I‘m a new Linux system user and developer running Ubuntu server. How do I check how long a process or pid has been running on my Ubuntu/Debian or CentOS Linux server?

You need to use the ps command to see information about a selection of the active processes. The pid command provide following two formatting options for a PID (process identification number). A PID automatically assigned when process is created on a Linux or Unix-like operating system. Linux ps command to find process runtimes options:

1. etime Display elapsed time since the process was started, in the form [[DD-]hh:]mm:ss.
2. etimes Display elapsed time since the process was started, in seconds.

How to check how long a process has been running?

You need to pass the -o etimes or -o etime to the ps command. The syntax is:
ps -p {PID-HERE} -o etime
ps -p {PID-HERE} -o etimes
The ps command used to provide information about the currently running processes on Linux, including their process identification numbers (PIDs) and other information. We can use other commands too. For example, pstree command, top command, htop command and more. Let us see examples to find out how long the OpenVPN process has been running on my server.

Step 1: Find PID of a process (say openvpn)

Open the shell prompt and then type the following pidof command:
$ pidof openvpn
6176
Please note down the PID 6176.

Step 2: How long a openvpn process has been running?

$ ps -p 6176 -o etime
OR
$ ps -p 6176 -o etimes
To hide header:
$ ps -p 6176 -o etime=
$ ps -p 6176 -o etimes=

The 6176 is the PID of the process you want to check. In this case I’m looking into openvpn process. Feel free to replace openvpn and PID # 6176 as per your own requirements. In this find example, I am printing PID, command, elapsed time, user ID, and group ID:
$ ps -p 6176 -o pid,cmd,etime,uid,gid
Sample outputs:

  PID CMD                             ELAPSED   UID   GID
 6176 /usr/sbin/openvpn --daemon        15:25 65534 65534

On many production systems, unprivileged users or developers are not allowed to look up process-related information due to security concerns. In such cases, it is necessary to run the ps as root user either using the su command or sudo command. See “Linux hide processes from other users and ps command” for more info.

How to find how long a process named mysqld has run in Linux

Find process id of mysqld by ysing the ps command or pidof command:
$ sudo pidof mysqld
## Alternatively, use the grep command as filter ##
$ sudo ps aux | grep '/usr/sbin/[m]ysqld'
Now, you have the PID for the mysqld, let us find and print the PID creation date. In other words find out when when the process was started on Linux, enter:
$ sudo ps -p {PID} -o start,etimes,etime
$ sudo ps -p 1861 -o start,etimes,etime
$ sudo ps -p 1861 -o pid,cmd,start,etimes,etime
Our final example directly find PID by giving process name such as lighttpd, nginx, mysqld and so on:
$ sudo ps -C {process-name} -o pid,cmd,start,etimes,etime
$ sudo ps -C lighttpd -o pid,cmd,start,etimes,etime
$ sudo ps -C nginx -o pid,cmd,start,etimes,etime
$ sudo ps -C mysqld -o pid,cmd,start,etimes,etime

Multiple processes or PID means you are running the same process using Docker/LXD or Linux containers. Same daemons (long-running background process) such as Nginx uses the parent-child process model so you might see many PIDs too. For instance, here is outputs from server that is running multiple Linux containers with php-fpm7 master process per container:
sudo ps -C php-fpm7 -o pid,cmd,lstart,etime,etimes
Sample outputs:

  PID CMD                                          STARTED     ELAPSED ELAPSED
 5329 php-fpm: master process (/e Sun Feb  2 11:37:30 2020 98-00:47:03 8470023
 8907 php-fpm: pool www           Fri May  1 21:25:46 2020  8-14:58:47  745127
15269 php-fpm: pool www           Wed Apr 15 11:28:58 2020 25-00:55:35 2163335
15271 php-fpm: pool www           Wed Apr 15 11:28:59 2020 25-00:55:34 2163334

My PID 5329 (php-fpm master process) was started on “Sun Feb 2 11:37:30 2020” and 8470023 seconds ago. The 98-00:47:03 indicates that it was started 98 days ago:

• Today’s date when the ps was executed: Sunday, 10 May 2020
• PID started on: Sunday, 2 Feb 2020
• Time elapsed in seconds: 8470023
• Days elapsed: 98 days

Understanding ps options

Linux find how long a process running with ps:

Options	Short description	Examples
-C	Select process by command name instead of PID	ps -C firefox ps -C memcached ps -C redis-server
-o pid	A number representing the process ID	ps -C nginx -o pid ps -p PID_HERE -o pid
-o cmd	Command with all its arguments as a string	ps -C google-chrome -o cmd
-o start	Time the command started. If the process was started less than 24 hours ago, the output format is “HH:MM:SS”, else it is ” Mmm dd” (where Mmm is a three-letter month name)	ps -C mysqld -o start
-o lstart	Find time the background command started	ps -C lighttpd -o lstart
-o etimes	Get elapsed time since the process was started, in seconds	ps -C php-fpm7 -o etimes
-o etime	See elapsed time since the process was started, in the form [[DD-]hh:]mm:ss	ps -C php-fpm7 -o etime
-o etime= -o cmd= -o $opt=	Hide the ps command header	ps -C php-fpm7 -o pid,cmd,lstart,etimes=

Conclusion

In this tutorial, you learned steps to identify process IDs (PID) in Linux systems and then determine how long they have been running on your machine or cloud server. See ps command man page by typing the following ps command or read it online here:
man ps