在Bash测试中是否声明了关联数组 [英] In Bash test if associative array is declared

问题描述

如何测试Bash中是否声明了关联数组?我可以测试像这样的变量:

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

[ -z $FOO ] && echo nope

但我似乎不适用于关联数组:

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

谢谢您的回答，两者似乎都可以奏效，所以我让速度来决定:

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

和另一个:

$ 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

让我们比较一下Chepner的解决方案与以前的解决方案:

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

那太快了.

再次感谢大家.

推荐答案

在bash 4.2或更高版本中，可以使用-v选项:

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

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

请注意，在任何版本中，使用

Note that in any version, using

declare -A FOO

实际上并没有立即创建一个关联数组.它只是在名称FOO上设置一个属性，该属性允许分配给该名称作为关联数组.直到第一次分配，数组本身才存在.

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.

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