Bash mapfile array count different

Question

I did the following:

mapfile -t array < <(commands)

where commands will produce new-line separated output.

Then based on the result, if there's nothing in the array, exit, otherwise continue:

[[ "${#array[@]}" -eq 0 ]] && exit

...

The problem is that the condition is never met. Because when the said commands returns nothing, the ${#array[@]} returns 1 instead of 0.

But it counts correctly if I assign the array as follows:

array=($(commands))

My tests:

$ declare -a array
$ array=($(echo))
$ echo ${#array[@]}
0
$ unset array
$ mapfile -t array < <(echo)
$ echo ${#array[@]}
1

Why?

Initially I used the array=($(commands)) format but shellcheck suggested me to use mapfile instead.

GNU bash, version 5.1.16(1)-release (x86_64-pc-linux-gnu)

There's a newline in the input to mapfile in mapfile -t array < <(echo). It's not nothing - compare with the output of, say, true or :. — muru
– muru, Commented Sep 1, 2023 at 4:14
The behavior you describe indicates the commands don't output newline-separated lines. So this comes down to those particular commands. Without them, we can't help you solve this issue. — Sotto Voce
– Sotto Voce, Commented Sep 1, 2023 at 4:52
Check if first element is empty: [[ "${array[0]}" == "" ]] && exit — Cyrus
– Cyrus, Commented Sep 2, 2023 at 0:30

ilkkachu · Accepted Answer · 2023-09-01 05:38:12Z

This is what you get if the command inside the prints nothing:

$ mapfile -t array < <(printf ''); echo "num: ${#array[@]}"; declare -p array
num: 0
declare -a array=()

This is what you get if it prints a single empty line:

$ mapfile -t array < <(echo); echo "num: ${#array[@]}"; declare -p array
num: 1
declare -a array=([0]="")

The empty line produces an empty element, quite appropriately.

If your program produces such empty lines and you can't stop it, you could remove them with e.g. grep. Or check for the case of a single array element that's empty.

Remove the empty lines with grep:

$ mapfile -t array < <(echo | grep .); echo "num: ${#array[@]}"; declare -p array
num: 0
declare -a array=()

Check for the empty element:

$ mapfile -t array < <(echo);
$ if [[ "${#array[@]}" = 0 || "${#array[@]}" = 1 && "${array[0]}" = "" ]]; then 
    echo "empty list or only one empty element"; 
 fi
empty list or only one empty element

In

array=($(commands))

it's different, since the result of the command substitution goes through word splitting, which also removes any trailing whitespace. And splits lines with whitespace in them. (The command substitution itself also removes any trailing newlines, not that it matters here.)

@annahri, it is! Just note that there's an annoying difference between how declare -p prints regular variables vs. arrays. With arrays, it uses the $'' format, so e.g. a line with a newline looks like $'foo\n'. But with scalars, it prints a raw newline instead, making the output span two lines. And of course there's the usual backslash-escaping of special characters, which makes the output unambiguous but seems to confuse those who aren't familiar with it sometimes. — ilkkachu
– ilkkachu, Commented Sep 1, 2023 at 7:01

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