8.8. Trapping Signals

While your program is running, if you press Ctrl-C or Ctrl-\ , your program terminates as soon as the signal arrives. There are times when you would rather not have the program terminate immediately after the signal arrives. You could arrange to ignore the signal and keep running or perform some sort of cleanup operation before actually exiting the script. The trap command allows you to control the way a program behaves when it receives a signal.

A signal is defined as an asynchronous message that consists of a number that can be sent from one process to another, or by the operating system to a process if certain keys are pressed or if something exceptional happens.^[4] The trap command tells the shell to terminate the command currently in execution upon the receipt of a signal. If the trap command is followed by commands within quotes, the command string will be executed upon receipt of a specified signal. The shell reads the command string twice, once when the trap is set, and again when the signal arrives. If the command string is surrounded by double quotes, all variable and command substitution will be performed when the trap is set the first time. If single quotes enclose the command string, variable and command substitution do not take place until the signal is detected and the trap is executed.

^[4] Morris I. Bolsky and David G. Korn, The New KornShell Command and Programming Language (Englewood Cliffs, NJ: Prentice Hall PTR, 1995), p. 327.

Use the command kill –l to get a list of all signals. Table 8.5 provides a list of signal numbers and their corresponding names.

Table 8.5. Signal Numbers and Signals^{[a], [b]}

1) HUP	12) SYS	23) POLL
2) INT	13) PIPE	24) XCPU
3) QUIT	14) ALRM	25) XFSZ
4) ILL	15) TERM	26) VTALRM
5) TRAP	16) URG	27) PROF
6) IOT	17) STOP	28) WINCH
7) EMT	18) TSTP	29) LOST
8) FPE	19) CONT	30) USR1
9) KILL	20) CHLD	31) USR2
10) BUS	21) TTIN
11) SEGV	22) TTOU

^[a] The output of the kill command may differ depending on the OS and/or the shell.

^[b] For a complete list of UNIX signals and their meanings, go to www.cybermagician.co.uk/technet/unixsignals.htm. For Linux signals, go to www.comptechdoc.org/os/linux/programming/linux_pgsignals.html.

FORMAT

trap 'command; command' signal-number

Example 8.52.

trap 'rm tmp*; exit 1'  1 2 15

EXPLANATION

When any of the signals 1 (hangup), 2 (interrupt), or 15 (software termination) arrive, remove all the tmp files and exit.

If an interrupt signal comes in while the script is running, the trap command lets you handle the signal in several ways. You can let the signal behave normally (default), ignore the signal, or create a handler function to be called when the signal arrives.

8.8.1 Resetting Signals

To reset a signal to its default behavior, the trap command is followed by the signal name or number.

Example 8.53.

trap 2

EXPLANATION

Resets the default action for signal 2, SIGINT, which is used to kill a process (i.e., Ctrl-C).

Example 8.54.

trap 'trap 2' 2

EXPLANATION

Sets the default action for signal 2 (SIGINT) to execute the command string within quotes when the signal arrives. The user must press Ctrl-C twice to terminate the program. The first trap catches the signal, the second trap resets the trap back to its default action, which is to kill the process.

8.8.2 Ignoring Signals

If the trap command is followed by a pair of empty quotes, the signals listed will be ignored by the process.

Example 8.55.

trap " "  1  2

EXPLANATION

Signals 1 and 2 will be ignored by the shell.

8.8.3 Listing Traps

To list all traps and the commands assigned to them, type trap.

Example 8.56.

(The Script)

     #/bin/sh

     # Scriptname: trapping

     # Script to illustrate the trap command and signals

1    trap 'echo "Ctrl–C will not terminate $0."' 2

2    trap 'echo "Ctrl–\ will not terminate $0."' 3

3    echo "Enter any string after the prompt."

     echo "When you are ready to exit, type \"stop\"."

4    while true

     do

         echo  –n "Go ahead...> "

         read reply

5        if [ "$reply" = stop ]

         then

6            break

         fi

7        done



(The Output)

Enter any string after the prompt.

When you are ready to exit, type "stop".

Go ahead...> this is it^C

Ctrl–C will not terminate trapping.

Go ahead...> this is never it ^\

Ctrl–\ will not terminate trapping.

Go ahead...> stop

$

EXPLANATION

8.8.4 Traps in Functions

If you use a trap to handle a signal in a function, it will affect the entire script, once the function is called. The trap is global to the script. In the following example, the trap is set to ignore the interrupt key, Ctrl-C. This script had to be killed with the kill command to stop the looping. It demonstrates potential undesirable side effects when using traps in functions.

Example 8.57.

(The Script)

     #!/bin/sh

1    trapper () {

        echo "In trapper"

2       trap 'echo "Caught in a trap!"' 2

        # Once set, this trap affects the entire script. Anytime

        # ^C is entered, the script will ignore it.

    }

3   while :

    do

        echo "In the main script"

4       trapper

5       echo "Still in main"

        sleep 5

    done



(The Output)

In the main script

In trapper

Still in main

^CCaught in a trap!

In the main script

In trapper

Still in main

^CCaught in a trap!

In the main script

EXPLANATION

8.8.5 Debugging

By using the –n option to the sh command, you can check the sytnax of your scripts without really executing any of the commands. If there is a syntax error in the script, the shell will report the error. If there are no errors, nothing is displayed.

The most commonly used method for debugging scripts is to use the set command with the –x option, or to use the –x option as an argument to the sh command, followed by the script name. See Table 8.6 for a list of debugging options. These options allow an execution trace of your script. Each command from your script is displayed after substitution has been performed, and then the command is executed. When a line from your script is displayed, it is preceded with a plus (+) sign.

Table 8.6. Debugging Options

Command	Option	What It Does
sh –x scriptname	Echo option	Displays each line of script after variable substitutions and before execution
sh –v scriptname	Verbose option	Displays each line of script before execution, just as you typed it
sh –n scriptname	Noexec option	Interprets but does not execute commands
set –x	Turns on echo	Traces execution in a script
set +x	Turns off echo	Turns off tracing

With the verbose option turned on, or by invoking the Bourne shell with the –v option (sh –v scriptname), each line of the script will be displayed just as it was typed in the script, and then executed.

Example 8.58.

(The Script)

     #!/bin/sh

1    # Scriptname: todebug

     name="Joe Blow"

     if  [ "$name" = "Joe Blow" ]

     then

         echo "Hi $name"

     fi

     num=1

     while [ $num -lt 5 ]

     do

          num=`expr $num + 1`

     done

     echo The grand total is $num



(The Command Line and Output)

2    $ sh –x todebug

     + name=Joe Blow

     + [ Joe Blow =  Joe Blow ]

     + echo Hi Joe Blow



     Hi Joe Blow

     num=1

     + [ 1 -lt 5 ]

     + expr 1 + 1

     num=2

     + [ 2 -lt 5 ]

     + expr 2 + 1

     num=3

     + [ 3 -lt 5 ]

     + expr 3 + 1

     num=4

     + [ 4 -lt 5 ]

     + expr 4 + 1

     num=5

     + [ 5 -lt 5 ]

     + echo The grand total is 5

     The grand total is 5

EXPLANATION