What's the best way to send multiple commands via ssh whilst running a script?

Question

I'm making a script that occasionally sends commands to another machine over ssh. At the moment, im just calling this one function that opens up a new shell for each command it sends like so:

 #!/bin/bash

port='22'
user='user'
host='hostname'

send_ssh() {
        ssh -oBatchMode=yes -oConnectTimeout=5 -p "$1" -tq "$2"@"$3" "$4" || exit 1
}

send_ssh "$port" "$user" "$host" true
   # it worked so do some stuff locally blah blah

send_ssh "$port" "$user" "$host" anothercommand
   # rest of script

I've looked at keeping a tunnel open in the background and / or using a control socket so i can speed things up without having to open a new connection for each command but cant work out if and wether it's worth doing that in this case.

chepner · Accepted Answer · 2021-01-27 15:06:36Z

1

Setting up a control socket is so easy that I would argue it is absolutely worth doing, even with only two calls to send_ssh.

send_ssh() {
    ssh -o ControlMaster=auto -o ControlPersist=10 -oBatchMode=yes -oConnectTimeout=5 -p "$1" -tq "$2"@"$3" "$4" || exit 1
}

ControlMaster=auto means that an existing connection will be used if available, otherwise one will be opened for you.

The setting ControlPersist=10 means that the connection to the remote host will remain open for 10 seconds after the client exists, you can adjust this as desired. You can set it to 0 or yes to keep it open permanently, in which case you'll want to make sure something like

ssh -o exit -p "$1" "$2@$3"

executes before your script exits to close the master connection.

answered Jan 27, 2021 at 15:06

chepner

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2 Comments

ctac_ Over a year ago

I think it's cautious to set ControlPath with ControlPersist set to 0.

chepner Over a year ago

That's probably a good idea regardless of the value of ControlPersist.

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