Add CLI support for command templates/matrix commands

[cdrini]

It would be useful to be able to generate the commands dynamically based on an environment variable. For example:

concurrently --matrix "A B C" "echo {}"

Which would be equivalent to typing:

concurrently "echo A" "echo B" "echo C"

Or with multiple values you could do:

concurrently --matrix "A B C" --matrix "1 2 3" "echo {1}{2}"

Which would be equivalent to writing:

concurrently "echo A1" "echo "A2" "echo A3" "echo B1" "echo B2" "echo B3" "echo C1" "echo C2" "echo C3"

This would be useful for common repetitive tasks that can happen in parallel. For example, let's say I want to download some docker image on a few different servers. I could do:

SERVERS="server1 server2"
IMAGES="python:3.11 node:20"

concurrently --group --matrix "$SERVERS" --matrix "$IMAGES" "ssh {1} 'docker pull {2}'"

This would help making concurrently a drop-in replacement for bash for-loops; the above would replace

for server in $SERVERS; do
    for image in $IMAGES; do
        ssh $server "docker pull $image"
    done
done

It would also make concurrently a drop-in for a lot of gnu parallel usages, eg:

parallel --quote ssh {1} "docker pull {2}" ::: $SERVERS ::: $IMAGES

Another example: processing a lot files in parallel, eg less compiling a number of files:

concurrently --matrix "$(ls *.less)" --group "lessc {}"

[gustavohenke]

Hey, thanks for the suggestion.

We already have the syntax parsing built into concurrently, it's the passthrough args (docs - https://github.com/open-cli-tools/concurrently/blob/8d3f9761bf6269ae9abdd5dbb661314b9980d62d/docs/cli/passthrough-arguments.md), though where it lacks is combining all the input combinations, which is what you're asking for.

As for which flag should activate it, I'm on the fence between reusing --passthrough-arguments/-P with a matrix value, or adding a separate --matrix, even if as a shortcut. Thoughts?

Tagged it as pull requests welcome, if anyone is keen on implementing this!

[cdrini]

I think --passthrough-arguments might work differently ; that will always still only run the number of commands specified, but change their arguments. --matrix will always change (multiply) the commands specified.

--passthrough-arguments is kind of like an append to each of the existing commands, whereas --matrix is like multiplying the commands.

I think :P If I'm understanding that feature correctly.

[cdrini]

Ok having implemented it now I think I see what you mean :P There is something between the two features, but I'm not quite finding the way to DRY them up. I think they might be different enough to warrant different names -- and passthrough only allows one list as input, whereas matrix can take multiple lists, which I'm having trouble finding a way to reconcile.