The /proc Filesystem
Several Unix flavors have borrowed an idea developed at
Bell Labs: the /proc
filesystem.
Instead of supplying access to kernel data via myriad system calls that
need continual updating, kernel data is made available through a special
device driver that implements a standard filesystem interface in the
/proc
directory. Each running process
has a subdirectory there, named with the process number, and inside each
subdirectory are various small files with kernel data. The contents of
this filesystem are described in the manual pages for
proc(4) (most systems) or
proc(5) (GNU/Linux).
GNU/Linux has developed this idea more than most other Unix
flavors, and its ps command gets all
of the required process information by reading files under /proc
, which you can readily verify by running
a system-call trace with strace -e
trace=file ps aux
.
Here's an example of the process files for a text-editor session:
$ls /proc/16521
List proc files for process 16521 cmdline environ fd mem root statm cwd exe maps mounts stat status $ls -l /proc/16521
List them again, verbosely total 0 -r--r--r-- 1 jones devel 0 Oct 28 11:38 cmdline lrwxrwxrwx 1 jones devel 0 Oct 28 11:38 cwd -> /home/jones -r-------- 1 jones devel 0 Oct 28 11:38 environ lrwxrwxrwx 1 jones devel 0 Oct 28 11:38 exe -> /usr/bin/vi dr-x------ 2 jones devel 0 Oct 28 11:38 fd -r--r--r-- 1 jones devel 0 Oct 28 11:38 maps -rw------- 1 jones devel 0 Oct 28 11:38 mem -r--r--r-- 1 jones devel 0 Oct 28 11:38 mounts lrwxrwxrwx ...
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