How to check whether a process with a given PID is running

Introduction

As a system administrator or developer, it's important to be able to monitor and manage processes running on your system. One of key pieces of information you need to know is whether a particular process is currently running. In this article, we'll explore different ways to check whether a process with a given PID is running.

What is a PID?

Before we dive into ways to check if a process with a given PID is running, let's first define what a PID is. A PID (Process Identifier) is a unique identification number assigned to each process running on a system. This number is used by operating system to keep track of and manage processes.

Using ps Command

One of easiest ways to check whether a process with a given PID is running is to use ps command. ps command is used to display information about currently running processes on a system.

To check whether a process with a given PID is running using ps command, run following command −

ps -p <PID>

Replace <PID> with PID of process you want to check.

If process is running, you will see output similar to following −

PID TTY          TIME CMD
1234 pts/0    00:00:00 bash

The output shows process ID (PID), terminal it's running on (TTY), amount of CPU time it has used (TIME), and command that started process (CMD).

If process is not running, you will not see any output.

Using pgrep Command

Another way to check whether a process with a given PID is running is to use pgrep command. pgrep command is used to search for processes based on their name or other attributes.

To check whether a process with a given PID is running using pgrep command, run following command −

pgrep <PID>

Replace <PID> with PID of process you want to check.

If process is running, you will see PID printed to console.

If process is not running, you will not see any output.

Using kill Command

The kill command is used to send a signal to a process. One of signals that can be sent is 0 signal, which does not actually kill process, but instead checks whether it's running.

To check whether a process with a given PID is running using kill command, run following command −

kill -0 <PID>

Replace <PID> with PID of process you want to check.

If process is running, you will not see any output.

If process is not running, you will see an error message similar to following −

kill: (1234) - No such process

Using pidof Command

The pidof command is similar to pgrep command, but it only returns PID of process.

To check whether a process with a given PID is running using pidof command, run following command −

pidof <PID>

Replace <PID> with PID of process you want to check.

If process is running, you will see PID printed to console.

If process is not running, you will not see any output.

Here are some additional tips and tricks for checking whether a process with a given PID is running −

• Use htop command for a more interactive and user-friendly process viewer. htop allows you to sort and filter processes, and provides more detailed information about CPU usage, memory usage, and other metrics.

• If you want to check whether a process is running in a script, you can use ps command with q option to suppress output, and then check exit code of command. For example −

ps -p <PID> q >/dev/null
if [ $? -eq 0 ]; then
   echo "Process is running"
else
   echo "Process is not running"
fi

• You can use lsof command to check whether a process has open files or network connections. For example, to check whether process with PID 1234 has any open files, run following command −

lsof -p 1234

• You can use systemctl command to check whether a systemd service is running. For example, to check whether Apache web server service is running, run following command −

systemctl status apache2

• If you want to check whether a process is running on a remote system, you can use ssh command to run a command on remote system. For example, to check whether process with PID 1234 is running on a remote system with IP address 192.168.1.100, run following command −

ssh 192.168.1.100 ps -p 1234

These are just a few examples of many tools and commands available for checking whether a process with a given PID is running. By combining these tools with your own scripts and workflows, you can effectively monitor and manage processes running on your system.

Conclusion

In this article, we explored different ways to check whether a process with a given PID is running. We learned that we can use ps command to display information about currently running processes, pgrep command to search for processes based on their name or other attributes, kill command to send a signal to a process and check whether it's running, and pidof command to return PID of process.

Knowing how to check whether a process with a given PID is running is an essential skill for system administrators and developers. It can help in troubleshooting issues, monitoring system performance, and managing resources.