Run cron job only if it isn't already running

Use flock. It's new. It's better.

Now you don't have to write the code yourself. Check out more reasons here: https://serverfault.com/a/82863

/usr/bin/flock -n /tmp/my.lockfile /usr/local/bin/my_script

I do this for a print spooler program that I wrote, it's just a shell script:

#!/bin/sh
if ps -ef | grep -v grep | grep doctype.php ; then
        exit 0
else
        /home/user/bin/doctype.php >> /home/user/bin/spooler.log &
        #mailing program
        /home/user/bin/simplemail.php "Print spooler was not running...  Restarted." 
        exit 0
fi

It runs every two minutes and is quite effective. I have it email me with special information if for some reason the process is not running.

As others have stated, writing and checking a PID file is a good solution. Here's my bash implementation:

#!/bin/bash

mkdir -p "$HOME/tmp"
PIDFILE="$HOME/tmp/myprogram.pid"

if [ -e "${PIDFILE}" ] && (ps -u $(whoami) -opid= |
                           grep -P "^\s*$(cat ${PIDFILE})$" &> /dev/null); then
  echo "Already running."
  exit 99
fi

/path/to/myprogram > $HOME/tmp/myprogram.log &

echo $! > "${PIDFILE}"
chmod 644 "${PIDFILE}"

It's suprising that no one mentioned about run-one. I've solved my problem with this.

 apt-get install run-one

then add run-one before your crontab script

*/20 * * * * * run-one python /script/to/run/awesome.py

Check out this askubuntu SE answer. You can find link to a detailed information there as well.

Don't try to do it via cron. Have cron run a script no matter what, and then have the script decide if the program is running and start it if necessary (note you can use Ruby or Python or your favorite scripting language to do this)

You can also do it as a one-liner directly in your crontab:

* * * * * [ `ps -ef|grep -v grep|grep <command>` -eq 0 ] && <command>

The way I am doing it when I am running php scripts is:

The crontab:

* * * * * php /path/to/php/script.php &

The php code:

<?php
if (shell_exec('ps aux | grep ' . __FILE__ . ' | wc  -l') > 1) {
    exit('already running...');
}
// do stuff

This command is searching in the system process list for the current php filename if it exists the line counter (wc -l) will be greater then one because the search command itself containing the filename

so if you running php crons add the above code to the start of your php code and it will run only once.

As a follow up to Earlz answer, you need a wrapper script that creates a $PID.running file when it starts, and delete when it ends. The wrapper script calls the script you wish to run. The wrapper is necessary in case the target script fails or errors out, the pid file gets deleted..

With lockrun you don't need to write a wrapper script for your cron job. http://www.unixwiz.net/tools/lockrun.html

# one instance only (works unless your cmd has 'grep' in it)
ALREADY_RUNNING_EXIT_STATUS=0
bn=`basename $0`
proc=`ps -ef | grep -v grep | grep "$bn" | grep -v " $$ "`
[ $? -eq 0 ] && {
    pid=`echo $proc | awk '{print $2}'`
    echo "$bn already running with pid $pid"
    exit $ALREADY_RUNNING_EXIT_STATUS
}

UPDATE .. better way using flock:

/usr/bin/flock -n /tmp/your-app.lock /path/your-app args 

I would recommend to use an existing tool such as monit, it will monitor and auto restart processes. There is more information available here. It should be easily available in most distributions.

This one never failed me:

one.sh:

LFILE=/tmp/one-`echo "$@" | md5sum | cut -d\  -f1`.pid
if [ -e ${LFILE} ] && kill -0 `cat ${LFILE}`; then
   exit
fi

trap "rm -f ${LFILE}; exit" INT TERM EXIT
echo $$ > ${LFILE}

$@

rm -f ${LFILE}

cron job:

* * * * * /path/to/one.sh <command>

I'd suggest the following as an improvement to rsanden's answer (I'd post as a comment, but don't have enough reputation...):

#!/usr/bin/env bash

PIDFILE="$HOME/tmp/myprogram.pid"

if [ -e "${PIDFILE}" ] && (ps -p $(cat ${PIDFILE}) > /dev/null); then
  echo "Already running."
  exit 99
fi

/path/to/myprogram

This avoids possible false matches (and the overhead of grepping), and it suppresses output and relies only on exit status of ps.

Simple custom php is enough to achieve. No need to confuse with shell script.

lets assume you want to run php /home/mypath/example.php if not running

Then use following custom php script to do the same job.

create following /home/mypath/forever.php

<?php
    $cmd = $argv[1];
    $grep = "ps -ef | grep '".$cmd."'";
    exec($grep,$out);
    if(count($out)<5){
        $cmd .= ' > /dev/null 2>/dev/null &';
        exec($cmd,$out);
        print_r($out);
    }
?>

Then in your cron add following

* * * * * php /home/mypath/forever.php 'php /home/mypath/example.php'

Docs: https://www.timkay.com/solo/

solo is a very simple script (10 lines) that prevents a program from running more than one copy at a time. It is useful with cron to make sure that a job doesn't run before a previous one has finished.

Example

* * * * * solo -port=3801 ./job.pl blah blah

Consider using pgrep (if available) rather than ps piped through grep if you're going to go that route. Though, personally, I've got a lot of mileage out of scripts of the form

while(1){
  call script_that_must_run
  sleep 5
}

Though this can fail and cron jobs are often the best way for essential stuff. Just another alternative.