How to send expression with variables over ssh?

Question

I can run the followind loop locally

for i in A B C; do mycommand $i; done

Now I am trying to send it over ssh

The following version

ssh myhost for i in A B C; do mycommand $i; done

The following version

ssh myhost "for i in A B C; do mycommand $i; done"

also fails, because it wants i varibale locally.

How to handle?

Is there any general approach, for example, what if mycommand is another ssh?

ssh myhost 'for i in hos1 host2 host3; do ssh "$i" ... another for expression

Quoting in ssh $host $FOO and ssh $host "sudo su user -c $FOO" type constructs -- better yet, don't pass the commands over the command line so you avoid the quoting hell completely — ilkkachu, Feb 21 '23 at 14:18
For Bash, but gives some insight: How can I single-quote or escape the whole command line in Bash conveniently? A general approach can be summarized as (from this answer): "To be in control of what the remote shell will get as $command_line_built_by_ssh, you need to understand, predict and mastermind the parsing and interpreting that happens before. You need to craft your local command, so after the local shell and ssh digest it, it becomes the exact $command_line… you want to execute on the remote side." — Kamil Maciorowski, Feb 21 '23 at 14:31

score 6 · Answer 1 · edited Feb 21 '23 at 14:09

6

Just use single quote:

ssh myhost 'for i in A B C; do mycommand "$i"; done'

That will use the $i as literal characters to the ssh command instead of having the local shell substitute them. In general, you'll also want to double-quote the expansion to prevent issues with issues with word splitting, though it's not strictly necessary with the values here.

See:

edited Feb 21 '23 at 14:09

ilkkachu

138,973

answered Feb 21 '23 at 14:08

Lie Ryan

1,228

Hm, can't figure general approach, this is ad-hoc. – Dims Feb 21 '23 at 14:12
@Dims: the shell normally expands $whatever to replace it with the variable content before running the command. It also does so inside of double quotes like so: echo "$PWD". Here, you want the shell to treat $ as a literal symbol, and not expand it. Use single quotes like echo '$PWD' (try both commands!) or escape the $ character: echo \$PWD. – MayeulC Feb 21 '23 at 15:37

How to send expression with variables over ssh?

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