Source: https://mysqlentomologist.blogspot.com/2021/01/linux-proc-filesystem-for-mysql-dbas.html
Proc
Julia Evans
Basically, the proc filesystem is a pseudo-filesystem which provides an interface to kernel data structures. It is commonly mounted at /proc:
/etc/issue is a text file which contains a message or system identification to be printed before the login prompt.
The cat /etc/os-release command displays the operating system’s name and version information on a Linux system
openxs@ao756:~$ cat /etc/issue
Ubuntu 16.04.7 LTS \n \l
mount displays a list of all currently mounted filesystems.
The output typically includes the device, the mount point, the filesystem type, and mount options.
openxs@ao756:~$ mount | grep '/proc'
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
...
/proc is a temporary filesystem which linux initializes, in order to keep track of resources and states of each process,
as well as system wide resources.
Every command line tool like ps,top,vmstat,free derives its results from parsing /proc directory.
Most of it is read-only, but some files allow to change kernel variables.
openxs@ao756:~$ ps aux | grep mysqld
...
mysql 30580 0.7 8.1 746308 313984 ? Sl Jan02 9:55 /usr/sbin/mysqld --daemonize --pid-file=/var/run/mysqld/mysqld.pid
So there will be the /proc/30580 directory with the following content:
Highlighted are the files and directories are more useful.
Note also “Permission denied” messages above that you may get while accessing some files in /proc,
even related to the processes you own. You may still need root/sudo access (or belong to some dedicated group) to read them.