In unix/linux, what is the proc directory for? why does it exists?

Question

In unix/linux, what is the proc directory for? why does it exists?

What can I use it for (if not just for the system)?

Answer 1

• http://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard
• http://en.wikipedia.org/wiki/Procfs

procfs (or the proc filesystem) 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.

Examples:

• inside /proc, there is a directory for each running process, named with its process ID. These directories contain files that have useful information about the processes, such as
  • exe, which is a symbolic link to the file on disk the process was started from
  • cwd, which is a symbolic link to the working directory of the process
  • wchan, which, when read, returns the waiting channel the process is on
  • maps, which, when read, returns the memory maps of the process
• /proc/cmdline returns the command line passed to the running kernel
• /proc/cpuinfo returns information about the running CPUs
• /proc/uptime returns the uptime as two decimal values in seconds, separated by a space:
  • the amount of time since the kernel was started
  • the amount of time that the kernel has been idle
• /proc/version returns detailed information about the kernel; more than you'd get from uname -a

Answer 2

Its main use is for debuggers and other diagnostic programs. For example, "top" and "ps" examine its contents to determine what processes are running.

gdb and such like, also use proc (also ptrace system call) to examine the process's memory.

Additionally it provides some system-wide information (uptime, version etc) which may be useful for some programs.

And a bunch of other things - for example the "sysctl" kernel tunable parameters are in /proc/sys.

Answer 3

From the proc man page:

   The proc file system is a pseudo-file system which is used as an inter‐
   face to kernel data structures.  It is commonly mounted at /proc.  Most
   of  it  is  read-only,  but  some  files  allow  kernel variables to be
   changed.