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 bash$ fgrep Mem /proc/meminfo MemTotal: 515216 kB MemFree: 266248 kB

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 $5}' < /proc/cpuinfo ) if [ "$CPU" = "Pentium(R)" ] then run_some_commands ... else run_other_commands ... fi cpu_speed=$( fgrep "cpu MHz" /proc/cpuinfo | awk '{print $4}' ) # Current operating speed (in MHz) of the cpu on your machine. # On a laptop this may vary, depending on use of battery #+ or AC power.

#!/bin/bash # get-commandline.sh # Get the command-line parameters of a process. OPTION=cmdline # Identify PID. pid=$( echo $(pidof "$1") | awk '{ print $1 }' ) # Get only first ^^^^^^^^^^^^^^^^^^ of multiple instances. echo echo "Process ID of (first instance of) "$1" = $pid" echo -n "Command-line arguments: " cat /proc/"$pid"/"$OPTION" | xargs -0 echo # Formats output: ^^^^^^^^^^^^^^^ # (Thanks, Han Holl, for the fixup!) echo; echo # For example: # sh get-commandline.sh xterm

+

devfile="/proc/bus/usb/devices" text="Spd" USB1="Spd=12" USB2="Spd=480" bus_speed=$(fgrep -m 1 "$text" $devfile | awk '{print $9}') # ^^^^ Stop after first match. if [ "$bus_speed" = "$USB1" ] then echo "USB 1.1 port found." # Do something appropriate for USB 1.1. fi

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. (May not work on all Linux distros.) 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.

#!/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                   #+ 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

#!/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.

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