flock - Online in the Cloud

PROGRAM:

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

-c, --command command
Pass a single command, without arguments, to the shell with -c.

-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.

-e, -x, --exclusive
Obtain an exclusive lock, sometimes called a write lock. This is the default.

-n, --nb, --nonblock
Fail rather than wait if the lock cannot be immediately acquired. See the -E
option for the exit code used.

-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.

-s, --shared
Obtain a shared lock, sometimes called a read lock.

-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.

-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.

--verbose
Report how long it took to acquire the lock, or why the lock could not be obtained.

-V, --version
Display version information and exit.

-h, --help
Display help text 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.

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