Bash array expansion patterns for use with -u

_README.md

Expanding Bash arrays safely with set -u

Prior to Bash 4.4 set -u treated empty arrays as "unset", and terminates the process. There are a number of possible workarounds using array parameter expansion, however almost all of them fail in certain Bash versions.

This gist is a supplement to this StackOverflow post.

tl;dr

The only safe option for expanding an array across Bash versions under set -u is:

${array[@]+"${array[@]}"}

Notice the quotes are inside the expansion, not surrounding the whole expression, and that it uses +, not :+.

Alternatives

See results.txt for a detailed breakdown across several versions of Bash. Some of these alternatives are "obviously" wrong due to incorrect quoting, but they're included for completeness.

shorthand	$@ example	${arr[@]} example	broken in	notes
":+"	"${@:+"$@"}"	"${arr[@]:+"${arr[@]}"}"	4.2+
":+	"${@:+$@}"	"${arr[@]:+${arr[@]}}"	4.2+
"+"	"${@+"$@"}"	"${arr[@]+"${arr[@]}"}"	4.2
"+	"${@+$@}"	"${arr[@]+${arr[@]}}"	4.2
:+"	${@:+"$@"}	${arr[@]:+"${arr[@]}"}	*
:+	${@:+$@}	${arr[@]:+${arr[@]}}	*
+"	${@+"$@"}	${arr[@]+"${arr[@]}"}	N/A	works in all tested versions (> 3.0)
+	${@+$@}	${arr[@]+${arr[@]}}	*
":0/1	"${@:1}"	"${arr[@]:0}"	4.2	crashes, presumably a regression
:0/1	${@:1}	${arr[@]:0}	*	also crashes in 4.2
":-	"${@:-}"	"${arr[@]:-}"	*
:-	${@:-}	${arr[@]:-}	*
"-	"${@-}"	"${arr[@]-}"	*
-	${@-}	${arr[@]-}	*

If we exclude v4.2 several other expansions do work, including "${@+"$@"}" and "${@:1}", but so long you intend to support that version these expansions are not safe.

Reproduction

To reproduce the contents of results.txt run:

for v in 3.1 3.2 4.0 4.1 4.2 4.3 4.4 5.0; do
  docker run -v "$PWD:/mnt" "bash:$v" bash /mnt/expansions.sh
done

Bash 3.0 is intentionally excluded from the reported results, but you can run the script against that version too to see what breaks (hint: it's a lot).

expansions.sh

#!/bin/bash

echo "Bash Version: $BASH_VERSION"

expansion() {
  local incantation=$1 expected=$2 actual
  shift 2
  actual=$(
    set -u
    eval "$(printf 'copy=( %s ); printf "${#copy[@]}"' "$incantation")" 2>/dev/null
  ) || { printf '\t\e[1;33m%s\e[0m' "!"; return 0; }
  if (( actual == expected )); then
    printf '\t\e[32m%s\e[0m' "✓"
  else
    printf '\t\e[31m%s\e[0m' "✗"
  fi
}

test_expansion() {
  local arr=("$@")
  
  printf '$#:%s' "$#"
  for incantation in \
    '"${@:+"$@"}"' '"${@:+$@}"' '"${@+"$@"}"' '"${@+$@}"' '${@:+"$@"}' '${@:+$@}' \
    '${@+"$@"}' '${@+$@}' '"${@:1}"' '${@:1}'
  do
    expansion "$incantation" "$#" "$@"
  done
  printf '\n'
  
  printf '#arr:%s' "${#arr[@]}"
  for incantation in \
    '"${arr[@]:+"${arr[@]}"}"' '"${arr[@]:+${arr[@]}}"' '"${arr[@]+"${arr[@]}"}"' \
    '"${arr[@]+${arr[@]}}"' '${arr[@]:+"${arr[@]}"}' '${arr[@]:+${arr[@]}}' \
    '${arr[@]+"${arr[@]}"}' '${arr[@]+${arr[@]}}' '"${arr[@]:0}"' '${arr[@]:0}'
  do
    expansion "$incantation" "$#"
  done
  printf '\n'
}

printf '\t%s' '":+"' '":+' '"+"' '"+' ':+"' ':+' '+"' '+' '":0/1' ':0/1'
printf '\n'
test_expansion
test_expansion ''
test_expansion '' ''
test_expansion a 'b c'

results.txt

Bash Version: 3.1.23(1)-release
	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1	":-	:-	"-	-
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
#arr:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
$#:1	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:1	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
Bash Version: 3.2.57(1)-release
	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1	":-	:-	"-	-
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
#arr:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
$#:1	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:1	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
Bash Version: 4.0.44(1)-release
	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1	":-	:-	"-	-
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
#arr:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
$#:1	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:1	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
Bash Version: 4.1.17(2)-release
	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1	":-	:-	"-	-
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
#arr:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
$#:1	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:1	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
Bash Version: 4.2.53(2)-release
	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1	":-	:-	"-	-
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
#arr:0	✓	✓	✓	✓	✓	✓	✓	✓	!	!	✗	✓	✗	✓
$#:1	✗	✗	✗	✗	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:1	✗	✗	✗	✗	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
Bash Version: 4.3.48(1)-release
	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1	":-	:-	"-	-
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
#arr:0	✓	✓	✓	✓	✓	✓	✓	✓	!	!	✗	✓	✗	✓
$#:1	✗	✗	✗	✗	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:1	✗	✗	✗	✗	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
Bash Version: 4.4.23(1)-release
	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1	":-	:-	"-	-
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
#arr:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
$#:1	✗	✗	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:1	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
Bash Version: 5.0.17(1)-release
	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1	":-	:-	"-	-
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
#arr:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓
$#:1	✗	✗	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:1	✗	✗	✓	✓	✗	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
$#:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗
#arr:2	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗	✓	✗	✓	✗

It's easier to see if you comment out the #arr tests entirely, to leave only the $# ones:

Bash Version: 3.1.23(1)-release
	.	"	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
$#:1	✗	✓	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
Bash Version: 3.2.57(1)-release
	.	"	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
$#:1	✗	✓	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
Bash Version: 4.0.44(1)-release
	.	"	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
$#:1	✗	✓	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
Bash Version: 4.1.17(2)-release
	.	"	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
$#:1	✗	✓	✓	✓	✓	✓	✗	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
Bash Version: 4.2.53(2)-release
	.	"	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
$#:1	✗	✓	✗	✗	✗	✗	✗	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
Bash Version: 4.3.48(1)-release
	.	"	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
$#:1	✗	✓	✗	✗	✗	✗	✗	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
Bash Version: 4.4.23(1)-release
	.	"	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
$#:1	✗	✓	✗	✗	✓	✓	✗	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
Bash Version: 5.0.18(1)-release
	.	"	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
$#:1	✗	✓	✗	✗	✓	✓	✗	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗

Ok yes I follow you now. You're right that $@ is not affected by the same set -u issue as other arrays, I believe I just included it in the table for completeness - as you've found $@ expansion sometimes behaves differently than ${arr[@]} expansion.