| Advanced Bash-Scripting Guide: A complete guide to shell scripting, using Bash | ||
|---|---|---|
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3.26. /dev and /proc
A Linux or UNIX machine typically has two special-purpose directories, /dev and /proc. The /dev directory contains entries for the physical devices that may or may not be present in the hardware. [1] 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 /dev/sndstat | grep Audio
Audio devices:
0: ESS ES1688 AudioDrive (rev 11) (3.01)
|
 | As of the 2.3 (and 2.4) kernel, /dev/sndstat has been deprecated. |
The /dev directory contains loopback devices, such as /dev/loop0. A loopback device is a gimmick allows an ordinary file to be accessed as if it were a block device. [2] This enables mounting an entire filesystem within a single large file. See Example 3-116 and Example 3-114.
Shell scripts may extract data from certain of the files in /dev and /proc. [3]
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 |
A few of the pseudo-devices in /dev have other specialized uses, such as /dev/zero.
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 3-156. Finding the process associated with a PID
1 #!/bin/bash
2 # Process id identifier:
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 $1" filters out this possibility.
22 if [ -z "$pidno" ] # If, after all the filtering, the result is a zero-length string,
23 then # no running process corresponds to the pid given.
24 echo "No such process running."
25 exit $E_NOSUCHPROCESS
26 fi
27
28 # Alternatively:
29 # if ! ps $1 > /dev/null 2>&1
30 # then # no running process corresponds to the pid given.
31 # echo "No such process running."
32 # exit $E_NOSUCHPROCESS
33 # fi
34
35
36 if [ ! -r "/proc/$1/$PROCFILE" ] # Check for read permission.
37 then
38 echo "Process $1 running, but..."
39 echo "Can't get read permission on /proc/$1/$PROCFILE."
40 exit $E_NOPERMISSION # Ordinary user can't access some files in /proc.
41 fi
42
43 # The last two tests may be replaced by:
44 # if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but
45 # this will test whether it is possible
46 # to send a signal to the process.
47 # then echo "PID doesn't exist or you're not its owner" >&2
48 # exit $E_BADPID
49 # fi
50
51
52
53 exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' )
54 # Or exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' )
55 #
56 # /proc/pid-number/exe is a symbolic link
57 # to the complete path name of the invoking process.
58
59 if [ -e "$exe_file" ] # If /proc/pid-number/exe exists...
60 then # the corresponding process exists.
61 echo "Process #$1 invoked by $exe_file."
62 else
63 echo "No such process running."
64 fi
65
66
67 # This elaborate script can *almost* be replaced by
68 # ps ax | grep $1 | awk '{ print $5 }'
69 # However, this will not work...
70 # because the fifth field of 'ps' is argv[0] of the process,
71 # not the executable file path.
72 #
73 # However, either of the following would work.
74 # find /proc/$1/exe -printf '%l\n'
75 # lsof -aFn -p $1 -d txt | sed -ne 's/^n//p'
76
77 # Additional commentary by Stephane Chazelas.
78
79 exit 0 |
Example 3-157. On-line connect status
1 #!/bin/bash
2
3 PROCNAME=pppd # ppp daemon
4 PROCFILENAME=status # Where to look.
5 NOTCONNECTED=65
6 INTERVAL=2 # Update every 2 seconds.
7
8 pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME | awk '{ print $1 }' )
9 # Finding the process number of 'pppd', the 'ppp daemon'.
10 # Have to filter out the process lines generated by the search itself.
11
12
13 if [ -z "$pidno" ] # If no pid, then process is not running.
14 then
15 echo "Not connected."
16 exit $NOTCONNECTED
17 else
18 echo "Connected."; echo
19 fi
20
21 while [ true ] # Endless loop, script can be improved here.
22 do
23
24 if [ ! -e "/proc/$pidno/$PROCFILENAME" ]
25 # While process running, then "status" file exists.
26 then
27 echo "Disconnected."
28 exit $NOTCONNECTED
29 fi
30
31 netstat -s | grep "packets received" # Get some connect statistics.
32 netstat -s | grep "packets delivered"
33
34
35 sleep $INTERVAL
36 echo; echo
37
38 done
39
40 exit 0
41
42 # As it stands, this script must be terminated with a Control-C.
43 # Exercises for the reader:
44 # Improve the script so it exits on a "q" keystroke.
45 # 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. |
Notes
| [1] | The entries in /dev provide mount points for physical and virtual devices. These entries use very little drive space. Some devices, such as /dev/null, /dev/zero, and /dev/urandom are virtual. They are not actual physical devices and exist only in software. |
| [2] | A block device reads and/or writes data in chunks, or blocks, in contrast to a character device, which acesses data in character units. Examples of block devices are a hard drive and CD ROM drive. An example of a character device is a keyboard. |
| [3] | Certain system commands, such as procinfo, free, and uptime do this as well. |