How can I avoid printing anything in Bash `printf` with an empty array?

Question

I often print out the contents of an array using a quick printf shorthand like this:

$ printf "%s\n" "${my_array[@]}"
$ printf "%s\0" "${my_array[@]}"

This works great in general:

$ my_array=(limburger stilton wensleydale)
$ printf "%s\n" "${my_array[@]}"
limburger
stilton
wensleydale
$

Works great, but when the array is empty it still outputs a single character (a newline or null byte):

$ my_array=()
$ printf "%s\n" "${my_array[@]}"

$

Of course, I can avoid that by testing first for an empty array:

$ [[ ${#my_array[@]} -gt 0 ]] && printf "%s\n" "${my_array[@]}"
$ (( ${#my_array[@]} )) && printf "%s\n" "${my_array[@]}"

But I use this idiom all the time and would prefer to keep it as short as possible. Is there a better (shorter) solution, maybe a different printf format, that won't print anything at all with an empty array?

It seems like the root problem is that printf fills in missing arguments. For example, with a quote spec, it prints the empty string: printf '%q\n' -> ''; and with a number spec, it prints zero: printf '%d\n' -> 0 or printf '%d %d\n' 5 -> 5 0. — wjandrea
– wjandrea, Commented Jun 12, 2021 at 19:15
(( ${#my_array[@]} )) fails in a set -e context. Instead, you may want the equivalent: (( ${#my_array[@]} == 0 )) || printf "%s\n" "${my_array[@]}" — Brian McBarron
– Brian McBarron, Commented Oct 11, 2023 at 8:13

Cyrus · Accepted Answer · 2021-06-12 19:15:40Z

7

Bash's parameter expansion might help:

printf "%b" "${my_array[@]/%/\\n}"

From help printf:

%b: expand backslash escape sequences in the corresponding argument

or

printf "%s" "${my_array[@]/%/$'\n'}"

answered Jun 12, 2021 at 19:15

Cyrus

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Léa Gris · Accepted Answer · 2021-06-13 10:44:20Z

4

For what it is worth:

(( ${#my_array[@]} )) && printf "%s\n" "${my_array[@]}"

(( ${#my_array[@]} )) is a Bash's stand-alone arithmetic expression that evaluates to true when array length is greater than 0.

Anyway, when I need to debug the content of an array. I prefer to use:

declare -p my_array

It will clearly show details that a simple printf '%s\n' cannot, like:

element key or index,
empty elements,
elements containing non-printable characters,
all of variable type or flags,
if array is undeclared, void declare -a my_array= or empty declare -a my_array=().

edited Jun 13, 2021 at 10:44

answered Jun 13, 2021 at 2:20

Léa Gris

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1 Comment

wjandrea Over a year ago

+1 for declare -p! I thought of it myself but neglected to mention it :) For (( ${#my_array[@]} )), though, OP has tried it already.

wjandrea · Accepted Answer · 2021-06-12 19:49:32Z

3

For what it's worth, a plain old for-loop works like you want. It's not shorter, but it is more obvious, so this is what I'd use for scripts instead of a shorthand.

$ my_array=(limburger stilton wensleydale)
$ for i in "${my_array[@]}"; do printf '%s\n' "$i"; done
limburger
stilton
wensleydale
$ my_array=()
$ for i in "${my_array[@]}"; do printf '%s\n' "$i"; done
$

answered Jun 12, 2021 at 19:49

wjandrea

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wjandrea · Accepted Answer · 2021-06-13 16:25:15Z

0

There's no better solution than to test if the array is not empty.

edited Jun 13, 2021 at 16:25

wjandrea

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answered Jun 13, 2021 at 4:51

konsolebox

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