for i in ${VAR} 如何在 bash 中工作，而不是 for i in "${VAR[@]}"？

Question

If I try on my machine (with bash 3,4 and 5) the following command:

bash-5.0$ VAR=(1 2 3)
bash-5.0$ for i in ${VAR}; do echo $i; done

I get only one line with the 1 .

If I do the same on ZSH for example, it nicely writes the three lines with progressive numbers.

However in one of our production servers I found this:

bash -c "for i in ${MY_VAR}; do stuff with $i; done"

And by checking the logs it seems that it is actually iterating correctly!

How is this possible? Is it a particular version of bash I'm not aware of? Or some flag I should set? Or maybe the array was populated in a particular way?

Answer 1

You should write:

var=(1 2 3)
for i in "${var[@]}"; do
    do stuff with "$i"
done

You need [@] as shown. And don't use uppercase variable names. Now as to why it works on your production server: possibly because MY_VAR is defined as MY_VAR="1 2 3" (or something analogous), ie, MY_VAR isn't an array (which is bad).

Answer 2

It "works" because the code isn't actually using an array at all.

export MY_VAR='1 2 3'
bash -c 'for i in ${MY_VAR}; do echo "Doing stuff with $i"; done'

...involves no arrays whatsoever; MY_VAR is a string being word-split and then glob-expanded.

Don't do that, ever , even if you really do need to iterate over items from a delimiter-separated string. The reliable alternative is to use read -r -a my_array <<<"$MY_VAR" to read your string into an array, and then for i in "${my_array[@]}"; do echo "Doing stuff with $i"; done for i in "${my_array[@]}"; do echo "Doing stuff with $i"; done for i in "${my_array[@]}"; do echo "Doing stuff with $i"; done to iterate over it.

Answer 3

Looks like Bash evaluates $arr to ${arr[0]} :

arr=(1 2 3)
echo $arr      # yields 1

arr[0]=999
echo $arr      # yields 999

With associative arrays:

declare -A h
h=([one]=1 [two]=2)
echo $h            # yields nothing

h=([0]=1 [two]=2)
echo $h            # yields 1

As others have pointed out, the right way to loop through an array is:

for i in "${arr[@]}"; do ...