How to control second script based on fisr script in main script on linux

Question

I have new to scripts .. I want to monitor service on the server - we have two scripts 1 - for checking service is running or not (checking.sh) 2 - start the service (start.sh)

I would like to merge two scripts into single (monitor.sh) and schedule/ cron it. how do I run second script based on first script result IF first script result is 0 need to start the service (second script need to run) ,if first script result is 1 second script no need to run and exit main script.

Answer 1

This is what exit codes are for. So, for your monitor script, we could do something like:

#!/bin/bash
# monitor.sh  -- checks if a Thing is doing its Thing
if [[ -r /var/run/myjob.pid ]]; then
    if kill -0 $( cat /var/run/myjob.pid ); then
        exit 0   # The process is alive, job is presumably running
    else
        exit 1   # Well, we had a PID file, but it was orphaned.
    fi
else
    exit 2 # no PID file, job presumably not running
fi

We use a different exit code for each state we wish to handle. Then, for our service checker:

#!/bin/bash
# check.sh -- Checks to see if Thing is Thinging and, if not, start it
if ! /path/to/monitor.sh; then
    /path/to/start.sh
fi

And now, the script that runs the job:

#!/bin/bash
# start.sh - do a Thing
if [[ -r /var/run/myjob.pid ]]; then
    echo "A Thing is already being done!" 1>&2
    exit 1 
else
    echo $$ > /var/run/myjob.pid
    trap 'rm /var/run/myjob.pid' EXIT
    do_Thing_related_things
fi