flock [-sxun] [-w timeout] fd

This utility manages (2) locks from within shell scripts or the command line.

The first form wraps the lock around the executing a command, in a manner similar to su(1) or newgrp(1). It locks a specified file, which is created (assuming appropriate permissions), if it does not already exist.

The second form is conveninent inside shell scripts, and is usually used the following manner:

( flock -s 200 # ... commands executed under lock ... ) 200>/var/lock/mylockfile

The mode used to open the file doesn't matter to flock; using > or >> allows the lockfile to be created if it does not already exist, however, write permission is required; using < requires that the file already exists but only read permission is required.

By default, if the lock cannot be immediately acquired, flock waits until the lock is available.

-s, --shared

Obtain a shared lock, sometimes called a read lock.

-x, -e, --exclusive

Obtain an exclusive lock, sometimes called a write lock. This is the default.

-u, --unlock

Drop a lock. This is usually not required, since a lock is automatically dropped when the file is closed. However, it may be required in special cases, for example if the enclosed command group may have forked a background process which should not be holding the lock.

-n, --nb, --nonblock

Fail (with an exit code of 1) rather than wait if the lock cannot be immediately acquired.

-w, --wait, --timeout seconds

Fail (with an exit code of 1) if the lock cannot be acquired within seconds seconds. Decimal fractional values are allowed.

-o, --close

Close the file descriptor on which the lock is held before executing command. This is useful if command spawns a child process which should not be hold ing the lock.

-c, --command command

Pass a single command to the shell with -c.

-h, --help

Print a help message.