Linux/Unix Shell Scripting Examples: SSH

WIP Alert This is a work in progress. Current information is correct but more content may be added in the future.

Unless otherwise defined, examples are written in the bash dialect

SSH using a private key

Your key is located under ~/.ssh, e.g. ~/.ssh/id_rsa.

$ ssh -i /path/to/your/key username@hostnameorip

Remember:

• permissions for the .ssh directory must be 700

• the public key (e.g. id_rsa.pub) must be 644

• the private key (e.g. id_rsa) must be 600

• authorized_keys must be 600

SSH command flags

• -i path/to/key: uses the given private key to login using key-based authentication

• -q: quiet mode. Useful when you need to use ssh inside scripts.

• -t: forces a pseudo-terminal to be used. This is useful to fix errors and warning messages about there not being a tty set when you use SSH to run remote commands.

SSH with simple remote command

$ ssh myusername@remotehost 'ls -lha'

SSH with multiline remote command

Note the use of the -t flag, to avoid errors

The "ENDSSH"...ENDSSH block is a HEREDOC multi-line string.

The quotes around the first ENDSSH marker tell bash you don't want it to perform variable expansion within the block.

#!/usr/bin/env bash

# you can add other options to the ssh command too
ssh -t myusername@remotehost << "ENDSSH"
cd /home/myusername
ls -lha
ENDSSH

SSH with multiline remote command and save result into a variable

If you also want to retrieve the output and store it in a variable, do this:

#!/usr/bin/env bash

output=$(ssh -t myusername@remotehost << "ENDSSH"
cd /home/myusername
ls -lha
ENDSSH
)

# do something with the results
echo "$output"

Force a remote command to be run on bash

For the cases when bash is not the default shell in the machine you're SSH'ing to, you can call it as a command:

output=$(ssh myusername@remotehost /bin/bash << ENDSSH
# some bash-specific code
ENDSSH
)

# do something with the results
echo "$output"

Troubleshooting: "here-document at line N delimited by end of-of-file wanted endssh"

Make sure there are no spaces before or after the final ENDSSH

References

• OpenBSD docs: SSH