Check script double execution [duplicate] - linux

Question

This question already has answers here:
Quick-and-dirty way to ensure only one instance of a shell script is running at a time
(43 answers)
Closed 4 years ago.
I have a bash script and sometimes happened that, even my script was scheduled, it was executed two times. So I added few code lines to check if the script is already in execution. Initially I hadn't a problem, but in the last three days I had got it again
PID=`echo $$`
PROCESS=${SL_ROOT_FOLDER}/xxx/xxx/xxx_xxx_$PID.txt
ps auxww | grep $scriptToVerify | grep -v $$ | grep -v grep > $PROCESS
num_proc=`awk -v RS='\n' 'END{print NR}' $PROCESS`
if [ $num_proc -gt 1 ];
then
sl_log "---------------------------Warning---------------------------"
sl_log "$scriptToVerify already executed"
sl_log "num proc $num_proc"
sl_log "--------"
sl_log $PROCESS
sl_log "--------"
exit 0;
fi
In this way I check how many rows I've got into my log and if the result is more than one, then I have two process in execution and one will be stopped.
This method doesn't work correctly, though. How can I fix my code to check how many scripts in execution I have?

Answer 1

Anything that involves:
read some state information
check results
do action based on results
finish
must do all three steps at the same time otherwise there is a "race" condition. For example:
(A) reads state
(A) checks results (ok)
(A) does action (ok)
(A) finishes
(B) reads state
(B) checks results (bad)
(B) does action (bad)
(B) finishes
but if timing is slightly different:
(A) reads state
(A) checks results (ok)
(B) reads state
(B) checks results (ok)
(A) does action (ok)
(B) does action (ok)
(A) finishes
(B) finishes
The usual example people give is updating bank balances.
Using your method, you may be able to reduce the frequency of your code running when it shouldn't but you can never avoid it entirely.
A better solution is to use locking. This guarantees that only one process can run at a time. For example, using flock, you can wrap all calls to your script:
flock -x /var/lock/myscript-lockfile myscript
Or, inside your script, you can do something like:
exec 300>/var/lock/myscript-lockfile
flock -x 300
# rest of script
flock -u 300
or:
exec 300>/var/logk/myscript-lockfile
if ! flock -nx 300; then
# another process is running
exit 1
fi
# rest of script
flock -u 300

Answer 2

flock(1)
#!/bin/bash
# Makes sure we exit if flock fails.
set -e
(
# Wait for lock on /var/lock/.myscript.exclusivelock (fd 200) for 10 seconds
flock -x -w 10 200
# Do stuff
) 200>/var/lock/.myscript.exclusivelock
This ensures that code between "(" and ")" is run only by one process at a time and that the process does wait for a lock too long.
Credit goes to Alex B.