Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
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Prev | Chapter 27. /dev and /proc | Next |
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_different_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"/"$OPTION" 15 16 echo; echo 17 18 19 # For example: 20 # sh get-commandline.sh xterm |
+
1 devfile="/proc/bus/usb/devices" 2 text="Spd" 3 USB1="Spd=12" 4 USB2="Spd=480" 5 6 7 bus_speed=$(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.
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
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 $1 -d txt | sed -ne 's/^n//p' 82 83 # Additional commentary by Stephane Chazelas. 84 85 exit 0 |
Example 27-3. On-line connect status
1 #!/bin/bash 2 3 PROCNAME=pppd # ppp daemon 4 PROCFILENAME=status # Where to look. 5 NOTCONNECTED=65 6 INTERVAL=2 # Update every 2 seconds. 7 8 pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME | awk '{ print $1 }' ) 9 # Finding the process number of 'pppd', the 'ppp daemon'. 10 # Have to filter out the process lines generated by the search itself. 11 # 12 # However, as Oleg Philon points out, 13 #+ this could have been considerably simplified by using "pidof". 14 # pidno=$( pidof $PROCNAME ) 15 # 16 # Moral of the story: 17 #+ When a command sequence gets too complex, look for a shortcut. 18 19 20 if [ -z "$pidno" ] # If no pid, then process is not running. 21 then 22 echo "Not connected." 23 exit $NOTCONNECTED 24 else 25 echo "Connected."; echo 26 fi 27 28 while [ true ] # Endless loop, script can be improved here. 29 do 30 31 if [ ! -e "/proc/$pidno/$PROCFILENAME" ] 32 # While process running, then "status" file exists. 33 then 34 echo "Disconnected." 35 exit $NOTCONNECTED 36 fi 37 38 netstat -s | grep "packets received" # Get some connect statistics. 39 netstat -s | grep "packets delivered" 40 41 42 sleep $INTERVAL 43 echo; echo 44 45 done 46 47 exit 0 48 49 # As it stands, this script must be terminated with a Control-C. 50 51 # Exercises: 52 # --------- 53 # Improve the script so it exits on a "q" keystroke. 54 # 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. |
[1] | Certain system commands, such as procinfo, free, vmstat, lsdev, and uptime do this as well. |