flock — Manage locks from shell scripts
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 ... ) 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, −−sharedObtain a shared lock, sometimes called a read lock.
−x, −e, −−exclusiveObtain an exclusive lock, sometimes called a write lock. This is the default.
−u, −−unlockDrop 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, −−nonblockFail (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 seconds.
Decimal fractional values are allowed.
−o, −−closeClose 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 commandPass a single command to the shell
with −c.
−h, −−helpPrint a help message.
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.
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