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/proc

27.2. /proc

The /proc directory is actually a pseudo-filesystem. The files in /proc 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 /proc/interrupts
           CPU0       
   0:      84505          XT-PIC  timer
   1:       3375          XT-PIC  keyboard
   2:          0          XT-PIC  cascade
   5:          1          XT-PIC  soundblaster
   8:          1          XT-PIC  rtc
  12:       4231          XT-PIC  PS/2 Mouse
  14:     109373          XT-PIC  ide0
 NMI:          0 
 ERR:          0


bash$ cat /proc/partitions
major minor  #blocks  name     rio rmerge rsect ruse wio wmerge wsect wuse running use aveq

    3     0    3007872 hda 4472 22260 114520 94240 3551 18703 50384 549710 0 111550 644030
    3     1      52416 hda1 27 395 844 960 4 2 14 180 0 800 1140
    3     2          1 hda2 0 0 0 0 0 0 0 0 0 0 0
    3     4     165280 hda4 10 0 20 210 0 0 0 0 0 210 210
    ...



bash$ cat /proc/loadavg
0.13 0.42 0.27 2/44 1119



bash$ cat /proc/apm
1.16 1.2 0x03 0x01 0xff 0x80 -1% -1 ?



bash$ cat /proc/acpi/battery/BAT0/info
present:                 yes
 design capacity:         43200 mWh
 last full capacity:      36640 mWh
 battery technology:      rechargeable
 design voltage:          10800 mV
 design capacity warning: 1832 mWh
 design capacity low:     200 mWh
 capacity granularity 1:  1 mWh
 capacity granularity 2:  1 mWh
 model number:            IBM-02K6897
 serial number:            1133
 battery type:            LION
 OEM info:                Panasonic
         

Shell scripts may extract data from certain of the files in /proc. [1]

FS=iso                       # ISO filesystem support in kernel?

grep $FS /proc/filesystems   # iso9660

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

devfile="/proc/bus/usb/devices"
USB1="Spd=12"
USB2="Spd=480"


bus_speed=$(grep Spd $devfile | awk '{print $9}')

if [ "$bus_speed" = "$USB1" ]
then
  echo "USB 1.1 port found."
  # Do something appropriate for USB 1.1.
fi

Note

It is even possible to control certain peripherals with commands sent to the /proc directory.

	  root# echo on > /proc/acpi/ibm/light
          
This turns on the Thinklight in certain models of IBM/Lenovo Thinkpads.

Of course, caution is advised when writing to /proc.

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 27-2. Finding the process associated with a PID

#!/bin/bash
# pid-identifier.sh: 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 $1" filters out this possibility.
#
#    pidno=$( ps ax | awk '{ print $1 }' | grep $1 )
#    also works, as Teemu Huovila, points out.

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

# To simplify the entire process, use "pidof".


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 27-3. 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 $1 }' )
# Finding the process number of 'pppd', the 'ppp daemon'.
# Have to filter out the process lines generated by the search itself.
#
#  However, as Oleg Philon points out,
#+ this could have been considerably simplified by using "pidof".
#  pidno=$( pidof $PROCNAME )
#
#  Moral of the story:
#+ When a command sequence gets too complex, look for a shortcut.


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:
#    ---------
#    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.

Notes

[1]

Certain system commands, such as procinfo, free, vmstat, lsdev, and uptime do this as well.