Bash Sub Shells

When writing bash scripts you sometimes need to run commands in the background. This is easily accomplished by appending the command line to be run in the background with an ampersand "&". But what do you do if you need to run multiple commands in the background? You could put them all into a separate script file and then execute that script followed by an ampersand, or you can keep the commands in your main script and run them as a sub-shell.

Creating sub-shells in bash is simple: just put the commands to be run in the sub-shell inside parentheses. This causes bash to start the commands as a separate process. This group of commands essentially acts like a separate script file, their input/output can be collectively redirected and/or they can be executed in the background by following the closing parenthesis with an ampersand.

As a somewhat contrived example, let's say that we want to start a "server" and then once it's running we want monitor it in the background to make sure it's still running. We'll assume that the server itself becomes a daemon and creates a PID file which we can use to monitor it. When the PID file disappears we assume the server has exitted and we send an email to somebody.

Now you could start the server from the main script and then create a second script that does the monitoring and then execute it in the background from the main script, but you can do the whole thing from the same script:

#!/bin/bash

server_cmd=server
pid_file=$(basename $server_cmd .sh).pid
log_file=$(basename $server_cmd .sh).log

(
    echo "Starting server"
    echo "Doing some init work"
    $server_cmd   # server becomes a daemon

    while true
    do
        if [[ -f $pid_file ]]; then
            sleep 15
        else
            break
        fi
    done
    mail -s "Server exitted" joe@blow.com <<<CRAP

) 2>&1 >> $log_file &

echo "Server started"

With this simple example you could of course just execute the whole script in the background and dispense with the sub-shell part, but that may not work if the script is part of a larger script. It's also nice not to require the user to have to remember to start the script in the background, not to mention having to remember to redirect its output to a log file. And of course there are numerous other things a real world script should do: check to see if the server is already running before starting it, delete the PID file if it's stale, check to see if the server has died without removing its PID file, etc. However, that's the real world, this is the example world.

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