trap – Ryan Schulze

Bash snippet, verify ctrl+c

July 10, 2013August 27, 2013Ryanbash, ctrl+c, trap

Lately I’ve been working on a pair of more elaborate scripts using ncat and openssl to transfer data between hosts. I’ll get around to posting it eventually, but until then a few small snippets that people may find useful.

Today we will catch ctrl+c and ask the user if he really want’s to terminate the script.

#!/bin/bash

initialize() { #{{{
  # Treat unset variables as an error
  set -o nounset
  trap verify_quit INT # verify ending script
  trap cleanup TERM EXIT # clean up if script exits
} #}}} 

verify_quit() { #{{{
  local timeout=10
  echo "Press ctrl+c again within ${timeout} seconds to end script"
  sleep ${timeout}
  [[ $? -gt 0 ]] &amp;&amp; cleanup
} #}}}

cleanup() { #{{{
  exit 0
} #}}} 

initialize
while [ true ]
do 
  # do stuff
  sleep 10
done

#!/bin/bash

initialize() { #{{{

# Treat unset variables as an error

set -o nounset

trap verify_quit INT # verify ending script

trap cleanup TERM EXIT # clean up if script exits

} #}}}

verify_quit() { #{{{

local timeout=10

echo "Press ctrl+c again within ${timeout} seconds to end script"

sleep ${timeout}

[[ $? -gt 0 ]] && cleanup

} #}}}

cleanup() { #{{{

exit 0

} #}}}

initialize

while [ true ]

# do stuff

sleep 10

done

The initialize() and cleanup() are my usual function names I have in every script, making sure general settings and variables are defined and that on exit any tempfiles get deleted.
What has been added was a trap for the INT signal (ctrl+c) which calls the verify_quit() function, giving the user 10 seconds to press ctrl+c again to exit (via cleanup()) or return back to wherever we were in the code. There is one unavoidable caveat, the first ctrl+c will kill whatever the script was doing before it jumps into the verify_quit() function.

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