安装MongoDB，尝试配置成服务后，启动一直不成功

# systemctl start mongod
# systemctl status mongod
● mongod.service - High-performance, schema-free document-oriented database
   Loaded: loaded (/usr/lib/systemd/system/mongod.service; enabled; vendor preset: disabled)
   Active: inactive (dead) since 五 2021-10-29 18:14:10 CST; 4s ago
     Docs: https://docs.mongodb.org/manual
  Process: 3921 ExecStart=/usr/local/mongodb/bin/mongod $OPTIONS (code=exited, status=0/SUCCESS)
 Main PID: 3921 (code=exited, status=0/SUCCESS)

10月 29 18:14:09 abce systemd[1]: Started High-performance, schema-free document-oriented database.
10月 29 18:14:09 abce mongod[3921]: about to fork child process, waiting until server is ready for connections.
10月 29 18:14:09 abce mongod[3921]: forked process: 3925
10月 29 18:14:10 abce mongod[3921]: child process started successfully, parent exiting

启动的时候，进程会自动被kill掉。

看看mongodb的日志可以看到，会自动发送kill信号

{"t":{"$date":"2021-10-29T18:14:10.140+08:00"},"s":"I",  "c":"CONTROL",  "id":23377,   "ctx":"SignalHandler","msg":"Received signal","attr":{"signal":15,"error":"Terminated"}}
{"t":{"$date":"2021-10-29T18:14:10.140+08:00"},"s":"I",  "c":"CONTROL",  "id":23378,   "ctx":"SignalHandler","msg":"Signal was sent by kill(2)","attr":{"pid":1,"uid":0}}
{"t":{"$date":"2021-10-29T18:14:10.140+08:00"},"s":"I",  "c":"CONTROL",  "id":23381,   "ctx":"SignalHandler","msg":"will terminate after current cmd ends"}

原因和解决方法：
在/usr/lib/systemd/system/mongod.service文件中添加

PIDFile=mongod.pid(实际pid文件名和路径)