Linux Shell Scripting Tutorial (LSST) v1.05r3
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Chapter 4: Advanced Shell Scripting Commands
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trap command

Consider following script example:

$ cat > testsign
ls -R /

Save and run it as
$ chmod +x testsign
$ ./testsign

Now if you press ctrl + c , while running this script, script get terminated. The ctrl + c here work as signal, When such signal occurs its send to all process currently running in your system. Now consider following shell script:

$ cat > testsign1
#
# Why to trap signal, version 1
#
Take_input1()
{
 recno=0
 clear
 echo "Appointment Note keeper Application for Linux"
 echo -n "Enter your database file name : "
 read filename
if [ ! -f $filename ]; then
  echo "Sorry, $filename does not exit, Creating $filename database"
  echo "Appointment Note keeper Application database file" > $filename
fi
echo "Data entry start data: `date`" >/tmp/input0.$$
#
# Set a infinite loop
#
while :
do
     echo -n "Appointment Title:"
     read na
     echo -n "time :"
     read ti
     echo -n "Any Remark :"
     read remark
     echo -n "Is data okay (y/n) ?"
     read ans
if [ $ans = y -o $ans = Y ]; then
    recno=`expr $recno + 1`
    echo "$recno. $na $ti $remark" >> /tmp/input0.$$
fi
echo -n "Add next appointment (y/n)?"
read isnext
 if [ $isnext = n -o $isnext = N ]; then
     cat /tmp/input0.$$ >> $filename
     rm -f /tmp/input0.$$
    return # terminate loop
 fi
done
}
#
#
# Call our user define function : Take_input1
#
Take_input1

Save it and run as
$ chmod +x testsign1
$ ./testsign1

It first ask you main database file where all appointment of the day is stored, if no such database file found, file is created, after that it open one temporary file in /tmp directory, and puts today's date in that file. Then one infinite loop begins, which ask appointment title, time and remark, if this information is correct its written to temporary file, After that, script asks user , whether he/she wants to add next appointment record, if yes then next record is added , otherwise all records are copied from temporary file to database file and then loop will be terminated. You can view your database file by using cat command. Now problem is that while running this script, if you press CTRL + C, your shell script gets terminated and temporary file are left in /tmp directory. For e.g. try it as follows
$./testsign1
After given database file name and after adding at least one appointment record to temporary file press CTRL+C, Our script get terminated, and it left temporary file in /tmp directory, you can check this by giving command as follows
$ ls /tmp/input*
Our script needs to detect such signal (event) when occurs; To achieve this we have to first detect Signal using trap command.
Syntax:
trap {commands} {signal number list}

Signal NumberWhen occurs
0shell exit 
1hangup
2interrupt (CTRL+C)
3quit 
9kill (cannot be caught)

To catch signal in above script, put trap statement before calling Take_input1 function as trap del_file 2 ., Here trap command called del_file() when 2 number interrupt ( i.e. CTRL+C ) occurs. Open above script in editor and modify it so that at the end it will look like as follows:

$ vi testsign1
#
# signal is trapped to delete temporary file , version 2
#
del_file()
{
  echo "* * * CTRL + C Trap Occurs (removing temporary file)* * *"
  rm -f /tmp/input0.$$
  exit 1
}


Take_input1()
{
recno=0
clear
echo "Appointment Note keeper Application for Linux"
echo -n "Enter your database file name : "
read filename
if [ ! -f $filename ]; then
  echo "Sorry, $filename does not exit, Creating $filename database"
  echo "Appointment Note keeper Application database file" > $filename
fi
echo "Data entry start data: `date`" >/tmp/input0.$$
#
# Set a infinite loop
#
while :
do
  echo -n "Appointment Title:"
  read na
  echo -n "time :"
  read ti
  echo -n "Any Remark :"
  read remark
  echo -n "Is data okay (y/n) ?"
  read ans
  if [ $ans = y -o $ans = Y ]; then
   recno=`expr $recno + 1`
   echo "$recno. $na $ti $remark" >> /tmp/input0.$$
  fi
  echo -n "Add next appointment (y/n)?"
  read isnext
  if [ $isnext = n -o $isnext = N ]; then
    cat /tmp/input0.$$ >> $filename
    rm -f /tmp/input0.$$
    return # terminate loop
  fi
done # end_while
}
#
# Set trap to for CTRL+C interrupt i.e. Install our error handler
# When occurs it first it calls del_file() and then exit
#
trap del_file 2
#
# Call our user define function : Take_input1
#
Take_input1

Run the script as:
$ ./testsign1

After giving database file name and after giving appointment title press CTRL+C, Here we have already captured this CTRL + C signal (interrupt), so first our function del_file() is called, in which it gives message as "* * * CTRL + C Trap Occurs (removing temporary file)* * * " and then it remove our temporary file and then exit with exit status 1. Now check /tmp directory as follows
$ ls /tmp/input*
Now Shell will report no such temporary file exit.

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