Advanced Bash-Scripting Guide:
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# 29.2. `/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.

Example 29-3. 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                   #+ 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 29-4. On-line connect status

    #!/bin/bash
    # connect-stat.sh
    #  Note that this script may need modification
    #+ to work with a wireless connection.

    PROCNAME=pppd        # ppp daemon
    PROCFILENAME=status  # Where to look.
    NOTCONNECTED=85
    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.
    #    Fix the script to work with wireless/DSL connections.

| 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.

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