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{{short description|Filesystem in Unix-like operating systems}}
{{more citations needed|date=March 2010}}
{{lowercase title|title=procfs}}
{{Use dmy dates|date=July 2021}}
The '''proc filesystem''' ('''procfs''') is a special filesystem in [[Unix-like]] operating systems that presents information about [[process (computing)|process]]es 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 (software)|tracing]] methods or direct access to [[kernel (operating system)|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 about running processes in the kernel. In [[Linux kernel|Linux]], it can also be used to obtain information about the kernel and to change certain kernel parameters at runtime ([[sysctl]]).
Many Unix-like operating systems support the proc filesystem, including [[UNIX System V|System V]], [[Solaris (operating system)|Solaris]], [[IRIX]], [[Tru64 UNIX]], [[Berkeley Software Distribution|BSD]], [[Linux]], [[IBM AIX]],<ref name=":0">{{Cite book |last1=Nemeth |first1=Evi |url=https://books.google.com/books?id=rgFIAnLjb1wC&pg=PA136 |title=UNIX and Linux System Administration Handbook |last2=Snyder |first2=Garth |last3=Hein |first3=Trent R. |last4=Whaley |first4=Ben |date=2010-07-14 |publisher=Pearson Education |isbn=978-0-13-211736-4 |pages=136 |language=en}}</ref> [[QNX]], and [[Plan 9 from Bell Labs]]. [[OpenBSD]] dropped support in version 5.7, released in May 2015. It is absent from [[HP-UX]]<ref name=":0" /> and [[macOS]].<ref name=":1" />
The [[Linux kernel]] extends it to non–process-related data.
The proc filesystem provides a method of communication between [[user space and kernel space|kernel space]] and [[user space and kernel space|user space]]. For example, the [[GNU]] version of the process reporting utility [[ps (Unix)|ps]] uses the proc file system to obtain its data, without using any specialized [[system call]]s.
== History ==
=== UNIX 8th Edition ===
[[Tom J. Killian]] implemented the [[Version 8 Unix|UNIX 8th Edition]] (V8) version of ''/proc'': he presented a paper titled [http://lucasvr.gobolinux.org/etc/Killian84-Procfs-USENIX.pdf "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 [http://man.cat-v.org/unix_8th/4/proc proc(4) manual page].
=== SVR3 ===
The original AT&T System V Release 3 (SVR3) operating system (available internally to AT&T in 1986 and generally in 1987) did not come with the ''/proc'' filesystem, but a subsequent incremental version of it did. It only contained files representing the processes rather than the now common subdirectories.
=== SVR4 ===
[[Roger Faulkner]] and [[Ron Gomes]] ported V8 ''/proc'' to [[System V Release 4|SVR4]], and published a paper called [https://www.usenix.org/sites/default/files/usenix_winter91_faulkner.pdf "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 (Unix)|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 from Bell Labs|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 and derivatives ===
[[4.4BSD]] cloned its implementation of {{Mono|/proc}} from Plan&nbsp;9.{{citation needed|date=December 2010}} {{As of|2011|2}}, procfs is gradually becoming phased out in FreeBSD,<ref>{{cite web|url=http://lists.freebsd.org/pipermail/freebsd-fs/2011-February/010760.html|title=Why is procfs deprecated in favor of procstat?|work=freebsd.org|date=22 February 2011 }}</ref> and it has turned to use the ''[[sysctl]]'' interface instead for process-related information.
To provide binary compatibility with Linux user space programs, the FreeBSD kernel also provides '''linprocfs''' that is similar to the Linux procfs.<ref>{{cite web |title=linprocfs(5) |url=https://www.freebsd.org/cgi/man.cgi?query=linprocfs&apropos=0&sektion=5&manpath=FreeBSD+13.0-RELEASE&arch=default&format=html |website=FreeBSD Manual Pages |publisher=The FreeBSD Project |accessdate=2021-06-12 |language=en |date=2019-11-13}}</ref>
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".<ref>{{cite web|url=http://www.openbsd.org/plus57.html|title=Detailed changes between OpenBSD 5.6 and 5.7|work=openbsd.org}}</ref>
[[macOS]] did not implement procfs and user space programs have to use the ''sysctl'' interface for retrieving process data.<ref name=":1">{{cite web |author1=Amit Singh |title=/proc on Mac OS X |url=http://osxbook.com/book/bonus/ancient/procfs/ |website=Mac OS X Internals: The Book |access-date=2021-07-10 |archive-url=https://web.archive.org/web/20120504052329/http://osxbook.com/book/bonus/ancient/procfs/ |archive-date=2012-05-04 |date=2003}}</ref>
=== Solaris ===
/proc in Solaris was available from the beginning (June 1992). Solaris 2.6 in 1996 introduced procfs2 from Roger Faulkner.
=== Linux ===
Linux first added a /proc filesystem in [https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?h=0.97.3 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 (operating system)|kernel]] processes, in directories named {{Mono|/proc/PID}}, where {{Mono|PID}} is the process number. Each directory contains information about one process, including:
* {{Mono|/proc/PID/cmdline}}, the command that originally started the process.
* {{Mono|/proc/PID/cwd}}, a [[symbolic link|symlink]] to the [[Working directory|current working directory]] of the process.
* {{Mono|/proc/PID/environ}} contains the names and values of the environment variables that affect the process.
* {{Mono|/proc/PID/exe}}, a symlink to the original [[executable]] file, if it still exists (a process may continue running after its original executable has been deleted or replaced).
* {{Mono|/proc/PID/fd}}, a directory containing a symbolic link for each open [[file descriptor]].
* {{Mono|/proc/PID/fdinfo}}, a directory containing entries which describe the position and flags for each open file descriptor.
* {{Mono|/proc/PID/maps}}, a text file containing information about [[memory map|mapped]] files and blocks (like heap and stack).
* {{Mono|/proc/PID/mem}}, a binary image representing the process's [[virtual memory]], can only be accessed by a [[ptrace]]'ing process.
* {{Mono|/proc/PID/root}}, a symlink to the root path as seen by the process. For most processes this will be a link to / unless the process is running in a [[chroot]] jail.
* {{Mono|/proc/PID/status}} contains basic information about a process including its run state and memory usage.
* {{Mono|/proc/PID/task}}, a directory containing [[hard link]]s to any tasks that have been started by this (i.e.: the parent) process.
(Users may obtain the [[Process identifier|PID]] with a utility such as [[pgrep]], [[pidof]] or [[ps (Unix)|ps]]:
<syntaxhighlight lang="console">
$ 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
</syntaxhighlight>)
{{Mono|/proc}} 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 {{Mono|/sys}}:
* depending on the mode of power management (if at all), either directory, {{Mono|/proc/acpi}} or {{Mono|/proc/apm}}, which predate sysfs and contain various bits of information about the state of power management.
* {{Mono|/proc/buddyinfo}}, information about the [[Buddy memory allocation|buddy algorithm]] that handles memory fragmentation.<ref>{{cite web
|url=http://www.centos.org/docs/5/html/5.2/Deployment_Guide/s2-proc-buddyinfo.html
|title=3.2.2. /proc/buddyinfo
|work=centos.org
|access-date=23 May 2011
|archive-url=https://web.archive.org/web/20130902032109/http://www.centos.org/docs/5/html/5.2/Deployment_Guide/s2-proc-buddyinfo.html
|archive-date=2 September 2013
|url-status=dead
}}</ref>
* {{Mono|/proc/bus}}, containing directories representing various buses on the computer, such as [[Peripheral Component Interconnect|PCI]]/[[USB]]. This has been largely superseded by [[sysfs]] under /sys/bus which is far more informative.
* {{Mono|/proc/fb}}, a list of the available framebuffers
* {{Mono|/proc/cmdline}}, giving the boot options passed to the kernel
* {{anchor|cpuinfo}} {{Mono|/proc/cpuinfo}}, containing information about the [[Central processing unit|CPU]], such as its vendor (and CPU family, model and model names which should allow users to identify the CPU) and its speed (CPU [[Clock rate|clockspeed]]), cache size, number of siblings, cores, and [[CPU flag (x86)|CPU flags]]. The format and available information is highly architecture-depended. On some architectures, {{Mono|/proc/cpuinfo}} includes a value for "[[bogomips]]", frequently misconstrued as a measure of CPU speed, like a benchmark, but it does not actually measure any sensible (for end-users) value at all. It occurs as a side-effect of kernel timer calibration and yields highly varying values depending on CPU type, even at equal clock speeds.
{{sxhl|2=console|
$ cat /proc/cpuinfo
processor   : 0
vendor_id  : AuthenticAMD
cpu family : 16
model      : 6
model name : AMD Athlon(tm) II X2 270 Processor
stepping   : 3
microcode  : 0x10000c8
cpu MHz    : 2000.000
cache size : 1024 KB
...
processor  : 1
vendor_id  : AuthenticAMD
cpu family : 16
model      : 6
model name : AMD Athlon(tm) II X2 270 Processor
stepping   : 3
microcode  : 0x10000c8
cpu MHz    : 800.000
cache size : 1024 KB
...
}}
On multi-core CPUs, /proc/cpuinfo contains the fields for "siblings" and "cpu cores" which represent the following calculation is applied:<ref>{{cite web
|last=Baron
|first=Jason
|title=HT vs. dual-core
|url=http://www.redhat.com/archives/nahant-list/2006-January/msg00176.html
|access-date=28 June 2011
|archive-date=13 May 2016
|archive-url=https://web.archive.org/web/20160513191756/http://www.redhat.com/archives/nahant-list/2006-January/msg00176.html
|url-status=dead
}}</ref>
<pre>"siblings" = (HT per CPU package) * (# of cores per CPU package)
"cpu cores" = (# of cores per CPU package)</pre>
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.<ref>{{cite web
|title=Understanding Linux /proc/cpuinfo
|url=http://www.richweb.com/cpu_info
|archive-url=https://web.archive.org/web/20120403230159/http://www.richweb.com/cpu_info
|access-date=2015-04-21
|archive-date=2012-04-03
|website=richweb.com
}}</ref> For instance, a CPU package with siblings=2 and "cpu cores"=2 is a dual-core CPU but does not support hyper-threading.
* {{Mono|/proc/crypto}}, a list of available cryptographic modules
* {{Mono|/proc/devices}}, a list of character and block devices sorted by device ID but giving the major part of the {{Mono|/dev}} name too
* {{Mono|/proc/diskstats}}, giving some information (including device numbers) for each of the logical disk devices
* {{Mono|/proc/filesystems}}, a list of the file systems supported by the kernel at the time of listing
* {{Mono|/proc/interrupts}}, {{Mono|/proc/iomem}}, {{Mono|/proc/ioports}} and the directory {{Mono|/proc/irq}}, giving some details about the devices (physical or logical) using the various [[system resource]]s
* {{Mono|/proc/kmsg}}, holding messages output by the kernel<ref>
{{cite web
| url = https://books.google.com/books?id=wLJWBQAAQBAJ
| title = Linux Filesystem Hierarchy
| last1 = Nguyen
| first1 = Binh
| date = 2004-07-30
| publisher = Binh Nguyen
| page = 63
| access-date = 2016-07-18
| quote = /proc/kmsg[:] Messages output by the kernel. These are also routed to syslog.
}}
</ref>
* {{Mono|/proc/loadavg}}, containing stats about the current [[Load (computing)|load]] average in the last minutes.
* {{Mono|/proc/meminfo}}, containing a summary of how the kernel is managing its memory.
* {{Mono|/proc/modules}}, one of the most important files in {{Mono|/proc}}, containing a list of the kernel modules currently loaded . It gives some indication (not always entirely correct) of dependencies.
* {{Mono|/proc/mounts}}, a symlink to self/mounts which contains a list of the currently mounted devices and their mount points (and which file system is in use and what mount options are in use).
* {{Mono|/proc/net/}}, a directory containing useful information about the network stack, in particular {{Mono|/proc/net/nf_conntrack}}, which lists existing network connections (particularly useful for tracking routing when [[iptables]] FORWARD is used to redirect network connections)
* {{Mono|/proc/partitions}}, a list of the device-numbers, their size and {{Mono|/dev}} names which the kernel has identified as existing [[Disk partitioning|partitions]]
* {{Mono|/proc/scsi}}, giving information about any devices connected via a [[SCSI]] or [[RAID]] controller
* a symbolic link to the current (traversing) process at {{Mono|/proc/self}} (i.e. {{Mono|/proc/PID/}} where [[Process identifier|PID]] is that of the current process).
* {{Mono|/proc/slabinfo}}, listing statistics on the caches for frequently-used objects in the Linux kernel
* {{Mono|/proc/swaps}}, a list of the active swap partitions, their various sizes and priorities
* Access to dynamically configurable kernel options under {{Mono|/proc/sys}}.  Under {{Mono|/proc/sys}} appear directories representing the areas of kernel, containing readable and writable [[virtual file]]s.<br> For example, a commonly referenced virtual file is  {{Mono|/proc/sys/net/ipv4/ip_forward}}, because it is necessary for routing firewalls or tunnels. The file contains either a '1' or a '0': if it is 1, the IPv4 stack forwards packets not meant for the local host, if it is 0 then it does not.
* {{Mono|/proc/sysvipc}}, containing memory-sharing and [[inter-process communication]] (IPC) information.
* {{Mono|/proc/tty}}, containing information about [[Terminal emulator|terminals]]; {{Mono|/proc/tty/driver}} contains a list of TTY drivers and their usage<ref>{{cite web |title=The /proc filesystem |url=https://www.kernel.org/doc/html/latest/filesystems/proc.html#tty-info-in-proc-tty |website=Linux Kernel documentation |access-date=9 August 2024}}</ref>
* {{Mono|/proc/uptime}}, the length of time the kernel has been running since boot and spent in idle mode (both in seconds)
* {{Mono|/proc/version}}, containing the Linux kernel version, distribution number, [[GNU Compiler Collection|gcc]] version number (used to build the kernel) and any other pertinent information relating to the version of the kernel currently running
* other files depending on various hardware, module configurations, and changes to the kernel.
The basic utilities that use /proc under Linux come in the [[procps]] ({{Mono|/proc}} processes) package, and only function in conjunction with a mounted {{Mono|/proc}}.
=== CYGWIN ===
[[Cygwin]] implemented a procfs that is basically the same as the Linux procfs.
==References==
{{Reflist}}
==Sources==
* [http://man.cat-v.org/unix_8th/4/proc Unix 8th Edition proc(2) manual page] - Description of the original procfs.
* [http://man.cat-v.org/plan_9/3/proc Plan 9 procfs manual page] - Plan 9 greatly expanded the procfs concept, providing a much expanded interface to control and manipulate processes.
* [https://web.archive.org/web/20160303182044/http://manpages.courier-mta.org/htmlman5/proc.5.html Linux Manual Pages Proc(5)] Linux manual documentation for procfs
* [http://kernel.org/doc/Documentation/filesystems/proc.txt Documentation/filesystems/proc.txt] Linux kernel documentation for procfs
== External links ==
* [http://dtrace.org/blogs/eschrock/2004/06/25/a-brief-history-of-proc/ A brief history of /proc] Eric Schrock's Weblog
* [http://www.ibm.com/developerworks/library/l-proc.html Access the Linux kernel using the Procfs] An IBM developerWorks article by M. Tim Jones
* [http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html Linux-Filesystem-Hierarchy] Linux Documentation Project
* [https://web.archive.org/web/20130310084458/http://archive09.linux.com/feature/126718 Discover the possibilities of the /proc directory] by Federico Kereki
{{File systems}}
{{Operating system}}
[[Category:Unix file system technology]]
[[Category:Linux kernel features]]
[[Category:Special-purpose file systems]]
[[Category:Interfaces of the Linux kernel]]
[[Category:Pseudo file systems supported by the Linux kernel]]