Check if program is already running with bash

Small tip of the day: Sometimes you need to run a program via cron or command line, but the important is that the program should not run if another istance it’s already active.

Or perhaps you have a process that every X minutes or hours crash and you need a wrapper that check for it’s existence and if not running restart it.

The fastest way to achieve this that i’ve found is :

if [ -z "$(pgrep foo)" ]
  then
     # foo is not running
  else
     # foo is running
fi

pgrep looks through the currently running processes and lists the process IDs which matches the selection criteria to stdout. The nice thing is that pgrep take a lot of options, for example

-u Only match processes whose effective user ID is listed. Either the numerical or symbolical value may be used

-f The pattern is normally only matched against the process name. When -f is set, the full command line is used.

So for example you can use:

#!/bin/bash
 
if [ -z "$(pgrep -u mysql -f -- -port=3316 )" ]
  then
	/etc/init.d/mysqld start
  else
	echo "Process already running" >> /tmp/mysql.log
fi

To check if mysql is already running and if not start it.

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