In Bash test if associative array is declared

问题:

How can I test if an associative array is declared in Bash? I can test for a variable like:

[ -z $FOO ] && echo nope

but I doesn't seem to work for associative arrays:

$unset FOO
$declare -A FOO
$[ -z $FOO ] && echo nope
nope
$FOO=([1]=foo)
$ [ -z $FOO ] && echo nope
nope
$ echo ${FOO[@]}
foo

EDIT:

Thank you for your answers, both seem to work so I let the speed decide:

$ cat test1.sh
#!/bin/bash
for i in {1..100000}; do
    size=${#array[@]}
    [ "$size" -lt 1 ] && :
done
$ time bash test1.sh #best of five

real    0m1.377s
user    0m1.357s
sys     0m0.020s

and the other:

$ cat test2.sh
#!/bin/bash

for i in {1..100000}; do
    declare -p FOO >/dev/null 2>&1 && :
done
$ time bash test2.sh #again, the best of five

real    0m2.214s
user    0m1.587s
sys     0m0.617s

EDIT 2:

Let's speed compare Chepner's solution against the previous ones:

#!/bin/bash

for i in {1..100000}; do
    [[ -v FOO[@] ]] && :
done
$ time bash test3.sh #again, the best of five

real    0m0.409s
user    0m0.383s
sys     0m0.023s

Well that was fast.

Thanks again, guys.

回答1:

In bash 4.2 or later, you can use the -v option:

[[ -v FOO[@] ]] && echo "FOO set"

Note that in any version, using

declare -A FOO

doesn't actually create an associative array immediately; it just sets an attribute on the name FOO which allows you to assign to the name as an associative array. The array itself doesn't exist until the first assignment.

回答2:

You can use declare -p to check if a variable has been declared:

declare -p FOO >/dev/null 2>&1 && echo "exists" || echo "nope"

And to check specifically associative array:

[[ "$(declare -p FOO 2>/dev/null)" == "declare -A"* ]] &&
   echo "array exists" || echo "nope"

回答3:

One of the easiest ways is to the check the size of the array:

size=${#array[@]}
[ "$size" -lt 1 ] && echo "array is empty or undeclared"

You can easily test this on the command line:

$ declare -A ar=( [key1]=val1 [key2]=val2 ); echo "szar: ${#ar[@]}"
szar: 2

This method allow you to test whether the array is declared and empty or undeclared altogether. Both the empty array and undeclared array will return 0 size.