Bash empty array expansion with `set -u`

Question

I\'m writing a bash script which has set -u, and I have a problem with empty array expansion: bash appears to treat an empty array as an unset variable during e

Answer 1

@ikegami's accepted answer is subtly wrong! The correct incantation is ${arr[@]+"${arr[@]}"}:

$ countArgs () { echo "$#"; }
$ arr=('')
$ countArgs "${arr[@]:+${arr[@]}}"
0   # WRONG
$ countArgs ${arr[@]+"${arr[@]}"}
1   # RIGHT
$ arr=()
$ countArgs ${arr[@]+"${arr[@]}"}
0   # Let's make sure it still works for the other case...

Answer 2

The most simple and compatible way seems to be:

$ set -u
$ arr=()
$ echo "foo: '${arr[@]-}'"

Answer 3

this may be another option for those who prefer not to duplicate arr[@] and are okay to have an empty string

echo "foo: '${arr[@]:-}'"

to test:

set -u
arr=()
echo a "${arr[@]:-}" b # note two spaces between a and b
for f in a "${arr[@]:-}" b; do echo $f; done # note blank line between a and b
arr=(1 2)
echo a "${arr[@]:-}" b
for f in a "${arr[@]:-}" b; do echo $f; done

Answer 4

Here are a couple of ways to do something like this, one using sentinels and another using conditional appends:

#!/bin/bash
set -o nounset -o errexit -o pipefail
countArgs () { echo "$#"; }

arrA=( sentinel )
arrB=( sentinel "{1..5}" "./*" "with spaces" )
arrC=( sentinel '$PWD' )
cmnd=( countArgs "${arrA[@]:1}" "${arrB[@]:1}" "${arrC[@]:1}" )
echo "${cmnd[@]}"
"${cmnd[@]}"

arrA=( )
arrB=( "{1..5}" "./*"  "with spaces" )
arrC=( '$PWD' )
cmnd=( countArgs )
# Checks expansion of indices.
[[ ! ${!arrA[@]} ]] || cmnd+=( "${arrA[@]}" )
[[ ! ${!arrB[@]} ]] || cmnd+=( "${arrB[@]}" )
[[ ! ${!arrC[@]} ]] || cmnd+=( "${arrC[@]}" )
echo "${cmnd[@]}"
"${cmnd[@]}"

Answer 5

@ikegami's answer is correct, but I consider the syntax ${arr[@]+"${arr[@]}"} dreadful. If you use long array variable names, it starts to looks spaghetti-ish quicker than usual.

Try this instead:

$ set -u

$ count() { echo $# ; } ; count x y z
3

$ count() { echo $# ; } ; arr=() ; count "${arr[@]}"
-bash: abc[@]: unbound variable

$ count() { echo $# ; } ; arr=() ; count "${arr[@]:0}"
0

$ count() { echo $# ; } ; arr=(x y z) ; count "${arr[@]:0}"
3

It looks like the Bash array slice operator is very forgiving.

So why did Bash make handling the edge case of arrays so difficult? Sigh. I cannot guarantee you version will allow such abuse of the array slice operator, but it works dandy for me.

Caveat: I am using GNU bash, version 3.2.25(1)-release (x86_64-redhat-linux-gnu) Your mileage may vary.

Answer 6

Turns out array handling has been changed in recently released (2016/09/16) bash 4.4 (available in Debian stretch, for example).

$ bash --version | head -n1
bash --version | head -n1
GNU bash, version 4.4.0(1)-release (x86_64-pc-linux-gnu)

Now empty arrays expansion does not emits warning

$ set -u
$ arr=()
$ echo "${arr[@]}"

$ # everything is fine