How to use flock on Linux

Question

I am looking at the flock docs:

http://www.tutorialspoint.com/unix_system_calls/flock.htm

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

I have this:

#!/usr/bin/env bash

temp_dir="$HOME/temperton/tmp";
mkdir -p "$temp_dir"

(

  flock -x "$temp_dir/a"
  echo '111'

) &

(

  flock -x "$temp_dir/a"
  echo '222'

) &

but when I run it I get this error:

flock: bad file descriptor: '/home/ratt/temperton/tmp/a'
flock: 111
bad file descriptor: '/home/ratt/temperton/tmp/a'
222

user_11167950 · Answer

The 'flock' is the most useful then operates with a file descriptor. You should also provide a command to execute if locking on on file/dir-name. I guess, internally it works as the following:

    exec 4<$name # open
    flock 4
    
    exec 4<&-    # close

In your example you can replace locks with

    flock $temp_dir sleep 5s

to check the difference.

How to use flock on Linux

1 Answers