Programming – Page 6 – Ryan Schulze

bash scripting … switching into directory of the script

November 23, 2009March 12, 2011Ryanbash, readlink, snippet

Sometimes it is useful to switch into the directory of the script e.g. when we need to call or include further files and don’t want to go through the hassle of searching for the script in the file system. Especially when symlinks are involved everything get a bit more interesting. This little snippet switches into the directory of the script, using readlink to dereference symlinks if the script is called via a symlink.

## a small snippet to switch into script directory
SCRIPTDIR=${0}
if [[ -L ${SCRIPTDIR} ]]
then
# dereference the symlink
SCRIPTDIR="$(readlink -e ${0})"
fi
# chop off the scriptname and switch into the directory
cd ${SCRIPTDIR%/*}

## a small snippet to switch into script directory

SCRIPTDIR=${0}

if [[ -L ${SCRIPTDIR} ]]

then

# dereference the symlink

SCRIPTDIR="$(readlink -e ${0})"

# chop off the scriptname and switch into the directory

cd ${SCRIPTDIR%/*}

Bash scripting, traps and sleep

October 9, 2009March 12, 2011Ryan1 Commentbackground, bash, kill, trap

Today I ran into any old problem: you have a script that should do something when it recieves a signal (e.g. if someone sends it USR1 it should write to a log/syslog), but the script uses a long sleep because it normally only checks/calculates stuff every x min. If you send it a kill -USR1 $pid it will normally execute the trap AFTER the sleep is done, not so great. I figured of the following solution today: put the sleep in a while loop that checks if the full time was slept, and inside the loop a sleep that sleeps the X seconds remaing in the background followed by a wait.

If the script now recieves a USR1 it can kill the wait, execute the trap and will continue the remaining sleep on the next iteration of the loop.

#!/bin/bash

initialize() {
set -o nounset
trap cleanup INT TERM EXIT
trap print_foo USR1
}

cleanup() {
exit 0
}

print_foo() {
echo "foo"
}

initialize

# sleep 1 min
SLEEPTILL=$(date '+%s' --date="+1 min")

while [[ $(date '+%s') -lt ${SLEEPTILL} ]]
do
sleep $(bc &lt;&lt;&lt; "${SLEEPTILL} - $(date '+%s')") &amp;
BACKGROUND=$!
wait $BACKGROUND
done

#!/bin/bash

initialize() {

set -o nounset

trap cleanup INT TERM EXIT

trap print_foo USR1

}

cleanup() {

exit 0

}

print_foo() {

echo "foo"

}

initialize

# sleep 1 min

SLEEPTILL=$(date '+%s' --date="+1 min")

while [[ $(date '+%s') -lt ${SLEEPTILL} ]]

sleep $(bc <<< "${SLEEPTILL} - $(date '+%s')") &

BACKGROUND=$!

wait $BACKGROUND

done

Reboot Script for Linksys WAP200 access points

September 15, 2009May 13, 2013Ryan7 Commentsbash, cisco, linksys, reboot

Since the Linksys WAP200 has a tendency to hang and not let any users connect anymore, this little script to reboot an accesspoint (webinterface still works fine). Replace user and password with correct values.

#!/bin/bash

if [[ "$#" == "1" ]]
then
nc -w1 -z $1 443
if [[ "$?" == "0" ]]
then
wget -q -O - --user=foouser --password=foopass --no-check-certificate --post-data='DeviceReboot=1&amp;page=AdminReboot.htm' https://${1}/reboot.cgi &gt;/dev/null
else
echo "Could not connect to https webinterface on ${1}"
fi
else
echo "Usage: $(basename $0) "
fi

#!/bin/bash

if [[ "$#" == "1" ]]

then

nc -w1 -z $1 443

if [[ "$?" == "0" ]]

then

wget -q -O - --user=foouser --password=foopass --no-check-certificate --post-data='DeviceReboot=1&page=AdminReboot.htm' https://${1}/reboot.cgi >/dev/null

else

echo "Could not connect to https webinterface on ${1}"

else

echo "Usage: $(basename $0) "

rrdtool tricks / tips

May 28, 2009May 13, 2013Ryan1 Commentbash, rrdtool

There are various different ways to generate rrdtool images. And most people will probably be generating elaborate ones out of perl or php scripts. The ones I am currently generating are built in a shell script (bash) and I was really having a problem with quotes and white space here for a while. If you generate the query and save it in a variable, the quotes go to hell and your white space (e.g. a space in a COMMENT) starts causing problems. my solution may not be the best, but it works without any trouble: Throw the whole command line into a temporary file and then use eval to create the graph. The following is not a complete statement, just a few lines as an example.

TmpFile="$(mktemp)"
echo -n "--vertical-label example --title \"Title with spaces\"  " &gt;&gt; ${TmpFile}
echo -n 'COMMENT:"           Comment with spaces\l" ' &gt;&gt; ${TmpFile}
eval "/usr/bin/rrdtool graph output.png $(cat ${TmpFile}) &gt;/dev/null"

TmpFile="$(mktemp)"

echo -n "--vertical-label example --title \"Title with spaces\" " >> ${TmpFile}

echo -n 'COMMENT:" Comment with spaces\l" ' >> ${TmpFile}

eval "/usr/bin/rrdtool graph output.png $(cat ${TmpFile}) >/dev/null"