# man > proc(5) [_proc_(5)](https://www.chedong.com/phpMan.php/man/proc/5/markdown) File Formats Manual [_proc_(5)](https://www.chedong.com/phpMan.php/man/proc/5/markdown) ## NAME proc - process information, system information, and sysctl pseudo-filesystem ## DESCRIPTION The **proc **filesystem is a pseudo-filesystem which provides an interface to kernel data struc‐ tures. It is commonly mounted at _/proc_. Typically, it is mounted automatically by the sys‐ tem, but it can also be mounted manually using a command such as: mount -t proc proc /proc Most of the files in the **proc **filesystem are read-only, but some files are writable, allowing kernel variables to be changed. ### Mount options The **proc **filesystem supports the following mount options: **hidepid**=_n_ (since Linux 3.3) This option controls who can access the information in _/proc/_pid directories. The ar‐ gument, _n_, is one of the following values: 0 Everybody may access all _/proc/_pid directories. This is the traditional behavior, and the default if this mount option is not specified. 1 Users may not access files and subdirectories inside any _/proc/_pid directories but their own (the _/proc/_pid directories themselves remain visible). Sensitive files such as _/proc/_pid_/cmdline_ and _/proc/_pid_/status_ are now protected against other users. This makes it impossible to learn whether any user is running a specific program (so long as the program doesn't otherwise reveal itself by its behavior). 2 As for mode 1, but in addition the _/proc/_pid directories belonging to other users become invisible. This means that _/proc/_pid entries can no longer be used to dis‐ cover the PIDs on the system. This doesn't hide the fact that a process with a specific PID value exists (it can be learned by other means, for example, by "kill -0 $PID"), but it hides a process's UID and GID, which could otherwise be learned by employing [**stat**(2)](https://www.chedong.com/phpMan.php/man/stat/2/markdown) on a _/proc/_pid directory. This greatly complicates an at‐ tacker's task of gathering information about running processes (e.g., discovering whether some daemon is running with elevated privileges, whether another user is running some sensitive program, whether other users are running any program at all, and so on). **gid**=_gid_ (since Linux 3.3) Specifies the ID of a group whose members are authorized to learn process information otherwise prohibited by **hidepid **(i.e., users in this group behave as though _/proc_ was mounted with _hidepid=0_). This group should be used instead of approaches such as putting nonroot users into the [**sudoers**(5)](https://www.chedong.com/phpMan.php/man/sudoers/5/markdown) file. ### Overview Underneath _/proc_, there are the following general groups of files and subdirectories: _/proc/_pid subdirectories Each one of these subdirectories contains files and subdirectories exposing informa‐ tion about the process with the corresponding process ID. Underneath each of the _/proc/_pid directories, a _task_ subdirectory contains subdirecto‐ ries of the form _task/_tid, which contain corresponding information about each of the threads in the process, where _tid_ is the kernel thread ID of the thread. The _/proc/_pid subdirectories are visible when iterating through _/proc_ with [**getdents**(2)](https://www.chedong.com/phpMan.php/man/getdents/2/markdown) (and thus are visible when one uses [**ls**(1)](https://www.chedong.com/phpMan.php/man/ls/1/markdown) to view the contents of _/proc_). _/proc/_tid subdirectories Each one of these subdirectories contains files and subdirectories exposing informa‐ tion about the thread with the corresponding thread ID. The contents of these direc‐ tories are the same as the corresponding _/proc/_pid_/task/_tid directories. The _/proc/_tid subdirectories are _not_ visible when iterating through _/proc_ with **get‐** [**dents**(2)](https://www.chedong.com/phpMan.php/man/dents/2/markdown) (and thus are _not_ visible when one uses [**ls**(1)](https://www.chedong.com/phpMan.php/man/ls/1/markdown) to view the contents of _/proc_). _/proc/self_ When a process accesses this magic symbolic link, it resolves to the process's own _/proc/_pid directory. _/proc/thread-self_ When a thread accesses this magic symbolic link, it resolves to the process's own _/proc/self/task/_tid directory. _/proc/[a-z]*_ Various other files and subdirectories under _/proc_ expose system-wide information. All of the above are described in more detail below. ## NOTES Many files contain strings (e.g., the environment and command line) that are in the internal format, with subfields terminated by null bytes ('\0'). When inspecting such files, you may find that the results are more readable if you use a command of the following form to display them: $ **cat _**file_ **| tr '\000' '\n'** ## SEE ALSO [**cat**(1)](https://www.chedong.com/phpMan.php/man/cat/1/markdown), [**dmesg**(1)](https://www.chedong.com/phpMan.php/man/dmesg/1/markdown), [**find**(1)](https://www.chedong.com/phpMan.php/man/find/1/markdown), [**free**(1)](https://www.chedong.com/phpMan.php/man/free/1/markdown), [**htop**(1)](https://www.chedong.com/phpMan.php/man/htop/1/markdown), [**init**(1)](https://www.chedong.com/phpMan.php/man/init/1/markdown), [**ps**(1)](https://www.chedong.com/phpMan.php/man/ps/1/markdown), [**pstree**(1)](https://www.chedong.com/phpMan.php/man/pstree/1/markdown), [**tr**(1)](https://www.chedong.com/phpMan.php/man/tr/1/markdown), [**uptime**(1)](https://www.chedong.com/phpMan.php/man/uptime/1/markdown), **ch‐** [**root**(2)](https://www.chedong.com/phpMan.php/man/root/2/markdown), [**mmap**(2)](https://www.chedong.com/phpMan.php/man/mmap/2/markdown), [**readlink**(2)](https://www.chedong.com/phpMan.php/man/readlink/2/markdown), [**syslog**(2)](https://www.chedong.com/phpMan.php/man/syslog/2/markdown), [**slabinfo**(5)](https://www.chedong.com/phpMan.php/man/slabinfo/5/markdown), [**sysfs**(5)](https://www.chedong.com/phpMan.php/man/sysfs/5/markdown), [**hier**(7)](https://www.chedong.com/phpMan.php/man/hier/7/markdown), [**namespaces**(7)](https://www.chedong.com/phpMan.php/man/namespaces/7/markdown), [**time**(7)](https://www.chedong.com/phpMan.php/man/time/7/markdown), [**arp**(8)](https://www.chedong.com/phpMan.php/man/arp/8/markdown), [**hdparm**(8)](https://www.chedong.com/phpMan.php/man/hdparm/8/markdown), [**ifconfig**(8)](https://www.chedong.com/phpMan.php/man/ifconfig/8/markdown), [**lsmod**(8)](https://www.chedong.com/phpMan.php/man/lsmod/8/markdown), [**lspci**(8)](https://www.chedong.com/phpMan.php/man/lspci/8/markdown), [**mount**(8)](https://www.chedong.com/phpMan.php/man/mount/8/markdown), [**netstat**(8)](https://www.chedong.com/phpMan.php/man/netstat/8/markdown), [**procinfo**(8)](https://www.chedong.com/phpMan.php/man/procinfo/8/markdown), [**route**(8)](https://www.chedong.com/phpMan.php/man/route/8/markdown), [**sysctl**(8)](https://www.chedong.com/phpMan.php/man/sysctl/8/markdown) The Linux kernel source files: _Documentation/filesystems/proc.rst_, _Documentation/ad‐_ _min-guide/sysctl/fs.rst_, _Documentation/admin-guide/sysctl/kernel.rst_, _Documentation/ad‐_ _min-guide/sysctl/net.rst_, and _Documentation/admin-guide/sysctl/vm.rst_. Linux man-pages 6.7 2023-10-31 [_proc_(5)](https://www.chedong.com/phpMan.php/man/proc/5/markdown)