And this is the output (in verbose mode) of this script: bash -x /tmp/ + exec google_ad_height = 90; The syntax coloring in “vim” does not work properly. google_ad_width = 728; If you say that my implementation doesn’t work in Bash (I don’t know about “ksh”), please support your claim with a proof-of-concept example. set -e July 31, 2013 by Ivan Zahariev 13 Comments. flock(2) it’s used to apply advisory locks to open files. Sorry, your blog cannot share posts by email. The solution was almost correct. This is a kernel function which can be used with a tool called flock (yes, the same name as the kernel function). e.g. # your commands here 2. Zsh has zsystem flock call, but is there a more "shellish" method that's also available to Bash? sleep 60 Flock offers desktop, mobile, and web apps so you stay connected - in the office and on the go. I didn’t know about this Bash feature. Running this via “strace” shows the following system calls: This is a classical and 100% proof way to obtain a file lock. This is not 100% proof since you still have a race condition at `exec {lock_fd}>/var/lock/mylockfile`. NAME flock - Manage locks from shell scripts SYNOPSIS flock [-sxon] [-w timeout] lockfile [-c] command.... flock [-sxun] [-w timeout] fd DESCRIPTION. This is the default. While flock(2) does solely act on files (actually, on file handles), the file itself need not be the resource to which access is being controlled. } + flock -n 200 Because the command is called while the flock function wrapper is active, it uses some tricks to avoid trampling on BASH variables or declaring local variables that would affect final command. 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. One thing I've learned: Linux has flock. Categories: Development, Linux | Tags: bash, flock, Linux, performance, shell script | Permalink. After that i collect in the variable $pid the PID of this process and I write it in the lock file, i sleep 60 seconds (to test what happen if the script is run a second time) and at the end I give my message to the world. Change ), You are commenting using your Facebook account. it can be used to synchronize access to resources across multiple running processes. Example: Everything after Your code is only run if the file has the exclusive lock on the file handle 200. ( Log Out /  It's integrated into Roundcube, all settings can be done using the Webmail client. In such a case, your lock file will be unlocked once your process terminates. Decimal fractional values are allowed. Post was not sent - check your email addresses! Does “ksh” support the syntax “exec {lock_fd}>/var/lock/mylockfile”? 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. Note that there must be no space after the {lock_fd}, or else the magic won’t work. Get the latest posts delivered right to your inbox, 26 Aug 2020 – flock -o /var/lock/mylockfile -c do_something || { echo “ERROR: flock() failed.” >&2; exit 1; }. I discovered flock recently and use it every time the system boots and runs many iptables rules from the script. then bash will find a spare fd and assign it to $lock_fd, Great hint! Flock works on Windows, Mac, and Linux operating system devices. 3 min read, 2 May 2020 – restarting a daemon… sadly bash doesn’t allow setting fd to CLOEXEC. /* Leader-text-image */ This lets you synchronize your Bash scripts with all your other applications written in Perl, Python, C, etc. What happens if the script crashes and the line ‘flock -u 200’ is never reached? Drop a lock. Normally “exec” in a shell script is used to turn over control of the script to some other program. You can actually skip the line “flock -u 200″ at the very end of the script, as it’s redundant. Change ), You are commenting using your Google account. This is a kernel function which can be used with a tool called flock (yes, the same name as the kernel function). //-->. flock does not use the fd from the parent shell; therefore, it must be duped flock in the command. flock -n 200 || exit 1

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