如何确保一次只运行一个busybox脚本的实例而没有锁(文件)？

Question

在一个具有相当有限的Busybox (版本1.24.2) shell的路由器中，我希望确保备份(rsync) shell脚本只在它尚未运行时执行。

一个完整的鞭子壳的答案如下：

1. Quick-and-dirty way to ensure only one instance of a shell script is running at a time
2. Shell script: Ensure that script isn't executed if already running

与残废的灰壳不兼容。

路由器固件提供这些busybox命令：

[, [[, addgroup, adduser, arp, arping, ash, awk, basename, bash, brctl, bunzip2, bzcat, bzip2, cat, chgrp, chmod, chown, chpasswd, chroot, clear, cp, crond, crontab, cut, date, dd, df,
dhcp6c, dirname, dmesg, dnsdomainname, dos2unix, du, echo, egrep, eject, env, ether-wake, expr, false, fdisk, fgrep, find, flock, free, fuser, grep, gunzip, gzip, head, hostname,
ifconfig, inetd, insmod, kill, killall, klogd, ln, logger, login, ls, lsmod, lsof, md5sum, mdev, microcom, mkdir, mknod, mkswap, modprobe, more, mount, mountpoint, mv, netstat, nice,
nslookup, ntpd, passwd, pgrep, pidof, ping, ping6, printf, ps, pwd, rm, rmdir, rmmod, route, sed, sendmail, seq, sh, sleep, sort, start-stop-daemon, stat, strings, swapoff, swapon,
switch_root, sync, sysctl, syslogd, tail, tar, tee, telnetd, test, time, top, touch, tr, traceroute, traceroute6, true, udhcpc, umount, uname, uniq, unix2dos, unlink, uptime, usleep,
vconfig, vi, watch, wc, wget, which, whoami, xargs, yes, zcat, zcip 

self-invocation solution确实在第一次调用时返回exit 1。使用的测试脚本：

#!/bin/sh
[ "${FLOCKER}" != "$0" ] && exec /usr/bin/env FLOCKER="$0" /usr/bin/flock "$0" "$0" "$@" || exit 1
echo "servus!"
/bin/sleep 30

ps建议来自https://unix.stackexchange.com/questions/48505/how-to-make-sure-only-one-instance-of-a-bash-script-runs#answer-48511的一个答案是有希望的，但仍然不兼容路由器减少的Busybox外壳。Busybox附带了ps命令，但只附带了选项w、l和T。

有什么建议吗，不要用这个脚本捕捉nano或其他编辑器？

Answer 1

flock似乎有必要避免创建一个竞赛条件。一种基于flock的解决方案，适用于busybox 1.24.2：

#!/bin/bash

scriptname=$(basename $0)
lock="/var/run/${scriptname}"

exec 200>$lock
flock -n 200 || { echo "Error: Another copy of $0 is running. Exiting" ; exit 1; } 

pid=$$
echo $pid 1>&200

## Sample code
sleep 60
echo "Hello world"

来源：https://linuxaria.com/howto/linux-shell-introduction-to-flock