Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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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]

       1 FS=iso                       # ISO filesystem support in kernel?
       2
       3 grep $FS /proc/filesystems   # iso9660

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

       1 CPU=$( awk '/model name/ {print $5}' < /proc/cpuinfo )
       2
       3 if [ "$CPU" = "Pentium(R)" ]
       4 then
       5   run_some_commands
       6   ...
       7 else
       8   run_other_commands
       9   ...
      10 fi
      11
      12
      13
      14 cpu_speed=$( fgrep "cpu MHz" /proc/cpuinfo | awk '{print $4}' )
      15 #  Current operating speed (in MHz) of the cpu on your machine.
      16 #  On a laptop this may vary, depending on use of battery
      17 #+ or AC power.

       1 #!/bin/bash
       2 # get-commandline.sh
       3 # Get the command-line parameters of a process.
       4
       5 OPTION=cmdline
       6
       7 # Identify PID.
       8 pid=$( echo $(pidof "$1") | awk '{ print $1 }' )
       9 # Get only first            ^^^^^^^^^^^^^^^^^^ of multiple instances.
      10
      11 echo
      12 echo "Process ID of (first instance of) "$1" = $pid"
      13 echo -n "Command-line arguments: "
      14 cat /proc/"$pid"/"$OPTION" | xargs -0 echo
      15 #   Formats output:        ^^^^^^^^^^^^^^^
      16 #   (Thanks, Han Holl, for the fixup!)
      17
      18 echo; echo
      19
      20
      21 # For example:
      22 # sh get-commandline.sh xterm

+

       1 devfile="/proc/bus/usb/devices"
       2 text="Spd"
       3 USB1="Spd=12"
       4 USB2="Spd=480"
       5
       6
       7 bus_speed=$(fgrep -m 1 "$text" $devfile | awk '{print $9}')
       8 #                 ^^^^ Stop after first match.
       9
      10 if [ "$bus_speed" = "$USB1" ]
      11 then
      12   echo "USB 1.1 port found."
      13   # Do something appropriate for USB 1.1.
      14 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

       1 #!/bin/bash
       2 # pid-identifier.sh:
       3 # Gives complete path name to process associated with pid.
       4
       5 ARGNO=1  # Number of arguments the script expects.
       6 E_WRONGARGS=65
       7 E_BADPID=66
       8 E_NOSUCHPROCESS=67
       9 E_NOPERMISSION=68
      10 PROCFILE=exe
      11
      12 if [ $# -ne $ARGNO ]
      13 then
      14   echo "Usage: `basename $0` PID-number" >&2  # Error message >stderr.
      15   exit $E_WRONGARGS
      16 fi
      17
      18 pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 )
      19 # Checks for pid in "ps" listing, field #1.
      20 # Then makes sure it is the actual process, not the process invoked by this script.
      21 # The last "grep $1" filters out this possibility.
      22 #
      23 #    pidno=$( ps ax | awk '{ print $1 }' | grep $1 )
      24 #    also works, as Teemu Huovila, points out.
      25
      26 if [ -z "$pidno" ]  #  If, after all the filtering, the result is a zero-length string,
      27 then                #+ no running process corresponds to the pid given.
      28   echo "No such process running."
      29   exit $E_NOSUCHPROCESS
      30 fi
      31
      32 # Alternatively:
      33 #   if ! ps $1 > /dev/null 2>&1
      34 #   then                # no running process corresponds to the pid given.
      35 #     echo "No such process running."
      36 #     exit $E_NOSUCHPROCESS
      37 #    fi
      38
      39 # To simplify the entire process, use "pidof".
      40
      41
      42 if [ ! -r "/proc/$1/$PROCFILE" ]  # Check for read permission.
      43 then
      44   echo "Process $1 running, but..."
      45   echo "Can't get read permission on /proc/$1/$PROCFILE."
      46   exit $E_NOPERMISSION  # Ordinary user can't access some files in /proc.
      47 fi
      48
      49 # The last two tests may be replaced by:
      50 #    if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but
      51                                       # this will test whether it is possible
      52                                       # to send a signal to the process.
      53 #    then echo "PID doesn't exist or you're not its owner" >&2
      54 #    exit $E_BADPID
      55 #    fi
      56
      57
      58
      59 exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' )
      60 # Or       exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' )
      61 #
      62 #  /proc/pid-number/exe is a symbolic link
      63 #+ to the complete path name of the invoking process.
      64
      65 if [ -e "$exe_file" ]  #  If /proc/pid-number/exe exists,
      66 then                   #+ then the corresponding process exists.
      67   echo "Process #$1 invoked by $exe_file."
      68 else
      69   echo "No such process running."
      70 fi
      71
      72
      73 #  This elaborate script can *almost* be replaced by
      74 #       ps ax | grep $1 | awk '{ print $5 }'
      75 #  However, this will not work...
      76 #+ because the fifth field of 'ps' is argv[0] of the process,
      77 #+ not the executable file path.
      78 #
      79 # However, either of the following would work.
      80 #       find /proc/$1/exe -printf '%l\n'
      81 #       lsof -aFn -p $1 -d txt | sed -ne 's/^n//p'
      82
      83 # Additional commentary by Stephane Chazelas.
      84
      85 exit 0

Example 29-4. On-line connect status

       1 #!/bin/bash
       2 # connect-stat.sh
       3 #  Note that this script may need modification
       4 #+ to work with a wireless connection.
       5
       6 PROCNAME=pppd        # ppp daemon
       7 PROCFILENAME=status  # Where to look.
       8 NOTCONNECTED=85
       9 INTERVAL=2           # Update every 2 seconds.
      10
      11 pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME |
      12 awk '{ print $1 }' )
      13
      14 # Finding the process number of 'pppd', the 'ppp daemon'.
      15 # Have to filter out the process lines generated by the search itself.
      16 #
      17 #  However, as Oleg Philon points out,
      18 #+ this could have been considerably simplified by using "pidof".
      19 #  pidno=$( pidof $PROCNAME )
      20 #
      21 #  Moral of the story:
      22 #+ When a command sequence gets too complex, look for a shortcut.
      23
      24
      25 if [ -z "$pidno" ]   # If no pid, then process is not running.
      26 then
      27   echo "Not connected."
      28 # exit $NOTCONNECTED
      29 else
      30   echo "Connected."; echo
      31 fi
      32
      33 while [ true ]       # Endless loop, script can be improved here.
      34 do
      35
      36   if [ ! -e "/proc/$pidno/$PROCFILENAME" ]
      37   # While process running, then "status" file exists.
      38   then
      39     echo "Disconnected."
      40 #   exit $NOTCONNECTED
      41   fi
      42
      43 netstat -s | grep "packets received"  # Get some connect statistics.
      44 netstat -s | grep "packets delivered"
      45
      46
      47   sleep $INTERVAL
      48   echo; echo
      49
      50 done
      51
      52 exit 0
      53
      54 # As it stands, this script must be terminated with a Control-C.
      55
      56 #    Exercises:
      57 #    ---------
      58 #    Improve the script so it exits on a "q" keystroke.
      59 #    Make the script more user-friendly in other ways.
      60 #    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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