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Fake Signals
Fake signals are more sophisticated set of debugging aids. They can be used in trap statements to get the shell to act under certain conditions. Recall from the previous chapter that trap allows you to install some code that runs when a particular signal is sent to your script.
Fake signals work in the same way, but they are generated by the shell itself, as opposed to the other signals which are generated externally. They represent runtime events that are likely to be of interest to debuggers—both human ones and software tools—and can be treated just like real signals within shell scripts. Table 9-2 lists the four fake signals available in bash.
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Table 9-2. Fake signals
Fake signal
Sent when
EXIT
The shell exits from script
ERR
A command returning a non-zero exit status
DEBUG
The shell has executed a statement[4]
RETURN
A shell function or a script executed with the . or source builtins finishes executing[5]
[4] The DEBUG signal is not available in bash versions prior to 2.0.
[5] The RETURN signal is not available in bash versions prior to 3.0.
EXIT
The EXIT trap, when set, will run its code whenever the script within which it was set exits.[6]
Here's a simple example:
trap 'echo exiting from the script' EXIT
echo 'start of the script'
If you run this script, you will see this output:
start of the script
exiting from the script
In other words, the script starts by setting the trap for its own exit, then prints a message. The script then exits, which causes the shell to generate the signal EXIT, which in turn runs the code echo exiting from the script.
An EXIT trap occurs no matter how the script exits—whether normally (by finishing the last statement), by an explicit exit or return statement, or by receiving a "real" signal such as INT or TERM. Consider this inane number-guessing program:
trap 'echo Thank you for playing!' EXIT
magicnum=$(($RANDOM%10+1))
echo 'Guess a number between 1 and 10:'
while read -p 'Guess: ' guess ; do
sleep 4
if [ "$guess" = $magicnum ]; then
echo 'Right!'
exit
fi
echo 'Wrong!'
done
This program picks a number between 1 and 10 by getting a random number (the built-in variable RANDOM), extracting the last digit (the remainder when divided by 10), and adding 1. Then it prompts you for a guess, and after 4 seconds, it will tell you if you guessed right.
If you did, the program will exit with the message, "Thank you for playing!", i.e., it will run the EXIT trap code. If you were wrong, it will prompt you again and repeat the process until you get it right. If you get bored with this little game and hit CTRL-C or CTRL-D while waiting for it to tell you whether you were right, you will also see the message.
The EXIT trap is especially useful when you want to print out the values of variables at the point that your script exits. For example, by printing the value of loop counter variables, you can find the most appropriate places in a complicated script, with many nested for loops, to enable xtrace or place debug output.
ERR
The fake signal ERR enables you to run code whenever a command in the surrounding script or function exits with non-zero status. Trap code for ERR can take advantage of the built-in variable ?, which holds the exit status of the previous command. It survives the trap and is accessible at the beginning of the trap-handling code.
A simple but effective use of this is to put the following code into a script you want to debug:
function errtrap {
es=$?
echo "ERROR: Command exited with status $es."
}
trap errtrap ERR
The first line saves the nonzero exit status in the local variable es.
For example, if the shell can't find a command, it returns status 127. If you put the code in a script with a line of gibberish (like "nhbdeuje"), the shell responds with:
scriptname: line N: nhbdeuje: command not found
ERROR: command exited with status 127.
N is the number of the line in the script that contains the bad command. In this case, the shell prints the line number as part of its own error-reporting mechanism, since the error was a command that the shell could not find. But if the nonzero exit status comes from another program, the shell doesn't report the line number. For example:
function errtrap {
es=$?
echo "ERROR: Command exited with status $es."
}
trap errtrap ERR
function bad {
return 17
}
bad
This only prints ERROR: Command exited with status 17.
It would obviously be an improvement to include the line number in this error message. The built-in variable LINENO exists, but if you use it inside a function, it evaluates to the line number in the function, not in the overall file. In other words, if you used $LINENO in the echo statement in the errtrap routine, it would always evaluate to 2.
To get around this problem, we simply pass $LINENO as an argument to the trap handler, surrounding it in single quotes so that it doesn't get evaluated until the fake signal actually comes in:
function errtrap {
es=$?
echo "ERROR line $1: Command exited with status $es."
}
trap 'errtrap $LINENO' ERR
...
If you use this with the above example, the result is the message, ERROR line 12: Command exited with status 17. This is much more useful. We'll see a variation on this technique shortly.
This simple code is actually not a bad all-purpose debugging mechanism. It takes into account that a nonzero exit status does not necessarily indicate an undesirable condition or event: remember that every control construct with a conditional (if, while, etc.) uses a nonzero exit status to mean "false." Accordingly, the shell doesn't generate ERR traps when statements or expressions in the "condition" parts of control structures produce nonzero exit statuses. Also, an ERR trap is not inherited by shell functions, command substitutions, and commands executed in a subshell. However this inheritance behaviour can be turned on by using set -o errtrace (or set -E).[7]
One disadvantage is that exit statuses are not as uniform (or even as meaningful) as they should be, as we explained in Chapter 5. A particular exit status need not say anything about the nature of the error or even that there was an error.
DEBUG
Another fake signal, DEBUG, causes the trap code to be executed before every statement in a function or script.[8] This has two main uses. First is the use for humans, as a sort of "brute force" method of tracking a certain element of a program's state that you notice has gone awry.
For example, you notice the value of a particular variable is running amok. The naive approach is to put in a lot of echo statements to check the variable's value at several points. The DEBUG trap makes this easier by letting you do this:
function dbgtrap
{
echo "badvar
is
$badvar"
}
trap dbgtrap DEBUG
...section of code in which the problem occurs...
trap - DEBUG # turn off the DEBUG trap
This code will print the value of the wayward variable before every statement between the two traps.
One important point to remember when using DEBUG is that it is not inherited by functions called from the shell in which it is set. In other words, if your shell sets a DEBUG trap and then calls a function, the statements within the function will not execute the trap. There are three ways around this. Firstly you can set a trap for DEBUG explicitly within the function. Alternately you can declare the function with the -t option which turns on debug inheritance in functions and allows a function to inherit a DEBUG trap from the caller. Lastly you can use set -o functrace (or set -T) which does the same thing as declare but applies to all functions.[9]
The second use of the DEBUG signal is as a primitive for implementing a bash debugger. We'll look at doing just that shortly.
RETURN
A RETURN trap is executed each time a shell function or a script executed with the . or source commands finishes executing.
As with DEBUG, the RETURN trap is not inherited by functions. You again have the options of setting the trap for RETURN within the function, declare the function with the -t option so that that function inherits the trap, or use set -o functrace to turn on the inheritance for all functions.
Here is a simple example of a RETURN trap:
function returntrap {
echo "A return occurred"
}
trap returntrap RETURN
function hello {
echo "hello world"
}
hello
When the script is executed it executes the hello function and then runs the trap:
$ ./returndemo
hello world
A return occurred
$
Notice that it didn't trap when the script itself finished. The trap would only have run at the end of the script if we'd sourced the script. Normally, to trap at the exiting of the script we'd also need to define a trap for the EXIT signal that we looked at earlier.
In addition to these fake signals, bash 3.0 added some other features to help with writing a full-scale debugger for bash. The first of these is the extdebug option to the shopt command, which switches on certain things that are useful for a debugger. These include:
- The -F option to declare displays the source filename and line number corresponding to each function name supplied as an argument.
- If the command that is run by the DEBUG trap returns a non-zero value, the next command is skipped and not executed.
- If the command run by the DEBUG trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the . or source commands), a call to return is simulated.
The shell also has a new option, —debugger, which switches on both the extdebug and functrace functionality.
