flock [-sxun] [-w timeout] fd
This utility manages flock(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 ...
The mode used to open the file doesnt 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.
|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.|
|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.|
|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.|