MySQL: pid-file (process id file)

In previous articles have gone through options available with mysql_safe, one of the option is pid file i.e. process identifier file

• The server writes its process id into pid file when mysql services are started and removes file when stopped.
• The pid file is the means by which a server allows itself to be found by other processes.

Location
default location of  pid file is data directory or it can be specified by specific variable 
pid-file=file_name
generally pid file is given as hostname.pid

Content
The pid file content the process identification number of mysqld process, you can see content by using below command
cat

and also see the process id  of mysql daemon process using 
ps -ef|grep mysqld

you can see content of pid file and this process id identified using ps -ef command are same.