Advanced Bash-Scripting Guide: A complete guide to shell scripting, using Bash
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proc

A Linux or UNIX machine typically has two special-purpose directories, /dev and /proc. The /dev directory contains entries for the physical devices that may or may not be present in the hardware. [1] The /proc directory is actually a pseudo-filesystem. The files in the /proc directory mirror currently running system and kernel processes and contain information and statistics about them.

bash$ cat /proc/devices
Character devices:
   1 mem
   2 pty
   3 ttyp
   4 ttyS
   5 cua
   7 vcs
  10 misc
  14 sound
  29 fb
  36 netlink
 128 ptm
 136 pts
 162 raw
 254 pcmcia

 Block devices:
   1 ramdisk
   2 fd
   3 ide0
   9 md

bash$ cat /dev/sndstat | grep Audio
Audio devices:
 0: ESS ES1688 AudioDrive (rev 11) (3.01)

Note: As of the 2.3 (and 2.4) kernel, /dev/sndstat has been deprecated.

The /dev directory contains loopback devices, such as /dev/loop0. A loopback device is a gimmick allows an ordinary file to be accessed as if it were a block device. [2] This enables mounting an entire filesystem within a single large file. See Example 3-116 and Example 3-114.

Shell scripts may extract data from certain of the files in /dev and /proc. [3]

kernel_version=$( awk '{ print $3 }' /proc/version )

CPU=$( awk '/model name/ {print $4}' < /proc/cpuinfo )

if [ $CPU = Pentium ]
then
  run_some_commands
  ...
else
  run_different_commands
  ...
fi

A few of the pseudo-devices in /dev have other specialized uses, such as /dev/zero.

The /proc directory contains subdirectories with unusual numerical names. Every one of these names maps to the process ID of a currently running process. Within each of these subdirectories, there are a number of files that hold useful information about the corresponding process. The stat and status files keep running statistics on the process, the cmdline file holds the command-line arguments the process was invoked with, and the exe file is a symbolic link to the complete path name of the invoking process. There are a few more such files, but these seem to be the most interesting from a scripting standpoint.

Example 3-156. Finding the process associated with a PID

#!/bin/bash
# Process id identifier:
# gives complete path name to process associated with pid.

ARGNO=1  # Number of arguments the script expects.
E_WRONGARGS=65
E_BADPID=66
E_NOSUCHPROCESS=67
E_NOPERMISSION=68
PROCFILE=exe

if [ $# -ne $ARGNO ]
then
  echo "Usage: `basename $0` PID-number" >&2  # Error message >stderr.
  exit $E_WRONGARGS
fi  

pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 )
# Checks for pid in "ps" listing, field #1.
# Then makes sure it is the actual process, not the process invoked by this script.
# The last "grep $1" filters out this possibility.
if [ -z "$pidno" ]  # If, after all the filtering, the result is a zero-length string,
then                # no running process corresponds to the pid given.
  echo "No such process running."
  exit $E_NOSUCHPROCESS
fi  

# Alternatively:
#   if ! ps $1 > /dev/null 2>&1
#   then                # no running process corresponds to the pid given.
#     echo "No such process running."
#     exit $E_NOSUCHPROCESS
#    fi


if [ ! -r "/proc/$1/$PROCFILE" ]  # Check for read permission.
then
  echo "Process $1 running, but..."
  echo "Can't get read permission on /proc/$1/$PROCFILE."
  exit $E_NOPERMISSION  # Ordinary user can't access some files in /proc.
fi  

# The last two tests may be replaced by:
#    if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but
                                      # this will test whether it is possible
                                      # to send a signal to the process.
#    then echo "PID doesn't exist or you're not its owner" >&2
#    exit $E_BADPID
#    fi



exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' )
# Or       exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' )
#
# /proc/pid-number/exe is a symbolic link
# to the complete path name of the invoking process.

if [ -e "$exe_file" ]  # If /proc/pid-number/exe exists...
then                 # the corresponding process exists.
  echo "Process #$1 invoked by $exe_file."
else
  echo "No such process running."
fi  


# This elaborate script can *almost* be replaced by
# ps ax | grep $1 | awk '{ print $5 }'
# However, this will not work...
# because the fifth field of 'ps' is argv[0] of the process,
# not the executable file path.
#
# However, either of the following would work.
#       find /proc/$1/exe -printf '%l\n'
#       lsof -aFn -p $1 -d txt | sed -ne 's/^n//p'

# Additional commentary by Stephane Chazelas.

exit 0

Example 3-157. On-line connect status

#!/bin/bash

PROCNAME=pppd        # ppp daemon
PROCFILENAME=status  # Where to look.
NOTCONNECTED=65
INTERVAL=2  # Update every 2 seconds.

pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME | awk '{ print $1 }' )
# Finding the process number of 'pppd', the 'ppp daemon'.
# Have to filter out the process lines generated by the search itself.


if [ -z "$pidno" ]  # If no pid, then process is not running.
then
  echo "Not connected."
  exit $NOTCONNECTED
else
  echo "Connected."; echo
fi

while [ true ]  # Endless loop, script can be improved here.
do

  if [ ! -e "/proc/$pidno/$PROCFILENAME" ]
  # While process running, then "status" file exists.
  then
    echo "Disconnected."
    exit $NOTCONNECTED
  fi

netstat -s | grep "packets received"  # Get some connect statistics.
netstat -s | grep "packets delivered"


  sleep $INTERVAL
  echo; echo

done

exit 0

# As it stands, this script must be terminated with a Control-C.
# Exercises for the reader:
#    Improve the script so it exits on a "q" keystroke.
#    Make the script more user-friendly in other ways.

Warning

In general, it is dangerous to write to the files in /proc, as this can corrupt the filesystem or crash the machine.

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3.26. /dev and /proc

Notes