28.2. /proc
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. |