Check if a script is already running by saving its PID.

Rupadhya · 10-05-2012, 01:15 PM

Somebody might have a more elegant solution, but I thought I would share what I came up with. I made a script that saves its PID in /var/run and then if another instance of the script is executed, it exits without running. Check this out. I welcome any comments or suggestions.
- Raj Upadhyaya

Code:

#! /bin/bash
# back.sh - Backup Script.
#save the pid of this run
checkPID() {
while read a 
do 
echo "Checking for process: $a" 
for (( ; ; ))
do
if (ps -ef | awk '{ print $2 " " $3 }'|grep -q $a)
then
      echo "Found the back.sh process, exiting the script"
      exit 1
else
      break
fi
done
done < /var/run/back.pid
}
#
### Main program
#
if [ -f /var/run/back.pid ] 
then
   # Check if PID is still running, otherwise ignore it.
   checkPID
fi
echo $BASHPID > /var/run/back.pid

unSpawn · 10-06-2012, 06:40 AM

Unless I misunderstand this saving a Process Id can be done (should be done) inside the first-running script filling a variable with "$!" right after command execution and writing out the value to a PID file. Then any subsequent process can just check for existence of the PID file and if necessary, restart for instance, check its value with the PID of the current process. Note checking the PID of the current process may be found with 'pidof' if available (or compatible), or 'pgrep processname', or 'pgrep -f "/path/to/process --any-args" if you need to select from say several running processes doing away with any 'ps|grep|grep -v grep'-like gremlins.

Code:

function howto() { echo "Here's some Bash scripting guide help:
http://www.tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html 
http://www.tldp.org/LDP/Bash-Beginners-Guide/html/index.html 
http://www.gnu.org/software/bash/manual/html_node/index.html
http://www.grymoire.com/Unix/Sh.html
http://www.tldp.org/LDP/abs/html/ 
http://mywiki.wooledge.org/BashFAQ
http://mywiki.wooledge.org/BashPitfalls"; }

konsolebox · 10-06-2012, 08:45 AM

You could check if a process is running with 'kill -s 0' or by checking if directory /proc/$PID exists.

Code:

function check_process {
	local PID=$1
	[[ -e /proc/$PID ]] && kill -s 0 "$PID" # could be not both; best arrangement depends on system
}

* Wonders if any tutorial has this; I discovered it by my own * ... * Browsed and looks like there were some *