问题:

I recently come across a file with the extension .pid and explored inside it but didn't find much. The documentation says:

A Pid-File is a file containing the process identification number (pid) that is stored in a well-defined location of the filesystem thus allowing other programs to find out the pid of a running script.

Can anyone shed more light on this, or guide me to details of what's contained in the pid file?

回答1:

The pid files contains the process id (a number) of a given program. For example, Apache HTTPD may write its main process number to a pid file - which is a regular text file, nothing more than that - and later use the information there contained to stop itself. You can also use that information to kill the process yourself, using cat filename.pid | xargs kill

回答2:

To understand pid files, refer this DOC

Some times there are certain applications that require additional support of extra plugins and utilities. So it keeps track of these utilities and plugin process running ids using this pid file for reference.

That is why whenever you restart an application all necessary plugins and dependant apps must be restarted since the pid file will become stale.