Here Documents in Bash

using here documents in bash to feed multiple commands to ssh, with an example that dumps remote databases and restores them locally.

tl;dr

a here document is special-purpose code block that you can use to feed commands

1ssh otherhost << EOF
2  ls some_folder; 
3  ./someaction.sh 'some params'
4  pwd
5  ./some_other_action 'other params'
6EOF

• a limit string (usually EOF) indicates the start and finish of a here document. It doesn’t have to called EOF, you can call it whatever, e.g. JumanjiXYZ is a valid limit string
• Variables inside a Here document need to be quoted or else they won’t be substituted, like so: "${VAR}"
• The ending limit string needs to be at the beginning of the line

Here is an example where a here doc has been used to run multiple commands to the remote server. The script logs in to a remote server, takes a back up of all it’s databases and restores those databases directly on your current server, i.e. the server you ran the commands from

1# the limit string here is: SSHCOMMANDS
2# The variables inside the here doc are quoted
3ssh -i ${REMOTE_SSH_KEY} -p ${REMOTE_SSH_PORT} ${REMOTE_SSH_USER}@${REMOTE_SSH_HOST} /bin/bash << SSHCOMMANDS
4  mysqldump --user="${REMOTE_DB_USER}" --password="${REMOTE_DB_PASS}" --add-drop-database --all-databases | gzip -9 > "${DB_DUMP_FILENAME}".sql.gz
5  gunzip < "${DB_DUMP_FILENAME}".sql.gz | mysql --user="${DB_USER}" --password="${DB_PASS}" --host="${DB_HOST}"
6SSHCOMMANDS

Links

• Advanced Bash Scripting Guide: Chapter 19. Here Documents
• What is the cleanest way to ssh and run multiple commands in Bash?