Show HN: Linux CLI tool to provide mutex locks for long running bash ops | Hacker News Reader

Show HN: Linux CLI tool to provide mutex locks for long running bash ops

github.com

34 pointsbigattichouse20 comments

Been exploring claude and spec-based coding, I think it turned out fairly successful. It's just a simple unix-style tool that gives you a single command to use in bash scripts to simplify mutex or semaphore locking of execution.

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Why not https://man7.org/linux/man-pages/man2/flock.2.html?

Nit: Probably https://man7.org/linux/man-pages/man1/flock.1.html (shell command, not the underlying libc function)

This was my first thought and I suppose flock(1) could be used to recreate a lot of this. But it does come with some other quality-of-life improvements like being able to list all currently-used locks, having a lock holdable by N processes etc.

Because that's a syscall ;) https://man7.org/linux/man-pages/man1/flock.1.html is the command line manual.

I would say one good reason is that

  waitlock myapp &
  JOB_PID=$!
  # ... do exclusive work ...
  kill $JOB_PID

is a lot easier to use and remember than

  (; flock -n 9 || exit 1; # ... commands executed under lock ...; ) 9>/var/lock/mylockfile

Why

  (; flock -n 9

and not

  ( flock -n 9

?

It's a "for" loop.

Could you elaborate?

A for loop in a shell script may sometimes look like this:

`for ((i = 0 ; i < max ; i++ )); do echo "$i"; done`

Here this is essentially a "while" loop, meaning it will keep executing the commands as long as we don't reach `exit 1`.

(; flock -n 9 || exit 1; # ... commands executed under lock ...; )

It doesn't seem to work?

  [~] 0 $ ( flock -n 9 || exit 1; echo in loop ; sleep 3 ; echo done working ; ) 9>~/tmp/mylock
  in loop
  done working
  [~] 0 $ (; flock -n 9 || exit 1; echo in loop ; sleep 3 ; echo done working ; ) 9>~/tmp/mylock
  -bash: syntax error near unexpected token `;'
  [~] 2 $

(This is bash)

Flock can be used in a single line for example for cronjobs.

Flock -s file && script.

Pretty simple. (I forgot the argument, I think is -s..

just pushed a change so now it's:

waitlock myapp & #... do stuff waitlock --done myapp

flock is indeed built-in: `flock -xn /tmp/mylock.lock -c "echo running locked command"` does mutex locking in bash. Your tool might offer better ergonomics or features beyond flock's capabilities?

We already have lockfile: <https://manpages.debian.org/stable/procmail/lockfile.1.en.ht...>.

I don’t know the exact threshold at which you should use a real programming language instead of a bash script. But this type of work definitely exceeds it.

While in general I'd agree, this isn't necessarily just for bash scripts. It could just wrap the execution of another program allowing higher-level logic to handle concurrency and the low-level program to do it's one-at-a-time job

Sometimes you have a cron job that takes longer than it should (but inconsistently so), and another cron job that clobbers what that cron job is doing.

Useful project, I love all things terminal, so I also enjoyed your project.

You enjoyed the project because you love the terminal?

Good enough for me. I created the project because I love terminal, and wanted to make something using Claude (to learn how this tool works, strictly for personal enrichment) that solves a small problem I had with some overlapping cron job management.

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