This utility manages *flock*(2) locks from within shell scripts or from the command line.
The first and second of the above forms wrap the lock around the execution of a _command_, in a manner similar to *su*(1) or *newgrp*(1). They lock a specified _file_ or _directory_, which is created (assuming appropriate permissions) if it does not already exist. By default, if the lock cannot be immediately acquired, *flock* waits until the lock is available.
The third form uses an open file by its file descriptor _number_. See the examples below for how that can be used.
The exit status used when the *-n* option is in use, and the conflicting lock exists, or the *-w* option is in use, and the timeout is reached. The default value is *1*. The _number_ has to be in the range of 0 to 255.
Do not fork before executing _command_. Upon execution the flock process is replaced by _command_ which continues to hold the lock. This option is incompatible with *--close* as there would otherwise be nothing left to hold the lock.
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 holding the lock.
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.
Fail if the lock cannot be acquired within _seconds_. Decimal fractional values are allowed. See the *-E* option for the exit status used. The zero number of _seconds_ is interpreted as *--nonblock*.
The command uses <sysexits.h> exit status values for everything, except when using either of the options *-n* or *-w* which report a failure to acquire the lock with an exit status given by the *-E* option, or 1 by default. The exit status given by *-E* has to be in the range of 0 to 255.
When using the _command_ variant, and executing the child worked, then the exit status is that of the child command.
== EXAMPLES
Note that "shell> " in examples is a command line prompt.
The form is convenient inside shell scripts. 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.
This is useful boilerplate code for shell scripts. Put it at the top of the shell script you want to lock and it'll automatically lock itself on the first run. If the env var *$FLOCKER* is not set to the shell script that is being run, then execute *flock* and grab an exclusive non-blocking lock (using the script itself as the lock file) before re-execing itself with the right arguments. It also sets the FLOCKER env var to the right value so it doesn't run again.
This form is convenient for locking a file without spawning a subprocess. The shell opens the lock file for reading and writing as file descriptor 4, then flock is used to lock the descriptor.
Copyright {copyright} 2003-2006 H. Peter Anvin. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.