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Tags: bash linux locking flocker flock atomic unix.stackexchange.com

Clipped on: 2020-05-17

A solution that does not require additional tools would be prefered.

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asked Sep 18 '12 at 11:04

Almost like nsg's answer: use a lock directory. Directory creation is atomic under linux and unix and *BSD and a lot of other OSes.

if mkdir $LOCKDIR
then
    # Do important, exclusive stuff
    if rmdir $LOCKDIR
    then
        echo "Victory is mine"
    else
        echo "Could not remove lock dir" >&2
    fi
else
    # Handle error condition
    ...
fi

You can put the PID of the locking sh into a file in the lock directory for debugging purposes, but don't fall into the trap of thinking you can check that PID to see if the locking process still executes. Lots of race conditions lie down that path.

edited Feb 7 '17 at 20:33

To add to Bruce Ediger's answer, and inspired by this answer, you should also add more smarts to the cleanup to guard against script termination:

#Remove the lock directory
function cleanup {
    if rmdir $LOCKDIR; then
        echo "Finished"
    else
        echo "Failed to remove lock directory '$LOCKDIR'"
        exit 1
    fi
}

if mkdir $LOCKDIR; then
    #Ensure that if we "grabbed a lock", we release it
    #Works for SIGTERM and SIGINT(Ctrl-C)
    trap "cleanup" EXIT

    echo "Acquired lock, running"

    # Processing starts here
else
    echo "Could not create lock directory '$LOCKDIR'"
    exit 1
fi

edited May 23 '17 at 11:33

add a comment

This may be too simplistic, please correct me if I'm wrong. Isn't a simple ps enough?

#!/bin/bash 

me="$(basename "$0")";
running=$(ps h -C "$me" | grep -wv $$ | wc -l);
[[ $running > 1 ]] && exit;

# do stuff below this comment

edited Feb 22 '18 at 12:37

answered Sep 18 '12 at 12:19

I've used this condition for a week, and in 2 occasions it didn't prevent new process from starting. I figured what the problem is - new pid is a substring of the old one and gets hidden by grep -v $$. real examples: old - 14532, new - 1453, old - 28858, new - 858. – Naktibalda Feb 22 '18 at 11:30

I fixed it by changing grep -v $$ to grep -v "^${$} " – Naktibalda Feb 22 '18 at 11:52

@Naktibalda good catch, thanks! You could also fix it with grep -wv "^$$" (see edit). – terdon♦ Feb 22 '18 at 12:38

With this solution, if two instances of the same script are started at the same time, there's a chance that they will "see" each others and both will terminate. It may not be a problem, but it also may be, just be aware of it. – flagg19 Jan 25 at 11:19

show 2 more comments

One other way to make sure a single instance of bash script runs:

#!/bin/bash

# Check if another instance of script is running
pidof -o %PPID -x $0 >/dev/null && echo "ERROR: Script $0 already running" && exit 1

...

pidof -o %PPID -x $0 gets the PID of the existing script if its already running or exits with error code 1 if no other script is running

edited Oct 17 '17 at 20:56

answered Oct 17 '17 at 20:25

Although you've asked for a solution without additional tools, this is my favourite way using flock:

#!/bin/sh

[ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0" "$@" || :

echo "servus!"
sleep 10

This comes from the examples section of man flock, which further explains:

This is useful boilerplate code for shell scripts. Put it at the top of the shell script you want to lock and it'll automatically lock itself on the first run. If the env var $FLOCKER is not set to the shell script that is being run, then execute flock and grab an exclusive non-blocking lock (using the script itself as the lock file) before re-execing itself with the right arguments. It also sets the FLOCKER env var to the right value so it doesn't run again.

Points to consider:

Requires flock, the example script terminates with an error if it can't be found
Needs no extra lock file
May not work if the script is on NFS (see https://serverfault.com/questions/66919/file-locks-on-an-nfs)

edited Jan 27 '19 at 13:03

answered Feb 7 '17 at 20:08

I would use a lock file, as mentioned by Marco

#!/bin/bash

# Exit if /tmp/lock.file exists
[ -f /tmp/lock.file ] && exit

# Create lock file, sleep 1 sec and verify lock
echo $$ > /tmp/lock.file
sleep 1
[ "x$(cat /tmp/lock.file)" == "x"$$ ] || exit

# Do stuff
sleep 60

# Remove lock file
rm /tmp/lock.file

edited Sep 18 '12 at 11:51

answered Sep 18 '12 at 11:18

I have never used lsof for this purpose, I this it should work. Note that lsof is really slow in my system (1-2 sec) and most likely there is a lot of time for race conditions. – nsg Sep 18 '12 at 11:45

show 4 more comments

If you want to make sure that only one instance of your script is running take a look at:

Lock your script (against parallel run)

Otherwise you can check ps or invoke lsof <full-path-of-your-script>, since i wouldn't call them additional tools.

Supplement:

actually i thought of doing it like this:

for LINE in `lsof -c <your_script> -F p`; do 
    if [ $$ -gt ${LINE#?} ] ; then
        echo "'$0' is already running" 1>&2
        exit 1;
    fi
done

this ensures that only the process with the lowest pid keeps on running even if you fork-and-exec several instances of <your_script> simultaneously.

edited Sep 18 '12 at 12:56

answered Sep 18 '12 at 11:17

add a comment

This is a modified version of Anselmo's Answer. The idea is to create a read only file descriptor using the bash script itself and use flock to handle the lock.

SCRIPT=`realpath $0`     # get absolute path to the script itself
exec 6< "$SCRIPT"        # open bash script using file descriptor 6
flock -n 6 || { echo "ERROR: script is already running" && exit 1; }   # lock file descriptor 6 OR show error message if script is already running

echo "Run your single instance code here"

The main difference to all other answer's is that this code doesn't modify the filesystem, uses a very low footprint and doesn't need any cleanup since the file descriptor is closed as soon as the script finishes independent of the exit state. Thus it doesn't matter if the script fails or succeeds.

edited Nov 2 '18 at 6:37

answered Nov 2 '18 at 5:32

I am using cksum to check my script is truly running single instance, even I change filename & file path.

I am not using trap & lock file, because if my server suddenly down, I need to remove manually lock file after server goes up.

Note: #!/bin/bash in first line is required for grep ps

#!/bin/bash

checkinstance(){
   nprog=0
   mysum=$(cksum $0|awk '{print $1}')
   for i in `ps -ef |grep /bin/bash|awk '{print $2}'`;do 
        proc=$(ls -lha /proc/$i/exe 2> /dev/null|grep bash) 
        if [[ $? -eq 0 ]];then 
           cmd=$(strings /proc/$i/cmdline|grep -v bash)
                if [[ $? -eq 0 ]];then 
                   fsum=$(cksum /proc/$i/cwd/$cmd|awk '{print $1}')
                   if [[ $mysum -eq $fsum ]];then
                        nprog=$(($nprog+1))
                   fi
                fi
        fi
   done

   if [[ $nprog -gt 1 ]];then
        echo $0 is already running.
        exit
   fi
}

checkinstance 

#--- run your script bellow 

echo pass
while true;do sleep 1000;done

Or you can hardcoded cksum inside your script, so you no worry again if you want to change filename, path, or content of your script.

#!/bin/bash

mysum=1174212411

checkinstance(){
   nprog=0
   for i in `ps -ef |grep /bin/bash|awk '{print $2}'`;do 
        proc=$(ls -lha /proc/$i/exe 2> /dev/null|grep bash) 
        if [[ $? -eq 0 ]];then 
           cmd=$(strings /proc/$i/cmdline|grep -v bash)
                if [[ $? -eq 0 ]];then 
                   fsum=$(grep mysum /proc/$i/cwd/$cmd|head -1|awk -F= '{print $2}')
                   if [[ $mysum -eq $fsum ]];then
                        nprog=$(($nprog+1))
                   fi
                fi
        fi
   done

   if [[ $nprog -gt 1 ]];then
        echo $0 is already running.
        exit
   fi
}

checkinstance

#--- run your script bellow

echo pass
while true;do sleep 1000;done

edited May 24 '19 at 0:06

answered May 23 '19 at 23:54

My code to you

#!/bin/bash

script_file="$(/bin/readlink -f $0)"
lock_file=${script_file////_}

function executing {
  echo "'${script_file}' already executing"
  exit 1
}

(
  flock -n 9 || executing

  sleep 10

) 9> /var/lock/${lock_file}

Based on man flock, improving only:

the name of the lock file, to be based on the full name of the script
the message executing

Where I put here the sleep 10, you can put all the main script.

answered Sep 3 '18 at 20:38

You can use this: https://github.com/sayanarijit/pidlock

sudo pip install -U pidlock

pidlock -n sleepy_script -c 'sleep 10'

answered Jan 12 '18 at 18:32

This handy package does what you're looking for.

https://github.com/krezreb/singleton

once installed, just prefix your command with singleton LOCKNAME

e.g. singleton LOCKNAME PROGRAM ARGS...

answered Feb 19 at 18:07

Easiest one liner script rather than writing complex PID or lock statements

flock -xn LOCKFILE.lck -c SCRIPT.SH

where x denotes exclusive lock and n denotes non-blocking which fails rather than wait for the lock to be released. c runs the script that you want to launch.