How to use positional parameter in docker exec command with here strings (<<<)?

Question

Here is what I am trying to do:

    docker exec -it $1 /bin/bash <<< $CONTAINER_ID

But this doesn't work:

Output: "docker exec" requires at least 2 arguments.

It seems that <<< completely replaces STDIN ignoring $1. Is it possible to replace only one argument with <<<? I know about aliases, but it doesn't suit me. I need to be able to use only standard tools to pass to my colleague a command in which he will change only the last variable, which will replace one or more arguments in the center of the command.

Here is what I am trying to do: what is it trying to achieve? Please add examples — Jaromanda X, May 11 '23 at 07:02
I want to convert this:
`somecommand $var arg2`

to this:

`somecommand $1 arg2 <<< $var`

With <<< , I'm trying to replace only the first argument, instead of completely replacing standard input (STDIN). — Anton Kuznetsov, May 11 '23 at 07:18
"Is it possible to replace only one argument with <<<?" – <<< redirects stdin and has nothing to do with arguments. Maybe you should read What's the difference between STDIN and arguments passed to command? — Kamil Maciorowski, May 11 '23 at 07:44
that's what I thought, wasn't clear that you didn't understand the command line in the question — Jaromanda X, May 11 '23 at 08:11
Considering your answer and your comment under the answer, I see an XY problem here. — Kamil Maciorowski, May 11 '23 at 10:02
A decent solution to the actual problem revealed in your comment under your answer is sh -c 'exec docker exec -it "$1" /bin/bash' sh "$CONTAINER_ID". It's not an answer to the question in its current form though. — Kamil Maciorowski, May 11 '23 at 10:23

score 0 · Answer 1 · answered May 12 '23 at 09:22

<<< is the wrong tool for this job. It replaces standard input (stdin), which has nothing to do with passing arguments. Arguments and stdin are both forms of input to a command, but they're completely separate from each other. xargs can convert stdin to arguments, but at that point you're really trying to work around the problem instead of using a method that doesn't have this problem in the first place.

I'm not sure I understand what the actual goal and constraints here, but I think defining a function might be the cleanest way to accomplish what you want:

execit() {
    docker exec -it "$1" /bin/bash
}

(Replacing execit with whatever function name you prefer.) Then just run e.g. execit "$CONTAINER_ID".

If that doesn't fit what you're trying to do, the next cleanest solution is what Kamil Maciorowski suggested in a comment:

sh -c 'exec docker exec -it "$1" /bin/bash' sh "$CONTAINER_ID"

score -2 · Answer 2 · answered May 11 '23 at 09:36

-2

Thanks to Kamil Maciorowski, I figured out which way to look and found a solution. Here is the command that does what I need:

xargs -o -I{} docker exec -it {} /bin/bash <<< $CONTAINER_ID

answered May 11 '23 at 09:36

Anton Kuznetsov

1

2

What is wrong with docker exec -it "$CONTAINER_ID" /bin/bash? – Kamil Maciorowski May 11 '23 at 09:37
@KamilMaciorowski my task was to move the container name argument to the right. This makes it possible to copy a command from an internal work instruction, and then quickly replace the container name on the right with the desired one, without changing anything in the center. – Anton Kuznetsov May 11 '23 at 09:54
3

@AntonKuznetsov, that's not what your question says, though. – ilkkachu May 11 '23 at 09:54
3

Ditto. And frankly only after you published your answer I understood what you wanted the command to do. – Kamil Maciorowski May 11 '23 at 09:57
A oneline function is probably still better even for this: f() { docker exec -it "$1" /bin/bash ; }; f "$CONTAINER_ID" – muru May 12 '23 at 09:33

How to use positional parameter in docker exec command with here strings (<<<)?

2 Answers2