Run a shell script whenever a file in a certain directory changes? (lemmy.ml)

submitted 2 weeks ago* (last edited 2 weeks ago) by helloworld@lemmy.ml to c/linuxquestions@lemmy.zip

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Using a shell script, can I watch a folder and block program execution until a file in a certain folder changes?

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[-] running_ragged@lemmy.world -2 points 2 weeks ago

Can continuously loop over the file, examine the md5 hash for changes.

Run the script if it has changed.

https://stackoverflow.com/questions/6475252/bash-script-watch-folder-execute-command

daemon() {
chsum1=""

while [[ true ]]
do
    chsum2=`find src/ -type f -exec md5 {} \;`
    if [[ $chsum1 != $chsum2 ]] ; then           
        if [ -n "$chsum1" ]; then
            compile
        fi
        chsum1=$chsum2
    fi
    sleep 2
done
}

[-] hperrin@lemmy.ca 14 points 2 weeks ago

Oh god please don’t do this. Constantly reading the file is just stressing your IO for no reason.

Please inotify instead:

https://linux.die.net/man/1/inotifywait

[-] chonkyninja@lemmy.world 4 points 2 weeks ago

The fuck…

[-] testfactor@lemmy.world 2 points 2 weeks ago

Even if you wanted to implement a solution like this, which you shouldn't, why on earth monitor the MD5 sum instead of just the mtime of the file???? Like, doing a checksum is the least efficient method of checking this possible.

Like, you could do a simple while loop with a find myfile.txt +mmin 1; sleep 30 in it. Adjust numbers to your desired tolerance.

Again, don't do that. But if you must, definitely don't do an md5sum for godssake.

[-] helloworld@lemmy.ml -1 points 2 weeks ago

I really like this, replace compile with whatever command you desire I guess.

[-] hperrin@lemmy.ca 9 points 2 weeks ago

This is a terrible solution. You will stress your IO for no reason.

[-] helloworld@lemmy.ml 0 points 2 weeks ago* (last edited 2 weeks ago)

On the upside, you do not need to install the inotifywait package. md5sum already installed on my system haha

[-] hperrin@lemmy.ca 11 points 2 weeks ago

If you are a big fan of wasting disk performance, CPU cycles, and ultimately power.

[-] nublug@lemmy.blahaj.zone 2 points 2 weeks ago

why is that a plus

[-] helloworld@lemmy.ml 0 points 2 weeks ago

I do not need to install anything/can work on bare install without internet connection?

[-] possiblylinux127@lemmy.zip 1 points 2 weeks ago* (last edited 2 weeks ago)

You should be able to tie into the kernel with some C programming if you want to go extra small.

[-] nublug@lemmy.blahaj.zone 1 points 2 weeks ago

ah, fair.

[-] possiblylinux127@lemmy.zip 2 points 2 weeks ago

It isn't a terrible solution if you are checking infrequently just as ever 30 minutes.

this post was submitted on 25 Aug 2025

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