Bash – Is flock & exec safe in bash

bashlock

The "standard" locking snippet I've seen goes something like…

(
    flock -n 200 || exit 1;
    # do stuff
) 200>program.lock

Is it safe (testing seems to say so) to use exec at that point? Will the subprocess retain the lock?

(
    flock -n 200 || exit 1;
    exec /usr/bin/python vendors-notcoolstuff.py
) 200>program.lock

I vaguely remember exec'd processes retain open file descriptors and since flock uses file descriptors it should work. But I cannot find any documentation that makes that definitive and clear.

For the record, this is specific to Linux.

Best Answer

Yes, locks are preserved across exec. Locks are preserved across the underlying system call execve, as long as the file descriptor remains open. File descriptors remain open across execve unless they have been configured to be closed on exec, and file descriptors created by shell redirection are not marked as close-on-exec.

Related Solutions

Monitoring file locks, locked using flock

From man lsof:

FD is the File Descriptor number of the file or: FD is followed by one of these characters, describing the mode under which the file is open:

          The mode character is followed by one of these lock characters, describing the type of lock applied to the file:

               R for a read lock on the entire file;
               W for a write lock on the entire file;
               space if there is no lock.

So R in 3uR mean that read/shared lock is issued by 613 PID.

#lsof /tmp/file
COMMAND PID    USER   FD   TYPE DEVICE SIZE/OFF    NODE NAME
perl    613 turkish    3uR  REG    8,2        0 1306357 /tmp/file

Reading directly from /proc/locks is faster than lsof,

perl -F'[:\s]+' -wlanE'
  BEGIN { $inode = (stat(pop))[1]; @ARGV = "/proc/locks" }
  say "pid:$F[4] [$_]" if $F[7] == $inode
' /tmp/file

Shell – Pass multiple commands to flock

Invoke a shell explicitly.

flock -x -w 5 ~/counter.txt sh -c 'COUNTER=$(cat counter.txt); echo $((COUNTER + 1)) > ~/counter.txt'

Note that any variable that you change is local to that shell instance. For example, the COUNTER variable will not be updated in the calling script: you'll have to read it back from the file (but it may have changed in the meantime), or as the output of the command:

new_counter=$(flock -x -w 5 ~/counter.txt sh -c 'COUNTER=$(cat counter.txt); echo $((COUNTER + 1)) | tee ~/counter.txt')

Best Answer

Related Solutions

Monitoring file locks, locked using flock

Shell – Pass multiple commands to flock

Related Question