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@dimo414
Last active Jun 30, 2022
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Bash array expansion patterns for use with -u

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).

#!/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'
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 ✓ ✓ ✓ ✓ ✓ ✗ ✓ ✗ ✓ ✗ ✓ ✗ ✓ ✗
@dimo414
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dimo414 commented May 1, 2020

If the eval-magic feels a bit too handwavy for you, an earlier version of the script doesn't use it.

@quinncomendant
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quinncomendant commented Apr 21, 2022

Don't worry, ${arr[@]+"${arr[@]}"} will not have word-splitting applied, even though it is not surrounded by " … " (as you would normally do with "${arr[@]}", and as is done with the inferior "${arr[@]+"${arr[@]}"}").

Here's a test:

arr=(a 'b c');
for a in ${arr[@]+"${arr[@]}"}; do
    echo ">$a<";
done

The output is:

>a<
>b c<

@dimo414
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dimo414 commented Apr 22, 2022

@quinncomendant yes, that's exactly what this post is demonstrating. But ${arr[@]+"${arr[@]}"} is quoted, which is why word splitting doesn't occur. "${arr[@]+"${arr[@]}"}" ("+" in the table above) does not work correctly in some versions of bash, namely 4.2 and 4.3.

@andrewgdotcom
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andrewgdotcom commented Jun 29, 2022

@dimo414 Great stuff! But isn't the reference to ${@+"$@"} excessive? I've modified your script with '"$@"' included as a test case and it reports no issue on any of the bash versions listed. When would the longer form be required?

@dimo414
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dimo414 commented Jun 29, 2022

@andrewgdotcom can you share a more complete example of what you tried? As written, '"$@"' with enclosing single quotes would be a single string of the literal characters "$@" rather than an array expansion.

@andrewgdotcom
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andrewgdotcom commented Jun 29, 2022

@dimo414 I updated your script to read:

#!/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[@]+${arr[@]}}' '"${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'

and the output was:

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: 5.0.18(1)-release
	.	"	":+"	":+	"+"	"+	:+"	:+	+"	+	":0/1	:0/1
$#:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
#arr:0	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
$#:1	✗	✓	✗	✗	✓	✓	✗	✗	✓	✗	✓	✗
#arr:1	✗	✓	✗	✗	✓	✓	✗	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
#arr:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
$#:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗
#arr:2	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗

Second column, odd lines represent "$@" - no issues reported?

@andrewgdotcom
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andrewgdotcom commented Jun 29, 2022

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	✗	✓	✓	✓	✓	✓	✓	✗	✓	✗	✓	✗

@dimo414
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dimo414 commented Jun 30, 2022

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.

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