Classic Shell Scripting

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 ...