Shell中，实现单一实例(译)

代码

#!/bin/bash

## Copyright (C) 2009 Przemyslaw Pawelczyk <przemoc@gmail.com>
##
## This script is licensed under the terms of the MIT license.
## https://opensource.org/licenses/MIT
#
# Lockable script boilerplate

### HEADER ###

LOCKFILE="/var/lock/`basename $0`"
LOCKFD=99

# PRIVATE
_lock()             { flock -$1 $LOCKFD; }
_no_more_locking()  { _lock u; _lock xn && rm -f $LOCKFILE; }
_prepare_locking()  { eval "exec $LOCKFD>\"$LOCKFILE\""; trap _no_more_locking EXIT; }

# ON START
_prepare_locking

# PUBLIC
exlock_now()        { _lock xn; }  # obtain an exclusive lock immediately or fail
exlock()            { _lock x; }   # obtain an exclusive lock
shlock()            { _lock s; }   # obtain a shared lock
unlock()            { _lock u; }   # drop a lock

### BEGIN OF SCRIPT ###

# Simplest example is avoiding running multiple instances of script.
exlock_now || exit 1

# Remember! Lock file is removed when one of the scripts exits and it is
#           the only script holding the lock or lock is not acquired at all.

加锁过程

1. "/var/lock/`basename $0`": 根据当前脚本的名字，在/var/lock/目录下创建一个临时文件
2. eval "exec $LOCKFD>\"$LOCKFILE\"";: 打开这个临时文件，并获得句柄(LOCKFD)
3. _lock xn: 使用flock -xn $LOCKFD获得文件句柄的排它锁(Exclusive Lock)，如果获取失败，则状态码非0

解锁过程

1. trap _no_more_locking EXIT: 捕获脚本的退出事件(EXIT)，当脚本退出时(Ctrl + C、kill -s TERM $PID、正常结束等, 不包括kill -9 $PID)，会执行_no_more_locking
2. _lock u; _lock xn && rm -f $LOCKFILE;: 解锁，加锁(成功后)删除临时文件。_lock u解锁，以便其他进程对临时文件加锁；_lock xn && rm -f $LOCKFILE如果加锁成功，则删除临时文件，否则不删，因为此时有其他进程正在锁住临时文件。

注意

The flock command is part of the util-linux-ng package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/. macOS系统中没有这个命令

参考

原文: What is the best way to ensure only one instance of a Bash script is running?

Repo: przemoc/lockable_script_boilerplate.sh