Bash function that breaks loop

Question

I made a bash function that looks something like this:

keystroke()
{
    read -s -n1 -t0.1 key     #Read a single keystroke for 0.1 seconds
    [ "$key" = $'\e' ] &&     #If the pressed key is escape
    {
        echo Aborted by user  #Display message
        break                 #Break parent loop
    }
}

And whenever I needed to gracefully end a loop in other bash functions, i just called keystroke. I am no longer able to do this since bash v4.4.0 says:

-bash: break: only meaningful in a `for', `while', or `until' loop

How can I solve this without copying the same code over and over again more than 10x?

Return a status code that is checked by the caller of the function? — Some programmer dude
– Some programmer dude, Commented Jan 8, 2017 at 12:16

pawamoy · Accepted Answer · 2018-09-25 12:07:15Z

Indeed it seems that since Bash 4.4, the break keyword is not allowed anymore outside of a for, while or until loop.

I verified this with shenv and the following snippet. With Bash 4.3.30:

$ shenv shell bash-4.3.30
$ bash -c 'b() { break; }; for i in 1; do echo $i; b; done'
1

And with Bash 4.4:

$ shenv shell bash-4.4
$ bash -c 'b() { break; }; for i in 1; do echo $i; b; done'
1
environment: line 0: break: only meaningful in a `for', `while', or `until' loop

And the line in the changelog: https://github.com/samuelcolvin/bash/blob/a0c0a00fc419b7bc08202a79134fcd5bc0427071/CHANGES#L677.

xx. Fixed a bug that could allow break' orcontinue' executed from shell functions to affect loops running outside of the function.

So now you cannot use the break keyword in a function anymore to break the parent loop. The solution is to return a status code instead, and check that code in the parent loop:

keystroke()
{
    read -s -n1 -t0.1 key
    [ "$key" = $'\e' ] &&
    {
        echo Aborted by user
        return 1
    }
}

while true; do
    ...
    keystroke || break
    ...
done

However, we can see in the changelog another interesting information: https://github.com/samuelcolvin/bash/blob/a0c0a00fc419b7bc08202a79134fcd5bc0427071/CHANGES#L5954.

i. In POSIX mode, break' andcontinue' do not complain and return success if called when the shell is not executing a loop.

So it seems you can retain the old behavior if you enable POSIX-mode.

$ shenv shell bash-4.4
$ bash --posix -c 'b() { break; }; for i in 1; do echo $i; b; done'
1

heemayl · Accepted Answer · 2017-01-08 12:16:43Z

2

For functions you should use return:

keystroke() {
    ...
    return
}

Optionally add an integer (between 0 and 127) as the return value e.g.:

keystroke() {
    ...
    return 1
}

Note that, otherwise the exit status of the last command will be used as the return value.

answered Jan 8, 2017 at 12:16

heemayl

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8 Comments

Ulrik Over a year ago

I guess I could go that way, although in my simplified example I forgot to mention keystroke function forwards the exit status of whatever executes before it.

pawamoy Over a year ago

It does not really answer the question. The question is "How to write a function that will break its upper/parent loop?". Apparently in previous Bash versions, break was working in functions, and in Bash >=4.4 it's not. The first comment was a good answer: "Return a status code that is checked by the caller of the function". I guess there is no other way now: while true; do keystroke && break; ...; done, keystroke returning 0 when escape was hit, 1 (or more) otherwise.

heemayl Over a year ago

@pawamoy Please read OP's question, example and come again to re-read my answer. When you're inside a function, return is perfectly fine and preferable way to break out early; does not matter whether the function has regular commands or loop within. return also sets the exit status of the function FYI (like I mentioned).

pawamoy Over a year ago

I've done that, and I still don't think you're answering OP's question. Your answer only tells about the return keyword and what it does. OP wants to break the parent loop from within a function, not simply return early from a function.

heemayl Over a year ago

@pawamoy How do I return a status code from a function that the caller of the function can check?

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