NAME¶
flock - manage locks from shell scripts
SYNOPSIS¶
flock [options]
file|
directory command [
arguments]
flock [options]
file|
directory -c command
flock [options]
number
DESCRIPTION¶
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.
OPTIONS¶
- -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 rather than wait if the lock cannot be immediately acquired. See the
-E option for the exit code used.
- -w, --wait, --timeout seconds
- Fail if the lock cannot be acquired within seconds. Decimal
fractional values are allowed. See the -E option for the exit code
used. The zero number of seconds is interpreted as
--nonblock.
- -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.
- -E, --conflict-exit-code number
- The exit code 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.
- -c, --command command
- Pass a single command, without arguments, to the shell with
-c.
- -h, --help
- Display help text and exit.
- -V, --version
- Display version information and exit.
EXAMPLES¶
- 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 $? Set shared lock to
directory /tmp and the second command will not fail. Notice that
attempting to get exclusive lock with second command would fail.
- shell> flock -x local-lock-file echo 'a b c'
- Grab the exclusive lock "local-lock-file" before running echo
with 'a b c'.
- (
-
flock -n 9 || exit 1
# ... commands executed under lock ... ) 9>/var/lock/mylockfile The form
is convenient inside shell scripts. The mode used to open the file doesn't
matter to flock; using > or >> allows the
lockfile to be created if it does not already exist, however, write
permission is required. Using < requires that the file already
exists but only read permission is required.
- [ "${FLOCKER}" != "$0" ] && exec env
FLOCKER="$0" flock -en "$0" "$0"
"$@" || :
- 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.
EXIT STATUS¶
The command uses
sysexits.h return values for everything, except when
using either of the options
-n or
-w which report a failure to
acquire the lock with a return value given by the
-E option, or 1 by
default.
When using the
command variant, and executing the child worked, then the
exit status is that of the child command.
AUTHOR¶
H. Peter Anvin
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.
SEE ALSO¶
flock(2)
AVAILABILITY¶
The flock command is part of the util-linux package and is available from
Linux
Kernel Archive