#Bash 的 flock 命令

flock [OPTIONS]

功能

管理文件的锁。

类型

可执行文件（/usr/bin/flock），属于 util-linux。

参数

• OPTIONS 选项：

  • -c, --command command 通过 -c 将一个单独命令（不带参数）传递给 shell 执行。

  • -E, --conflict-exit-code number 当使用 -n（非阻塞）且锁冲突，或使用 -w（等待超时）且超时未获取锁时，指定返回的退出状态码。

  • 默认值为 1

  • 可用范围：0~255

  • -F, --no-fork 执行命令前 不 fork，flock 进程直接被命令替换，并继续持有锁。

  • 与 --close 不兼容，否则没有进程持锁。

  • -e, -x, --exclusive 获取独占锁（也称写锁），这是默认模式。

  • -n, --nb, --nonblock 如果锁无法立即获取，则直接失败而不等待。

  • 返回值可通过 -E 指定

  • -o, --close 在执行命令前关闭持锁的文件描述符。

  • 适用于命令生成子进程时，让子进程不持锁

  • -s, --shared 获取共享锁（也称读锁）。

  • -u, --unlock 释放锁。通常不需要手动调用，因为文件关闭时锁会自动释放。

  • 特殊情况需要，例如命令内部可能 fork 背景进程，不希望其持锁

  • -w, --wait, --timeout seconds 在指定秒数内未获取锁则失败。

  • 支持小数秒

  • 返回值可通过 -E 指定

  • 秒数为 0 时等同于 --nonblock

  • --verbose 输出获取锁所花的时间，或者说明无法获取锁的原因。

  • --help - 显示帮助

  • --version - 显示版本

#示例

阻塞式加锁

$ flock /tmp/lockfile -c "echo '任务开始'; sleep 5; echo '任务结束'"    # 加锁然后执行命令
任务开始
任务结束

• 如果文件已经被其它进程加锁，则会阻塞到锁释放
• flock 结束时自动释放锁

非阻塞式加锁

$ flock -n /tmp/lockfile -c "echo '任务开始'; sleep 5; echo '任务结束'"    # 加锁然后执行命令
任务开始
任务结束

• 如果文件已经被其它进程加锁，则会直接返回失败（不等的也不执行命令）
• flock 结束时自动释放锁

在脚本内加锁一段时间

#!/bin/bash
exec 200 > /tmp/lockfile        # 打开文件并指定文件描述符为 200 
flock 200                       # 加锁

echo "任务开始"
sleep 10
echo "任务结束"

• 脚本退出时，文件描述符 200 关闭，自动释放锁
• 也可以通过 flock -u 200 提前释放锁，或通过 exec 200>&- 关闭文件来释放锁

#推荐阅读

• flock(1) — Linux manual page

#手册

显示
FLOCK(1)                         User Commands                        FLOCK(1)

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 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.

       -F, --no-fork
           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.

       -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 status 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
           status 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.

       -h, --help
           Display help text and exit.

       -V, --version
           Print version and exit.

EXIT STATUS
       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.

NOTES
       flock does not detect deadlock. See flock(2) for details.

       Some file systems (e. g. NFS and CIFS) have a limited implementation of
       flock(2) and flock may always fail. For details see flock(2), nfs(5)
       and mount.cifs(8). Depending on mount options, flock can always fail
       there.

EXAMPLES
       Note that "shell> " in examples is a command line prompt.

       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 environment variable $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 environment variable to the
           right value so it doesn’t run again.

       shell> exec 4<>/var/lock/mylockfile; shell> flock -n 4
           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.

AUTHORS
       H. Peter Anvin <[email protected]>

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)

REPORTING BUGS
       For bug reports, use the issue tracker at
       https://github.com/util-linux/util-linux/issues.

AVAILABILITY
       The flock command is part of the util-linux package which can be
       downloaded from Linux Kernel Archive
       <https://www.kernel.org/pub/linux/utils/util-linux/>.

util-linux 2.39.3                 2023-10-23                          FLOCK(1)