在 Centos 系统中安装了mongodb，并且以后台服务的形式运行mongo，sudo service mongod start。安装完成，启动成功，一切都很顺利，但是在一次停止mongo 服务后再次启动时却提示一下错误信息：
Starting mongod (via systemctl): Job for mongod.service failed because the control process exited with error code. See "systemctl status mongod.service" and "journalctl -xe" for details. [FAILED]
命令行输入—systemctl status mongod.service查看错误日志如下：
mongod.service - SYSV: Mongo is a scalable, document-oriented database. Loaded: loaded (/etc/rc.d/init.d/mongod; bad; vendor preset: disabled) Active: failed (Result: exit-code) since Tue 2017-11-28 21:17:17 CST; 1min 19s ago
Docs: man:systemd-sysv-generator(8) Process: 14690 ExecStart=/etc/rc.d/init.d/mongod start (code=exited, status=1/FAILURE)Nov 28 21:17:16 iZuf62kvdczytdgvp41b7nZ systemd[1]: Starting SYSV: Mongo is a scalable, document-oriented database....Nov 28 21:17:16 iZuf62kvdczytdgvp41b7nZ runuser[14701]: pam_unix(runuser:session): session opened for user mongod by (uid=0)Nov 28 21:17:17 iZuf62kvdczytdgvp41b7nZ systemd[1]: mongod.service: control process exited, code=exited status=1Nov 28 21:17:17 iZuf62kvdczytdgvp41b7nZ mongod[14690]: Starting mongod: [FAILED]Nov 28 21:17:17 iZuf62kvdczytdgvp41b7nZ systemd[1]: Failed to start SYSV: Mongo is a scalable, document-oriented database..Nov 28 21:17:17 iZuf62kvdczytdgvp41b7nZ systemd[1]: Unit mongod.service entered failed state.Nov 28 21:17:17 iZuf62kvdczytdgvp41b7nZ systemd[1]: mongod.service failed.
从日志中并看不出什么有效的信息，之后求助 Google ，也没有搜寻到有效的解决方案，各种查找之后发现了一些答友说是文件夹权限的问题，再看日志发现有两行信息：
Nov 28 21:17:16 iZuf62kvdczytdgvp41b7nZ runuser[14701]: pam_unix(runuser:session): session opened for user mongod by (uid=0)Nov 28 21:17:17 iZuf62kvdczytdgvp41b7nZ systemd[1]: mongod.service: control process exited, code=exited status=1
看起来确实跟用户 （mongod）权限有关，mongo 需要执行的文件有两个，一个是数据存储文件/var/lib/mongo，一个是日志存储文件/var/log/mongodb。所以执行以下命令对这两个文件夹的用户权限进行修改：
chown -R mongod:mongod /var/lib/mongo
chown -R mongod:mongod /var/log/mongodb
之后再次执行sudo service mongod start，很令人失望，依然报错，再次执行systemctl status mongod.service查看错误日志如下：
● mongod.service - SYSV: Mongo is a scalable, document-oriented database. Loaded: loaded (/etc/rc.d/init.d/mongod; bad; vendor preset: disabled) Active: failed (Result: exit-code) since Tue 2017-11-28 21:33:30 CST; 34s ago
Docs: man:systemd-sysv-generator(8) Process: 16330 ExecStart=/etc/rc.d/init.d/mongod start (code=exited, status=1/FAILURE)Nov 28 21:33:30 iZuf62kvdczytdgvp41b7nZ systemd[1]: Starting SYSV: Mongo is a scalable, document-oriented database....Nov 28 21:33:30 iZuf62kvdczytdgvp41b7nZ systemd[1]: mongod.service: control process exited, code=exited status=1Nov 28 21:33:30 iZuf62kvdczytdgvp41b7nZ mongod[16330]: Error starting mongod. /var/run/mongodb/mongod.pid exists.Nov 28 21:33:30 iZuf62kvdczytdgvp41b7nZ systemd[1]: Failed to start SYSV: Mongo is a scalable, document-oriented database..Nov 28 21:33:30 iZuf62kvdczytdgvp41b7nZ systemd[1]: Unit mongod.service entered failed state.Nov 28 21:33:30 iZuf62kvdczytdgvp41b7nZ systemd[1]: mongod.service failed.
但是令人欣慰的是这次的错误信息不一样了，说明刚才修改权限的操作是有效的，小开心一下 :) ，继续看日志，发现有一行信息：
Nov 28 21:33:30 iZuf62kvdczytdgvp41b7nZ mongod[16330]: Error starting mongod. /var/run/mongodb/mongod.pid exists.
从字面意思看就是/var/run/mongodb/mongod.pid这个文件存在导致进程退出，查看 mongo 服务的执行脚本/etc/rc.d/init.d/mongod中有一段代码：
start(){... # Make sure the pidfile does not exist if [ -f "$PIDFILEPATH" ]; then echo "Error starting mongod. $PIDFILEPATH exists." RETVAL=1 return fi}
上面的错误日志正是出于此处，而且注释中说了这段代码的判断逻辑是确保 pidfile 不存在，这就进一步印证了错误是由于 pidfile 的存在导致的，果断删除/var/run/mongodb/mongod.pid
rm /var/run/mongodb/mongod.pid -f
再次重启服务：sudo service mongod start，成功了 ~~
Starting mongod (via systemctl): [ OK ]
最后整理了一个 shell 脚本mongoAuth.sh：
#!/bin/bashchown -R mongod:mongod /var/lib/mongo
chown -R mongod:mongod /var/log/mongodb
rm /var/run/mongodb/mongod.pid -f
之后再遇到 mongo 服务启动失败的问题，执行此脚本即可解决，我用了之后很好使，所以分享此文章，如果有小伙伴以后遇到这样的问题了，希望该篇文章可以帮到你。共同学习，共同进步！
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