What is the best way to ensure only one instance of a Bash script is running

Question

What is the simplest best way to ensure only one instance of a given script is running - assuming it s Bash on Linux   At the moment I m doing   ps -C script name sh  gt   dev null 2 gt  amp 1      script name sh   but it has several issues    it puts the check outside of script it doesn t let me run the same script from separate accounts - which I would like sometimes  -C checks only first 14 characters of process name   Of course  I can write my own pidfile handling  but I sense that there should be a simple way to do it

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Ubuntu Debian distros have the start-stop-daemon tool which is for the same purpose you describe  See also  etc init d skeleton to see how it is used in writing start stop scripts   -- Noah

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I d also recommend looking at chpst  part of runit     chpst -L  tmp your-lockfile loc   script name sh

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i found this in procmail package dependencies   apt install liblockfile-bin   To run  dotlockfile -l file lock   file lock will be created   To unlock  dotlockfile -u file lock  Use this to list this package files   command   dpkg-query -L liblockfile-bin

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first test example       lsof -t  0  wc -l   gt  1     amp  amp  echo  At least one of  0 is running    second test example  currsh  0 currpid    runpid   lsof -t  currsh  paste -s -d      if     runpid     currpid    then   sleep 11111111111111111 else   echo -e   nPID  runpid   currpid      At least one of    currsh   is running     n    false   exit 1 fi   explanation   lsof -t  to list all pids of current running scripts named   0    Command  lsof  will do two advantages    Ignore pids which is editing by editor such as vim  because vim edit its mapping file such as   file swp   Ignore pids forked by current running shell scripts  which most  grep  derivative command can t achieve it  Use  pstree -pH pidnum  command to see details about current process forking status

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from with your script   ps -ef   grep  0   grep   whoami

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One line ultimate solution        pgrep -fn  0   -ne    pgrep -fo  0      amp  amp  echo  At least 2 copies of  0 are running

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I think flock is probably the easiest  and most memorable  variant   I use it in a cron job to auto-encode dvds and cds    try to run a command  but fail immediately if it s already running flock -n  var lock myjob lock   my bash command   Use -w for timeouts or leave out options to wait until the lock is released   Finally  the man page shows a nice example for multiple commands             flock -n 9    exit 1            commands executed under lock          9 gt  var lock mylockfile

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Advisory locking has been used for ages and it can be used in bash scripts  I prefer simple flock  from util-linux -ng   over lockfile  from procmail   And always remember about a trap on exit  sigspec    EXIT or 0  trapping specific signals is superfluous  in those scripts   In 2009 I released my lockable script boilerplate  originally available at my wiki page  nowadays available as gist   Transforming that into one-instance-per-user is trivial  Using it you can also easily write scripts for other scenarios requiring some locking or synchronization   Here is the mentioned boilerplate for your convenience      bin bash   SPDX-License-Identifier  MIT     Copyright  C  2009 Przemyslaw Pawelczyk  lt przemoc gmail com gt        This script is licensed under the terms of the MIT license     https   opensource org licenses MIT     Lockable script boilerplate      HEADER      LOCKFILE   var lock  basename  0   LOCKFD 99    PRIVATE  lock                 flock - 1  LOCKFD     no more locking       lock u   lock xn  amp  amp  rm -f  LOCKFILE     prepare locking      eval  exec  LOCKFD gt    LOCKFILE     trap  no more locking EXIT       ON START  prepare locking    PUBLIC exlock now             lock xn       obtain an exclusive lock immediately or fail exlock                 lock x        obtain an exclusive lock shlock                 lock s        obtain a shared lock unlock                 lock u        drop a lock      BEGIN OF SCRIPT        Simplest example is avoiding running multiple instances of script  exlock now    exit 1    Remember  Lock file is removed when one of the scripts exits and it is             the only script holding the lock or lock is not acquired at all

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I found a pretty simple way to handle  one copy of script per system   It doesn t allow me to run multiple copies of the script from many accounts though  on standard Linux that is    Solution   At the beginning of script  I gave   pidof -s -o   PPID  -x    basename  0    gt   dev null 2 gt  amp 1  amp  amp  exit   Apparently pidof works great in a way that    it doesn t have limit on program name like ps -C     it doesn t require me to do grep -v grep   or anything similar     And it doesn t rely on lockfiles  which for me is a big win  because relaying on them means you have to add handling of stale lockfiles - which is not really complicated  but if it can be avoided - why not   As for checking with  one copy of script per running user   i wrote this  but I m not overly happy with it         pidof -s -o   PPID  -x    basename  0     tr       n      ps xo pid    tr -cd   0-9 n       sort   uniq -d   and then I check its output - if it s empty - there are no copies of the script from same user

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I had the same problem  and came up with a template that uses lockfile  a pid file that holds the process id number  and a kill -0   cat  pid file  check to make aborted scripts not stop the next run  This creates a foobar- USERID folder in  tmp where the lockfile and pid file lives   You can still call the script and do other things  as long as you keep those actions in alertRunningPS      bin bash  user id num   id -u  pid file   tmp foobar- user id num foobar- user id num pid  lock file   tmp foobar- user id num running lock  ps id     function alertRunningPS          local PID   cat   pid file  2 gt   dev null      echo  Lockfile present  ps id file   PID      echo  Checking if process is actually running or something left over from crash         if kill -0  PID 2 gt   dev null  then         echo  Already running  exiting          exit 1     else         echo  Not running  removing lock and continuing          rm -f   lock file          lockfile -r 0   lock file      fi    echo  Hello  checking some stuff before locking stuff     Lock further operations to one process mkdir -p  tmp foobar- user id num lockfile -r 0   lock file     alertRunningPS    Do stuff here echo -n  ps id  gt    pid file  echo  Running stuff in ONE ps   sleep 30s  rm -f   lock file  rm -f   pid file  exit 0

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I m not sure there s any one-line robust solution  so you might end up rolling your own   Lockfiles are imperfect  but less so than using  ps   grep   grep -v  pipelines   Having said that  you might consider keeping the process control separate from your script - have a start script  Or  at least factor it out to functions held in a separate file  so you might in the caller script have     my script control ksh    Function exits if cannot start due to lockfile or prior running instance  my start me up lockfile name  trap  rm -f  lockfile name  exit  0 2 3 15   in each script that needs the control logic  The trap ensures that the lockfile gets removed when the caller exits  so you don t have to code this on each exit point in the script   Using a separate control script means that you can sanity check for edge cases  remove stale log files  verify that the lockfile is associated correctly with a currently running instance of the script  give an option to kill the running process  and so on  It also means you ve got a better chance of using grep on ps output successfully  A ps-grep can be used to verify that a lockfile has a running process associated with it  Perhaps you could name your lockfiles in some way to include information about the process  user  pid  etc   which can be used by a later script invocation to decide whether the process that created the lockfile is still around

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If the script is the same across all users  you can use a lockfile approach  If you acquire the lock  proceed else show a message and exit   As an example    Terminal  1    lockfile -r 0  tmp the lock  Terminal  1       Terminal  2    lockfile -r 0  tmp the lock  Terminal  2  lockfile  Sorry  giving up on   tmp the lock    Terminal  1    rm -f  tmp the lock  Terminal  1       Terminal  2    lockfile -r 0  tmp the lock  Terminal  2       After  tmp the lock has been acquired your script will be the only one with access to execution  When you are done  just remove the lock  In script form this might look like      bin bash  lockfile -r 0  tmp the lock    exit 1    Do stuff here  rm -f  tmp the lock

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Use the set -o noclobber option and attempt to overwrite a common file  A short example if    set -o noclobber   echo  gt   tmp global lock    then     exit 1    the global lock already exists fi        remainder of script       A longer example This example will wait for the global lock file but timeout after too long   function lockfile waithold          declare -ir time beg   date    s       declare -ir time max 7140    7140 s   1 hour 59 min          poll for lock file up to   time max s       put debugging info in lock file in case of issues         while             set -o noclobber             echo -e  quot DATE   date  nUSER   whoami  nPID    quot   gt   tmp global lock             2 gt  dev null     do         if        date    s   -   time beg    -gt   time max      then             echo  quot Error  waited too long for lock file  tmp global lock quot  1 gt  amp 2             return 1         fi         sleep 1     done       return 0       function lockfile release          rm -f  tmp global lock       if   lockfile waithold   then       exit 1  fi  trap lockfile release EXIT          remainder of script       This has reliably worked for me on an Ubuntu 16 host with multiple instances of a bash script that used the same system-wide  quot lock quot  file   This is similar to this post by  Barry Kelly which was noticed afterward

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Here s our standard bit   It can recover from the script somehow dying without cleaning up it s lockfile   It writes the process ID to the lock file if it runs normally   If it finds a lock file when it starts running  it will read the process ID from the lock file and check if that process exists   If the process does not exist it will remove the stale lock file and carry on   And only if the lock file exists AND the process is still running will it exit   And it writes a message when it exits     lock to ensure we don t get two copies of the same job script name  myscript sh  lock   var run   script name  pid  if    -e    lock       then     pid   cat   lock       if    -e  proc   pid      then         echo    script name   Process   pid  is still running  exiting           exit 1     else           Clean up previous lock file         rm -f   lock     fi fi trap  rm -f   lock   exit     INT TERM EXIT   write     PID  to the lock file echo       gt    lock

[linux] What is the best way to ensure only one instance of a Bash script is running?

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