Alain Kelder is a Giant Dork

Make a Bash script quit if it detects multiple running instances of itself

Let's say I've got a script called by cron at regular intervals and I want it to exit if it detects that a previously called instance is still running. Placing the following at the top is one way to do it:

#!/bin/bash

if [ "$(pgrep -x $(basename $0))" != "$$" ]; then
 echo "Error: another instance of $(basename $0) is already running"
 exit 1
fi

The way this works is:

• Bash built-in "$$" returns the PID of its process
• Bash built-in "$0" returns the name of its process (e.g. /usr/local/bin/script.sh)
• Command "basename $0" discards the path (e.g. /usr/local/bin/script.sh becomes script.sh)
• Command "pgrep -x $(basename $0)" searches for PIDs of current script
• When more than a single instance of the script is running, multiple PIDs will be returned by "pgrep", no longer matching a single PID returned by "$$".

UPDATE 2012-11-22: Improved code to make a Bash script quit if it detects multiple running instances of itself