home | O'Reilly's CD bookshelfs | FreeBSD | Linux | Cisco | Cisco Exam  

Book HomeBook TitleSearch this book

Chapter 9. Debugging Shell Programs

We hope that we have convinced you that the Korn shell can be used as a serious Unix programming environment. It certainly has plenty of features, control structures, etc. But another essential part of a programming environment is a set of powerful, integrated support tools. For example, there is a wide assortment of screen editors, compilers, debuggers, profilers, cross-referencers, etc., for languages like C, C++ and Java. If you program in one of these languages, you probably take such tools for granted, and you would undoubtedly cringe at the thought of having to develop code with, say, the ed editor and the adb machine-language debugger.

But what about programming support tools for the Korn shell? Of course, you can use any editor you like, including vi and Emacs. And because the shell is an interpreted language, you don't need a compiler.[126] But there are no other tools available. The most serious problem is the lack of a debugger.

[126] Actually, if you are really concerned about efficiency, there are shell code compilers on the market; some convert shell scripts to C code that often runs quite a bit faster; however, these tools are usually for Bourne shell scripts. Other "compilers" simply convert the script into a binary form so that customers can't read the program.

This chapter addresses that lack. The shell does have a few features that help in debugging shell scripts; we'll see these in the first part of the chapter. The Korn shell also has a couple of new features, not present in most Bourne shells, that make it possible to implement a full-blown debugging tool. We show these features; more importantly, we present kshdb, a Korn shell debugger that uses them. kshdb is basic yet quite usable, and its implementation serves as an extended example of various shell programming techniques from throughout this book.

9.1. Basic Debugging Aids

What sort of functionality do you need to debug a program? At the most empirical level, you need a way of determining what is causing your program to behave badly and where the problem is in the code. You usually start with an obvious what (such as an error message, inappropriate output, infinite loop, etc.), try to work backwards until you find a what that is closer to the actual problem (e.g., a variable with a bad value, a bad option to a command), and eventually arrive at the exact where in your program. Then you can worry about how to fix it.

Notice that these steps represent a process of starting with obvious information and ending up with often obscure facts gleaned through deduction and intuition. Debugging aids make it easier to deduce and intuit by providing relevant information easily or even automatically, preferably without modifying your code.

The simplest debugging aid (for any language) is the output statement, print in the shell's case. Indeed, old-time programmers debugged their Fortran code by inserting WRITE cards into their decks. You can debug by putting lots of print statements in your code (and removing them later), but you will have to spend lots of time narrowing down not only what exact information you want but also where you need to see it. You will also probably have to wade through lots and lots of output to find the information that you really want.

9.1.1. Set Options

Luckily, the shell has a few basic features that give you debugging functionality beyond that of print. The most basic of these are options to the set -o command (as covered in Chapter 3). These options can also be used on the command line when running a script, as Table 9-1 shows.

The verbose option simply echoes (to standard error) whatever input the shell gets. It is useful for finding the exact point at which a script is bombing. For example, assume your script looks like this:


Table 9-1. Debugging options

set -o option

Command-line option


noexec -n

Don't run commands; check for syntax errors only

verbose -v

Echo commands before running them

xtrace -x

Echo commands after command-line processing

None of these commands are standard Unix programs, and they all do their work silently. Say the script crashes with a cryptic message like "segmentation violation." This tells you nothing about which command caused the error. If you type ksh -v scriptname, you might see this:

segmentation violation

Now you know that dave is the probable culprit -- though it is also possible that dave bombed because of something it expected fred or bob to do (e.g., create an input file) that they did incorrectly.

The xtrace option is more powerful: it echoes each command and its arguments, after the command has been through parameter substitution, command substitution, and the other steps of command-line processing (as listed in Chapter 7). If necessary, the output is quoted in such as a way as to allow it to be reused later as input to the shell.

Here is an example:

$ set -o xtrace
$ fred=bob
+ fred=bob
$ print "$fred"
+ print bob
$ ls -l $(whence emacs)
+ whence emacs
+ ls -l /usr/bin/emacs
-rwxr-xr-x    2 root     root      3471896 Mar 16 20:17 /usr/bin/emacs

As you can see, xtrace starts each line it prints with +. This is actually customizable: it's the value of the built-in shell variable PS4.[127] If you set PS4 to "xtrace-> " (e.g., in your .profile or environment file), you'll get xtrace listings that look like this:

[127] As with PS1 and PS3, this variable also undergoes parameter, command, and arithmetic substitution before its value is printed.

$ ls -l $(whence emacs)
xtrace-> whence emacs
xtrace-> ls -l /usr/bin/emacs
-rwxr-xr-x    2 root     root      3471896 Mar 16 20:17 /usr/bin/emacs

An even better way of customizing PS4 is to use a built-in variable we haven't seen yet: LINENO, which holds the number of the currently running line in a shell script. Put this line in your .profile or environment file:

PS4='line $LINENO: '

We use the same technique as we did with PS1 in Chapter 3: using single quotes to postpone the evaluation of the string until each time the shell prints the prompt. This prints messages of the form line N: in your trace output. You could even include the name of the shell script you're debugging in this prompt by using the positional parameter $0:

PS4='$0 line $LINENO: '

As another example, say you are trying to track down a bug in a script called fred that contains this code:

fndrs=$(cut -f3 -d' ' $dfbmq)

You type fred bob to run it in the normal way, and it hangs. Then you type ksh -x fred bob, and you see this:

+ dbfmq=bob.fmq
+ + cut -f3 -d

It hangs again at this point. You notice that cut doesn't have a filename argument, which means that there must be something wrong with the variable dbfmq. But it has executed the assignment statement dbfmq=bob.fmq properly... ah-hah! You made a typo in the variable name inside the command substitution construct.[128] You fix it, and the script works properly.

[128] We should admit that if you turned on the nounset option at the top of this script, the shell would have flagged this error.

When set at the global level, the xtrace option applies to the main script and to any POSIX-style functions (those created with the name () syntax). If the code you are trying to debug calls function-style functions that are defined elsewhere (e.g., in your .profile or environment file), you can trace through these in the same way with an option to the typeset command. Just enter the command typeset -ft functname, and the named function will be traced whenever it runs. Type typeset +ft functname to turn tracing off. You can also put set -o xtrace into the function body itself, which is good when the function is within the script being debugged.

The last option is noexec, which reads in the shell script and checks for syntax errors but doesn't execute anything. It's worth using if your script is syntactically complex (lots of loops, code blocks, string operators, etc.) and the bug has side effects (like creating a large file or hanging up the system).

You can turn on these options with set -o in your shell scripts, and, as explained in Chapter 3, turn them off with set +o option. For example, if you're debugging a script with a nasty side effect, and you have localized it to a certain chunk of code, you can precede that chunk with set -o xtrace (and, perhaps, close it with set +o xtrace) to watch it in more detail.

NOTE: The noexec option is a "one-way" option. Once turned on, you can't turn it off again! That's because the shell only prints commands and doesn't execute them. This includes the set +o noexec command you'd want to use to turn the option off. Fortunately, this only applies to shell scripts; the shell ignores this option when it's interactive.

9.1.2. Fake Signals

A more sophisticated set of debugging aids is the shell's "fake debugging signals," which 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 act like real ones, but they are generated by the shell (as opposed to real signals, which the underlying operating system generates). They represent runtime events that are likely to be interesting to debuggers -- both human ones and software tools -- and can be treated just like real signals within shell scripts. The four fake signals and their meanings are listed in Table 9-2.

Table 9-2. Fake signals

Fake signal When sent

The shell exits from a function or script


A command returns a non-zero exit status


Before every statement (after in ksh88)


When reading characters in the editing modes (not for debugging)

The KEYBD signal is not used for debugging. It is an advanced feature, for which we delay discussion until Chapter 10. EXIT

The EXIT trap, when set, runs its code when the function or script within which it was set exits. Here's a simple example:

function func {
    trap 'print "exiting from the function"' EXIT
    print 'start of the function'

trap 'print "exiting from the script"' EXIT
print 'start of the script'

If you run this script, you see this output:

start of the script
start of the function
exiting from the function
exiting from the script

In other words, the script starts by setting the trap for its own exit. Then it prints a message and finally calls the function. The function does the same -- sets a trap for its exit and prints a message. (Remember that function-style functions can have their own local traps that supersede any traps set by the surrounding script, while POSIX functions share traps with the main script.)

The function then exits, which causes the shell to send it the fake signal EXIT, which in turn runs the code print "exiting from the function". Then the script exits, and its own EXIT trap code is run. Note also that traps "stack;" the EXIT fake signal is sent to each running function in turn as each more recently called function exits.

An EXIT trap occurs no matter how the script or function 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 the following inane number-guessing program:

trap 'print "Thank you for playing!"' EXIT

print 'Guess a number between 1 and 10:'
while read guess'?number> '; do
    sleep 10
    if (( $guess == $magicnum )); then
        print 'Right!'
    print 'Wrong!'

This program picks a number between 1 and 10 by getting a random number (via the built-in variable RANDOM, see Appendix B), extracting the last digit (the remainder when divided by 10), and adding 1. Then it prompts you for a guess, and after 10 seconds, it tells you if you guessed right.

If you did, the program exits with the message, "Thank you for playing!", i.e., it runs the EXIT trap code. If you were wrong, it prompts you again and repeats the process until you get it right. If you get bored with this little game and hit CTRL-C while waiting for it to tell you whether you were right, you also see the message. 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 {
    typeset es=$?
    print "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 1. If you put the code in a script with a line of gibberish (like "lskdjfafd"), the shell responds with:

scriptname: line N: lskdjfafd:  not found
ERROR: command exited with status 1.

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 {
    typeset es=$?
    print "ERROR: Command exited with status $es."

trap errtrap ERR

function bad {
    return 17


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 print 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 {
    typeset es=$?
    print "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.

But a 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. Signal delivery order

It is possible for multiple signals to arrive simultaneously (or close to it). In that case, the shell runs the trap commands in the following order:

  1. DEBUG

  2. ERR

  3. Real Unix signals, in order of signal number

  4. EXIT

9.1.3. Discipline Functions

In Chapter 4, we introduced the Korn shell's compound variable notation, such as ${person.name}. Using this notation, ksh93 provides special functions, called discipline functions, that give you control over variables when they are referenced, assigned to, and unset. Simple versions of such functions might look like this:

dave=dave                       Create the variable
function dave.set {             Called when dave is assigned to
    print "dave just got assigned '${.sh.value}'"

function dave.get {             Called when $dave retrieved
    print "dave's value referenced, it's '$dave'"    # this is safe

    .sh.value="dave was here"   Change what $dave returns, dave not changed

function dave.unset {           Called when dave is unset
    print "goodbye dave!"
    unset dave   # actually make dave go away
NOTE: The unset discipline function must actually use the unset command to unset the variable -- this does not cause an infinite loop. Otherwise, the variable won't be unset, which in turn leads to very surprising behavior.

Here is what happens once all of these functions are in place:

$ print $dave
dave's value referenced, it's 'dave'                    From dave.get
dave was here                                           From print
$ dave='who is this dave guy, anyway?'
dave just got assigned 'who is this dave guy, anyway?'  From dave.set
$ unset dave
goodbye dave!                                           From dave.unset
$ print $dave


Discipline functions may only be applied to global variables. They may not be used with local variables -- those you create with typeset inside a function-style function.

Table 9-3 summarizes the built-in discipline functions.

Table 9-3. Predefined discipline functions

Name Purpose

Called when a variable's value is retrieved. Assigning to .sh.value changes the value returned but not the variable itself.


Called when a variable is assigned to. ${.sh.value} is the new value being assigned. Assigning to .sh.value changes the value being assigned.


Called when a variable is unset. This function must use unset on the variable to actually unset it.

As we've just seen, within the discipline functions, there are two special variables that the shell sets which give you information, as well as one variable that you can set to change how the shell behaves. Table 9-4 describes these variables and what they do.

Table 9-4. Special variables for use in discipline functions

Variable Purpose

The name of the variable for which the discipline function is being run.


The current subscript for an array variable. (The discipline functions apply to the entire array, not each subscripted element.)


The new value being assigned in a set discipline function. If assigned to in a get discipline function, changes the value returned.

At first glance, it's not clear what the value of discipline functions is. But they're perfect for implementing a very useful debugger feature, called watchpoints. We're now ready to get down to writing our shell script debugger.

Library Navigation Links

Copyright © 2003 O'Reilly & Associates. All rights reserved.