Procfs

The proc filesystem (procfs) is a special filesystem in Unix-like operating systems that presents information about processes and other system information in a hierarchical file-like structure, providing a more convenient and standardized method for dynamically accessing process data held in the kernel than traditional tracing methods or direct access to kernel memory. Typically, it is mapped to a mount point named "/proc" at boot time. The proc file system acts as an interface to internal data structures in the
kernel. It can be used to obtain information about the system and to change
certain kernel parameters at runtime (sysctl).
Many Unix-like operating systems support the proc filesystem, including Solaris, IRIX, Tru64 UNIX, BSD, Linux, IBM AIX, QNX, and Plan 9 from Bell Labs. OpenBSD dropped support in version 5.7, released in May 2015.
The Linux kernel extends it to non–process-related data.
The proc filesystem provides a method of communication between kernel space and user space. For example, the GNU version of the process reporting utility ps uses the proc file system to obtain its data, without using any specialized system calls.
History.
UNIX 8th Edition.
Tom J. Killian implemented the UNIX 8th Edition (V8) version of "/proc": he presented a paper titled "Processes as Files" at USENIX in June 1984. The design of procfs aimed to replace the "ptrace" system call used for process tracing. Detailed documentation can be found in the proc(4) manual page.
SVR4.
Roger Faulkner and Ron Gomes ported V8 "/proc" to SVR4, and published a paper called "The Process File System and Process Model in UNIX System V" at USENIX in January 1991. This kind of procfs supported the creation of "ps", but the files could only be accessed with functions "read()", "write()", and "ioctl()". Between 1995 and 1996, Roger Faulkner created the procfs-2 interface for Solaris-2.6 that offers a structured /proc filesystem with sub-directories.
Plan 9.
Plan 9 implemented a process file system, but went further than V8. V8's process file system implemented a single file per process. Plan 9 created a hierarchy of separate files to provide those functions, and made /proc a real part of the file system.
4.4BSD.
4.4BSD cloned its implementation of from Plan 9. , procfs is gradually becoming phased out in FreeBSD. It was removed from OpenBSD in version 5.7, which was released in May 2015, because it "always suffered from race conditions and is now unused".
Solaris.
/proc in Solaris was available from the beginning. Solaris 2.6 in 1996 introduced procfs2 from Roger Faulkner.
Linux.
Linux first added a /proc filesystem in v0.97.3, September 1992, and first began expanding it to non-process related data in v0.98.6, December 1992.
As of 2020, the Linux implementation includes a directory for each running process, including kernel processes, in directories named , where is the process number. Each directory contains information about one process, including:
(Users may obtain the PID with a utility such as pgrep, pidof or ps:
$ ls -l /proc/$(pgrep -n python3)/fd # List all file descriptors of the most recently started `python3' process
total 0
lrwx------ 1 baldur baldur 64 2020-03-18 12:31 0 -> /dev/pts/3
lrwx------ 1 baldur baldur 64 2020-03-18 12:31 1 -> /dev/pts/3
lrwx------ 1 baldur baldur 64 2020-03-18 12:31 2 -> /dev/pts/3
$ readlink /proc/$(pgrep -n python3)/exe # List executable used to launch the most recently started `python3' process 
/usr/bin/python3.8
 also includes non-process-related system information, although in the 2.6 kernel much of that information moved to a separate pseudo-file system, sysfs, mounted under :
On multi-core CPUs, /proc/cpuinfo contains the fields for "siblings" and "cpu cores" which represent the following calculation is applied:
A CPU package means physical CPU which can have multiple cores ("single core" for one, "dual core" for two, "quad core" for four).
This allows a distinction between hyper-threading and dual-core, i.e. the number of hyper-threads per CPU package can be calculated by "siblings / CPU cores". If both values for a CPU package are the same, then hyper-threading is not supported. For instance, a CPU package with siblings=2 and "cpu cores"=2 is a dual-core CPU but does not support hyper-threading.
The basic utilities that use /proc under Linux come in the procps ( processes) package, and only function in conjunction with a mounted .
Proprietary Expansions.
Several companies and projects added additional functions to "/proc" for their systems, e.g. "/proc/lcd", might be a file containing the contents of the front-panel LCD screen. Text written to this file would be displayed on the screen.