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

According to the documentation,

An array variable is considered set if a subscript has been assigned a value. The null string is a valid value.

No subscript has been assigned a value, so the array isn't set.

But while the documentation suggests an error is appropriate here, this is no longer the case since 4.4.

$ bash --version | head -n 1
GNU bash, version 4.4.19(1)-release (x86_64-pc-linux-gnu)

$ set -u

$ arr=()

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

---

There is a conditional you can use inline to achieve what you want in older versions: Use ${arr[@]+"${arr[@]}"} instead of "${arr[@]}".

$ function args { perl -E'say 0+@ARGV; say "$_: $ARGV[$_]" for 0..$#ARGV' -- "$@" ; }

$ set -u

$ arr=()

$ args "${arr[@]}"
-bash: arr[@]: unbound variable

$ args ${arr[@]+"${arr[@]}"}
0

$ arr=("")

$ args ${arr[@]+"${arr[@]}"}
1
0: 

$ arr=(a b c)

$ args ${arr[@]+"${arr[@]}"}
3
0: a
1: b
2: c

Tested with bash 4.2.25 and 4.3.11.