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3.3. Directories within /proc/

Common groups of information concerning the kernel are grouped into directories and subdirectories within the /proc/ directory.

3.3.1. Process Directories

Every /proc/ directory contains a number of directories with numerical names. A listing of them may be similar to the following: 
dr-xr-xr-x    3 root     root            0 Feb 13 01:28 1 
dr-xr-xr-x    3 root     root            0 Feb 13 01:28 1010
dr-xr-xr-x    3 xfs      xfs             0 Feb 13 01:28 1087
dr-xr-xr-x    3 daemon   daemon          0 Feb 13 01:28 1123
dr-xr-xr-x    3 root     root            0 Feb 13 01:28 11307
dr-xr-xr-x    3 apache   apache          0 Feb 13 01:28 13660
dr-xr-xr-x    3 rpc      rpc             0 Feb 13 01:28 637
dr-xr-xr-x    3 rpcuser  rpcuser         0 Feb 13 01:28 666
These directories are called process directories, as they are named after a program's process ID and contain information specific to that process. The owner and group of each process directory is set to the user running the process. When the process is terminated, its /proc/ process directory vanishes.
Each process directory contains the following files:
  • cmdline — Contains the command issued when starting the process.
  • cwd — A symbolic link to the current working directory for the process.
  • environ — A list of the environment variables for the process. The environment variable is given in all upper-case characters, and the value is in lower-case characters.
  • exe — A symbolic link to the executable of this process.
  • fd — A directory containing all of the file descriptors for a particular process. These are given in numbered links: 
    total 0 
lrwx------    1 root     root           64 May  8 11:31 0 -> /dev/null
lrwx------    1 root     root           64 May  8 11:31 1 -> /dev/null
lrwx------    1 root     root           64 May  8 11:31 2 -> /dev/null
lrwx------    1 root     root           64 May  8 11:31 3 -> /dev/ptmx
lrwx------    1 root     root           64 May  8 11:31 4 -> socket:[7774817] 
lrwx------    1 root     root           64 May  8 11:31 5 -> /dev/ptmx
lrwx------    1 root     root           64 May  8 11:31 6 -> socket:[7774829] 
lrwx------    1 root     root           64 May  8 11:31 7 -> /dev/ptmx
  • maps — A list of memory maps to the various executables and library files associated with this process. This file can be rather long, depending upon the complexity of the process, but sample output from the sshd process begins like the following: 
    08048000-08086000 r-xp 00000000 03:03 391479     /usr/sbin/sshd
08086000-08088000 rw-p 0003e000 03:03 391479	/usr/sbin/sshd
08088000-08095000 rwxp 00000000 00:00 0
40000000-40013000 r-xp 0000000 03:03 293205	/lib/ld-2.2.5.so
40013000-40014000 rw-p 00013000 03:03 293205	/lib/ld-2.2.5.so 
40031000-40038000 r-xp 00000000 03:03 293282	/lib/libpam.so.0.75
40038000-40039000 rw-p 00006000 03:03 293282	/lib/libpam.so.0.75
40039000-4003a000 rw-p 00000000 00:00 0
4003a000-4003c000 r-xp 00000000 03:03 293218	/lib/libdl-2.2.5.so 
4003c000-4003d000 rw-p 00001000 03:03 293218	/lib/libdl-2.2.5.so
  • mem — The memory held by the process. This file cannot be read by the user.
  • root — A link to the root directory of the process.
  • stat — The status of the process.
  • statm — The status of the memory in use by the process. Below is a sample /proc/statm file: 
    263 210 210 5 0 205 0
    The seven columns relate to different memory statistics for the process. From left to right, they report the following aspects of the memory used:
    1. Total program size, in kilobytes.
    2. Size of memory portions, in kilobytes.
    3. Number of pages that are shared.
    4. Number of pages that are code.
    5. Number of pages of data/stack.
    6. Number of library pages.
    7. Number of dirty pages.
  • status — The status of the process in a more readable form than stat or statm. Sample output for sshd looks similar to the following: 
    Name:	sshd 
State:	S (sleeping) 
Tgid:	797 
Pid:	797 
PPid:	1 
TracerPid:	0 
Uid:	0	0	0	0 
Gid:	0	0	0	0 
FDSize:	32 
Groups:	 
VmSize:	    3072 kB 
VmLck:	       0 kB 
VmRSS:	     840 kB 
VmData:	     104 kB 
VmStk:	      12 kB 
VmExe:	     300 kB 
VmLib:	    2528 kB 
SigPnd:	0000000000000000 
SigBlk:	0000000000000000 
SigIgn:	8000000000001000 
SigCgt:	0000000000014005 
CapInh:	0000000000000000 
CapPrm:	00000000fffffeff 
CapEff:	00000000fffffeff
    The information in this output includes the process name and ID, the state (such as S (sleeping) or R (running)), user/group ID running the process, and detailed data regarding memory usage.

3.3.1.1. /proc/self/

The /proc/self/ directory is a link to the currently running process. This allows a process to look at itself without having to know its process ID.
Within a shell environment, a listing of the /proc/self/ directory produces the same contents as listing the process directory for that process.