flock — manage locks from shell scripts
flock
[options]
<file> −c
<command>
flock
[options]
<directory> −c
<command>
flock
[options] <file
descriptor number>
This utility manages flock(2) locks from within shell scripts or the command line.
The first and second forms wrap the lock around the executing a command, in a manner similar to su(1) or newgrp(1). It locks 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 open file by file descriptor number. See examples how that can be used.
−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
secondsFail (with an exit code of 1) if the lock cannot be acquired within 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 holding the lock.
−c
, −−command
commandPass a single command, without
arguments, to the shell with −c
.
−h
, −−help
Print a help message.
−V,
−−version
Show version number and exit.
shell1> flock /tmp -c cat shell2> flock -w .007 /tmp -c echo; /bin/echo $?
Set exclusive lock to directory /tmp and the second command will fail.
shell1> flock -s /tmp -c cat shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?
Set shared lock to directory /tmp and the second command will not fail. Notice that attempting to get exclusive lock with second command would fail.
( flock -n 9 || exit 1 # ... commands executed under lock ... ) 9>/var/lock/mylockfile
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.
The command uses sysexits.h
return values for everything else but an options −n
or −w
failures which return 1.
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.
The flock command is part of the util-linux package and is available from Linux Kernel Archive
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