+ flock -n 200 $ yum install util-linux. By default flock will wait indefinitely to get a lock unless you use the -n (nonblock) option.Let's take a deeper look at the two important lines that make up the lock.This redirects the file descriptor 100 to the desired lock file. I use FLOCK when I need the queuing behavior and Simple Lock Files when I just want subsequent runs of the script to terminate instead of queuing. A basic lock file is when you create a file to be used as a lock. The last thing done in the script is removing the lock file.The major pro of using this method is that it is really quite straightforward to implement and works pretty darn well… as long as your script never fails before the lock file gets deleted on the last line… And that would be a big con.

This utility manages flock(2) locks from within shell scripts or from the command line.. It is recommended you use a relatively high number. So your example can be rewritten in an RC free way like this: if { set -C; 2>/dev/null >manlocktest.lock; }; thenI've no idea how this gets formatted here, but I hope the idea is clear.The solution with flock is interesting and surely more elegant. Recently a colleague of mine should run multiple parallel jobs on a server (around 20 in our specific case), every job does “an elaboration” and this can take a time longer or shorter in respect of the others, once a job finishes its task it must read from a text file the first line that indicate the ID of a new job, it removes that ID from the top of the text file and starts to elaborate it.Problem: On a long run (a run of 50K+ jobs) it happens “frequently” that 2 jobs finish at the same time and so both take the same ID causing problems to the whole process, we tried to use a simple lock file “touched” just after the job was opening the file with the list of ID, but it seems that this operation it’s too slow and we had some case of failed concurrency again.Perhaps a small example will help, this is my /tmp/hello.sh script:#!/bin/bash – Alex Dupuy Apr 7 '14 at 9:13 @tchrist, flock(1) isn't a shell builtin even on Linux, so this is obviously untrue.
The ls command is probably the first command most Linux users encounter. If the command fails midway, then the caller still calls unlock. cases, for example if the enclosed command group may have echo "Hello world"Some explanation of the interesting parts of the code:When this option is on, if a simple command fails for any of the reasons listed in Consequences of Shell Errors or returns an exit status value >0, and is not part of the compound list following a while, until, or if keyword, and is not a part of an AND or OR list, and is not a pipeline preceded by the ! An advice ? The first and second forms wraps the lock around the executing a command, in a manner similar to su(1) or newgrp(1).It locks a specified file or directory, which is created (assuming appropriate permissions), if it does not already exist. After a bit of digging around though I eventually came across FLOCK.The majority of examples around the net of Flock usage involve invoking a sub-shell within your script. Getting used to using this when appropriate should equal much more reliable/expected job/script execution with consistent results, especially for scripts that have to work with shared resources.The above ‘touch’ method is unsafe.

A six hundred line script is doing an awful lot of stuff for the duration of the lock - just a point. I will kick off the script and then send it to the background. Say what happened.

Please contact the loosy team who maintains and develops this wonderful site by clicking in the mighty feedback button on the side of the page. I used the default sub-shell method and was very confused because my script kept doing odd things.
+ set -e I will link to the pertinent man pages and some good bash scripting resources below for additional information.If you enjoyed this article please consider showing your support by following us on After the first example I was expecting a sentence like: "This is the naive approach, how you should never do it! I.e., Instance 1 is currently running, Instances 2 and 3 were just started. The flock command runs the locked commands (if passed a filename) or is passed a file descriptor number - it works!

However, we believe that this information will be easily transferable to other languages.Here is a real world example of how lock files could be used. + sleep 60 A six hundred line script is doing an awful lot of stuff for the duration of the lock - just a point. The existence of the file will be a signal to any subsequent instance of the script not to run.

Thanks! Conversely, you can use the -n (nonblock) option to instantly fail if it cannot immediately acquire a lock.Below you will see that I background the first instance of the test.sh script. rather than wait if the lock cannot be immediately It has two steps(commands) so it’s not atomic.Please do expand (if you have time to).


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