Proc File System in Linux

I’d like to share an interesting finding with you today. Perhaps, many of you must be already familiar with it but being a newbie, it really intrigued me.

I have recently enrolled myself in the DevOps Ninja program conducted by my organization- OpsTree. Everything that I’m learning here is new to me and often amuses me. One day, I was working on an assignment and executed the top command. I left the task as it is and meanwhile started browsing something on the internet. Suddenly a question struck my mind that from where the top command fetches its data ( you can see how I got digressed from my main assignment :p).

$ top

So, I started exploring (please make note of top command process ID here- 7564) and utilised lsof command to dig deeper.

$ lsof -p <process id>

Here, I happened to find /proc, went in and found a whole lot of process directories which contained a lot of useful information about system processes and its respective activity history.

WHAT’S /proc?

The /proc directory is present on all Linux systems, regardless of flavor or architecture. The /proc directory is NOT a real file system but a virtual file system that is created dynamically by Linux to provide access to certain types of hardware information and information about the running processes. It is mapped to /proc and mounted at boot time.

To display information about your CPU, you can use the cat /proc/cpuinfo command:

To display information about the file systems supported by currently running Linux kernel:, you can use the cat /proc/filesystems command:

To display statistics about memory usage on the system, use the cat /proc/meminfo command:

To display the Linux kernel version, distribution number and other kernel-related information, use the cat /proc/version command:

Within /proc’s numbered directories, you will find a few files and links. These directories’ numbers correlate to the PID of the command being run within them.

Let’s check what’s there inside in one of these numbered directories. Here, I chose 2016 and run cat status:

In any numbered directory, you will have a similar file structure. The most important ones, and their descriptions, are as follows:

1. cmdline — contains the command that started the process
2. environ — contains the names and content of the environment variables for the process
3. fd — file descriptors
4. limits — contains information about the limits of the process
5. mounts — related information

You will also notice a number of links in the numbered directory:

1. cwd — a link to the current working directory of the process
2. exe — link to the executable of the process
3. root — link to the work directory of the process

Just like with the hardware information, you can display the content of these files using the cat command.

Originally published at http://blog.opstree.com on August 24, 2021.