Run cron job only if it isn t already running

Question

I m trying to set up a cron job as a sort of watchdog for a daemon that I ve created  If the daemon errors out and fails  I want the cron job to periodically restart it    I m not sure how possible this is  but I read through a couple of cron tutorials and couldn t find anything that would do what I m looking for    My daemon gets started from a shell script  so I m really just looking for a way to run a cron job ONLY if the previous run of that job isn t still running  I found this post  which did provide a solution for what I m trying to do using lock files  not I m not sure if there is a better way to do it

User · Answer

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

User · Answer

You can also do it as a one-liner directly in your crontab                ps -ef grep -v grep grep  lt command gt   -eq 0    amp  amp   lt command gt

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

With lockrun you don t need to write a wrapper script for your cron job   http   www unixwiz net tools lockrun html

User · Answer

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    amp  amp        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

User · Answer

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   lt  php      cmd    argv 1        grep    ps -ef   grep     cmd          exec  grep  out       if count  out  lt 5            cmd       gt   dev null 2 gt  dev null  amp            exec  cmd  out           print r  out           gt    Then in your cron add following            php  home mypath forever php  php  home mypath example php

User · Answer

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

User · Answer

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

User · Answer

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  gt  gt   home user bin spooler log  amp           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

User · Answer

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      amp  amp   ps -u   whoami  -opid                               grep -P    s   cat   PIDFILE      amp  gt   dev null   then   echo  Already running     exit 99 fi   path to myprogram  gt   HOME tmp myprogram log  amp   echo     gt     PIDFILE   chmod 644    PIDFILE

User · Answer

This one never failed me   one sh   LFILE  tmp one- echo        md5sum   cut -d   -f1  pid if   -e   LFILE     amp  amp  kill -0  cat   LFILE    then    exit fi  trap  rm -f   LFILE   exit  INT TERM EXIT echo     gt    LFILE       rm -f   LFILE    cron job              path to one sh  lt command gt

User · Answer

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      amp  amp   ps -p   cat   PIDFILE    gt   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

User · Answer

The way I am doing it when I am running php scripts is   The crontab             php  path to php script php  amp    The php code    lt  php if  shell exec  ps aux   grep       FILE         wc  -l    gt  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

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