Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
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The /proc directory is actually a pseudo-filesystem. The files in the /proc directory 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 |
Shell scripts may extract data from certain of the files in /proc. [1]
bash$ cat /proc/filesystems | grep iso9660 iso9660 |
1 kernel_version=$( awk '{ print $3 }' /proc/version ) |
1 CPU=$( awk '/model name/ {print $4}' < /proc/cpuinfo ) 2 3 if [ $CPU = Pentium ] 4 then 5 run_some_commands 6 ... 7 else 8 run_different_commands 9 ... 10 fi |
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 28-1. Finding the process associated with a PID
1 #!/bin/bash 2 # pid-identifier.sh: Gives complete path name to process associated with pid. 3 4 ARGNO=1 # Number of arguments the script expects. 5 E_WRONGARGS=65 6 E_BADPID=66 7 E_NOSUCHPROCESS=67 8 E_NOPERMISSION=68 9 PROCFILE=exe 10 11 if [ $# -ne $ARGNO ] 12 then 13 echo "Usage: `basename $0` PID-number" >&2 # Error message >stderr. 14 exit $E_WRONGARGS 15 fi 16 17 pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 ) 18 # Checks for pid in "ps" listing, field #1. 19 # Then makes sure it is the actual process, not the process invoked by this script. 20 # The last "grep $1" filters out this possibility. 21 if [ -z "$pidno" ] # If, after all the filtering, the result is a zero-length string, 22 then # no running process corresponds to the pid given. 23 echo "No such process running." 24 exit $E_NOSUCHPROCESS 25 fi 26 27 # Alternatively: 28 # if ! ps $1 > /dev/null 2>&1 29 # then # no running process corresponds to the pid given. 30 # echo "No such process running." 31 # exit $E_NOSUCHPROCESS 32 # fi 33 34 # To simplify the entire process, use "pidof". 35 36 37 if [ ! -r "/proc/$1/$PROCFILE" ] # Check for read permission. 38 then 39 echo "Process $1 running, but..." 40 echo "Can't get read permission on /proc/$1/$PROCFILE." 41 exit $E_NOPERMISSION # Ordinary user can't access some files in /proc. 42 fi 43 44 # The last two tests may be replaced by: 45 # if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but 46 # this will test whether it is possible 47 # to send a signal to the process. 48 # then echo "PID doesn't exist or you're not its owner" >&2 49 # exit $E_BADPID 50 # fi 51 52 53 54 exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' ) 55 # Or exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' ) 56 # 57 # /proc/pid-number/exe is a symbolic link 58 # to the complete path name of the invoking process. 59 60 if [ -e "$exe_file" ] # If /proc/pid-number/exe exists... 61 then # the corresponding process exists. 62 echo "Process #$1 invoked by $exe_file." 63 else 64 echo "No such process running." 65 fi 66 67 68 # This elaborate script can *almost* be replaced by 69 # ps ax | grep $1 | awk '{ print $5 }' 70 # However, this will not work... 71 # because the fifth field of 'ps' is argv[0] of the process, 72 # not the executable file path. 73 # 74 # However, either of the following would work. 75 # find /proc/$1/exe -printf '%l\n' 76 # lsof -aFn -p $1 -d txt | sed -ne 's/^n//p' 77 78 # Additional commentary by Stephane Chazelas. 79 80 exit 0 |
Example 28-2. On-line connect status
1 #!/bin/bash 2 3 PROCNAME=pppd # ppp daemon 4 PROCFILENAME=status # Where to look. 5 NOTCONNECTED=65 6 INTERVAL=2 # Update every 2 seconds. 7 8 pidno=$( 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. |