SC2199 warns on [[ ${ARRAY[@]:x:1} == ... ]]

[floatingatoll]

For bugs

• Rule Id: SC2199
• My shellcheck version: online
• I tried on shellcheck.net and verified that this is still a problem on the latest commit

Here's a snippet or screenshot that shows the problem:

#!/bin/bash
A=(1 2 3); if [[ ${A[@]:0:1} == '1' ]]; then true; fi

Here's what shellcheck currently says:

SC2199: Arrays implicitly concatenate in [[ ]]. Use a loop (or explicit * instead of @).

Here's what I wanted or expected to see:

No warning, since the array is sliced to a single element before implicit concatenation occurs.

[koalaman]

You're right in this particular case because the array is statically defined, but in general this is not the case: it will expand to 1 or 0 elements.

Here's what happens in [ which doesn't implicitly concatenate:

$ A=(); [ "${A[@]:0:1}" == "foo" ]  && echo ok
bash: [: ==: unary operator expected

If you want 0 elements to be considered an empty string, you should explicitly concatenate the 0 or 1 elements so that you always end up with a string to compare. Alternatively, you can use "${A[0]}" which is shorter and amounts to the same.

[floatingatoll]

ooo. Thank you! Sorry to bother.

…

On Apr 22, 2017, at 19:33, koalaman ***@***.***> wrote: You're right in this particular case because the array is statically defined, but in general this is not the case: it will expand to 1 or 0 elements. Here's what happens in [ which doesn't implicitly concatenate: $ A=(); [ "${A[@]:0:1}" == "foo" ] && echo ok bash: [: ==: unary operator expected If you want 0 elements to be considered an empty string, you should explicitly concatenate the 0 or 1 elements so that you always end up with a string to compare. Alternatively, you can use "${A[0]}" which is shorter and amounts to the same. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or mute the thread.

[floatingatoll]

(I suppose there's an opportunity for ❌1 warning specifically recommending the simpler syntax.)