound variable \n\n"
done

for item in ${array[@]:-}; do
 printf "will not run \n\n"
done

for item in "${array[@]:-}"; do
 printf "will run at least ONE time\n\n"         #not intended !!
done

for item in ${array[@]+"${array[@]}"}; do
 printf "would be correct\n\n"
done

shouldn't that be Shellchecks recommendation for arrays ?

Originally posted by @PeterPitterling in https://github.com/koalaman/shellcheck/issues/2374#issuecomment-969056887

[koalaman]

This workaround is only required on Bash 3 and below. On Bash 4+, for item in "${array[@]}" will not trigger set -u when the array is empty.

ShellCheck currently does not differentiate between shell versions, so there's no way to indicate that you want Bash 3 support and check accordingly.

[PeterPitterling]

Just verified with bash 4.3.48 (SLES12.3) - still triggers "unbound variable"

for item in "${array[@]}"

array[@]: unbound variable

[koalaman]

You're right, this was fixed in bash 4.4+ and not bash 4.0+:

3. New Features in Bash
   a. Using ${a[@]} or ${a[*]} with an array without any assigned elements when
   the nounset option is enabled no longer throws an unbound variable error.

[karniemi]

Still, with bash 5.2.15, getting the length of an empty array fails for unbound variable: ${#a[*]}" . Of course, this is just another "missing feature" in bash just like using ${a[@]} was.

Edit: but declaring the array like this declare -A a=(), instead of declare -A a, makes also ${#a[*]}" work. Did not expect this, because according to the chats in internet, this had not had any impact for the unbound variable error in case of ${a[@]}.

[wileyhy]

What's the context? Also, there's a bug submission form among ShellCheck's GitHub pages. Using that would be helpful. Wiley

On Wed, Dec 18, 2024, 21:15 karniemi @.***> wrote:

Still, with bash 5.2.15, getting the length of an empty array fails for unbound variable: ${#a[*]}" . Of course, this is just another "missing feature" in bash just like using ${a[@]} was.

— Reply to this email directly, view it on GitHub https://github.com/koalaman/shellcheck/issues/2387#issuecomment-2552804437, or unsubscribe https://github.com/notifications/unsubscribe-auth/AUF2F227QRNEOWA3XXFNLBL2GJI53AVCNFSM6AAAAABT4EX7FCVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDKNJSHAYDINBTG4 . You are receiving this because you are subscribed to this thread.Message ID: @.***>

[wileyhy]

Nevermind. I saw the thread. W.

On Wed, Dec 25, 2024, 23:12 Wiley Young @.***> wrote:

What's the context? Also, there's a bug submission form among ShellCheck's GitHub pages. Using that would be helpful. Wiley

On Wed, Dec 18, 2024, 21:15 karniemi @.***> wrote:

Still, with bash 5.2.15, getting the length of an empty array fails for unbound variable: ${#a[*]}" . Of course, this is just another "missing feature" in bash just like using ${a[@]} was.

— Reply to this email directly, view it on GitHub https://github.com/koalaman/shellcheck/issues/2387#issuecomment-2552804437, or unsubscribe https://github.com/notifications/unsubscribe-auth/AUF2F227QRNEOWA3XXFNLBL2GJI53AVCNFSM6AAAAABT4EX7FCVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDKNJSHAYDINBTG4 . You are receiving this because you are subscribed to this thread.Message ID: @.***>