Advanced Bash-Scripting Guide echo -n "$g$e2$d$c$d$c$a$g$n$g$e$n$g$e2$d$c$c$b$c$cis$n$cis$d \
$n$g$e2$d$c$d$c$a$g$n$g$e$n$g$a$d$c$b$a$b$c" > /dev/dsp
# dsp = Digital Signal Processor exit # A "bonny" example of an elegant shell script!
29.2.
/proc
The
/proc
directory is actually a pseudo-filesystem. The files in
/proc
mirror currently running system and
kernel processes and contain information and statistics about them.
bash$
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
...
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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
. [121]
FS=iso # ISO filesystem support in kernel?
grep $FS /proc/filesystems # iso9660 kernel_version=$( awk '{ print $3 }' /proc/version )
CPU=$( awk '/model name/ {print $5}' < /proc/cpuinfo ) if [ "$CPU" = "Pentium(R)" ] then
run_some_commands
...
else
run_other_commands
...
fi cpu_speed=$( fgrep "cpu MHz" /proc/cpuinfo | awk '{print $4}' )
# Current operating speed (in MHz) of the cpu on your machine.
# On a laptop this may vary, depending on use of battery
#+ or AC power.
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#!/bin/bash
# get-commandline.sh
# Get the command-line parameters of a process.
OPTION=cmdline
# Identify PID.
pid=$( echo $(pidof "$1") | awk '{ print $1 }' )
# Get only first ^^^^^^^^^^^^^^^^^^ of multiple instances.
echo echo "Process ID of (first instance of) "$1" = $pid" echo -n "Command-line arguments: " cat /proc/"$pid"/"$OPTION" | xargs -0 echo
# Formats output: ^^^^^^^^^^^^^^^
# (Thanks, Han Holl, for the fixup!) echo; echo
# For example:
# sh get-commandline.sh xterm
+ devfile="/proc/bus/usb/devices" text="Spd"
USB1="Spd=12"
USB2="Spd=480" bus_speed=$(fgrep -m 1 "$text" $devfile | awk '{print $9}')
# ^^^^ Stop after first match.
if [ "$bus_speed" = "$USB1" ] then
echo "USB 1.1 port found."
# Do something appropriate for USB 1.1.
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
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#!/bin/bash
# pid-identifier.sh:
# Gives complete path name to process associated with pid.
ARGNO=1 # Number of arguments the script expects.
E_WRONGARGS=65
E_BADPID=66
E_NOSUCHPROCESS=67
E_NOPERMISSION=68
PROCFILE=exe if [ $# -ne $ARGNO ] then
echo "Usage: `basename $0` PID-number" >&2 # Error message >stderr.
exit $E_WRONGARGS fi pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 )
# Checks for pid in "ps" listing, field #1.
# Then makes sure it is the actual process, not the process invoked by this script.
# The last "grep $1" filters out this possibility.
#
# pidno=$( ps ax | awk '{ print $1 }' | grep $1 )
# also works, as Teemu Huovila, points out.
if [ -z "$pidno" ] # If, after all the filtering, the result is a zero-length string, then #+ no running process corresponds to the pid given.
echo "No such process running."
exit $E_NOSUCHPROCESS fi
# Alternatively:
# if ! ps $1 > /dev/null 2>&1
# then # no running process corresponds to the pid given.
# echo "No such process running."
# exit $E_NOSUCHPROCESS
# fi
# To simplify the entire process, use "pidof".
if [ ! -r "/proc/$1/$PROCFILE" ] # Check for read permission.
then
echo "Process $1 running, but..."
echo "Can't get read permission on /proc/$1/$PROCFILE."
exit $E_NOPERMISSION # Ordinary user can't access some files in /proc.
fi
# The last two tests may be replaced by:
# if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but
# this will test whether it is possible
# to send a signal to the process.
# then echo "PID doesn't exist or you're not its owner" >&2
# exit $E_BADPID
# fi exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' )
# Or exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' )
#
# /proc/pid-number/exe is a symbolic link
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#+ to the complete path name of the invoking process.
if [ -e "$exe_file" ] # If /proc/pid-number/exe exists, then #+ then the corresponding process exists.
echo "Process #$1 invoked by $exe_file." else
echo "No such process running." fi
# This elaborate script can *almost* be replaced by
# ps ax | grep $1 | awk '{ print $5 }'
# However, this will not work...
#+ because the fifth field of 'ps' is argv[0] of the process,
#+ not the executable file path.
#
# However, either of the following would work.
# find /proc/$1/exe -printf '%l\n'
# lsof -aFn -p $1 -d txt | sed -ne 's/^n//p'
# Additional commentary by Stephane Chazelas.
exit 0
Example 29-4. On-line connect status
#!/bin/bash
# connect-stat.sh
# Note that this script may need modification
#+ to work with a wireless connection.
PROCNAME=pppd # ppp daemon
PROCFILENAME=status # Where to look.
NOTCONNECTED=85
INTERVAL=2 # Update every 2 seconds.
pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME | awk '{ print $1 }' )
# Finding the process number of 'pppd', the 'ppp daemon'.
# Have to filter out the process lines generated by the search itself.
#
# However, as Oleg Philon points out,
#+ this could have been considerably simplified by using "pidof".
# pidno=$( pidof $PROCNAME )
#
# Moral of the story:
#+ When a command sequence gets too complex, look for a shortcut.
if [ -z "$pidno" ] # If no pid, then process is not running.
then
echo "Not connected."
# exit $NOTCONNECTED else
echo "Connected."; echo fi while [ true ] # Endless loop, script can be improved here.
do
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if [ ! -e "/proc/$pidno/$PROCFILENAME" ]
# While process running, then "status" file exists.
then
echo "Disconnected."
# exit $NOTCONNECTED
fi netstat -s | grep "packets received" # Get some connect statistics.
netstat -s | grep "packets delivered"
sleep $INTERVAL
echo; echo done exit 0
# As it stands, this script must be terminated with a Control-C.
# Exercises:
# ---------
# Improve the script so it exits on a "q" keystroke.
# Make the script more user-friendly in other ways.
# 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.
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