9.2.1.1 The driver script

Therefore the code has two parts: the part that implements the debugger's functionality, and the part that installs that functionality into the script being debugged. The second part, which we'll see first, is the script called kshdb. It's very simple:

# kshdb -- Korn Shell debugger
# Main driver: constructs full script (with preamble) and runs it

print 'Korn Shell Debugger version 1.0\n'
_guineapig=$1
if [[ ! -r $1 ]]; then		# file not found or readable
    print "Cannot read $_guineapig." >&2
    exit 1
fi
shift

_tmpdir=/tmp
_libdir=.
_dbgfile=$_tmpdir/kshdb$$	# temp file for script being debugged (copy)
cat $_libdir/kshdb.pre $_guineapig > $_dbgfile
exec ksh $_dbgfile $_guineapig $_tmpdir $_libdir "$@"

kshdb takes as argument the name of the script being debugged, which for the sake of brevity we'll call the guinea pig. Any additional arguments will be passed to the guinea pig as its positional parameters.

If the argument is invalid (the file isn't readable), kshdb exits with error status. Otherwise, after an introductory message, it constructs a temporary filename in the way we saw in the last chapter. If you don't have (or don't have access to) /tmp on your system, then you can substitute a different directory for _tmpdir. [4] Also, make sure that _libdir is set to the directory where the kshdb.pre and kshdb.fns files (which we'll see soon) reside. /usr/lib is a good choice if you have access to it.

[4] All function names and variables (except those local to functions) in kshdb have names beginning with an underscore (_), to minimize the possibility of clashes with names in the guinea pig.

The cat statement builds the temp file: it consists of a file we'll see soon called kshdb.pre, which contains the actual debugger code, followed immediately by a copy of the guinea pig. Therefore the temp file contains a shell script that has been turned into a debugger for itself.

9.2.1.2 exec

The last line runs this script with exec, a statement we haven't seen yet. We've chosen to wait until now to introduce it because-as we think you'll agree-it can be dangerous. exec takes its arguments as a command line and runs the command in place of the current program, in the same process. In other words, the shell running the above script will terminate immediately and be replaced by exec's arguments. The situations in which you would want to use exec are few, far between, and quite arcane-though this is one of them. [5]

[5] exec can also be used with an I/O redirector only; this causes the redirector to take effect for the remainder of the script or login session. For example, the line exec 2>errlog at the top of a script directs standard error to the file errlog for the entire script.

In this case, exec just runs the newly-constructed shell script, i.e., the guinea pig with its debugger, in another Korn shell. It passes the new script three arguments-the names of the original guinea pig ($_guineapig), the temp directory ($_tmpdir), and the directory where kshdb.pre and kshdb.fns are kept-followed by the user's positional parameters, if any.

9.2.3.1 Commands

We'll explain shortly how _steptrap determines these things; now we'll look at _cmdloop. It's a typical command loop, resembling a combination of the case statements we saw in Chapter 5 and the calculator loop we saw in the previous chapter.

# Debugger command loop.
# Here at start of debugger session, when breakpoint reached,
# or after single-step.
function _cmdloop {
    typeset cmd args

    while read -s cmd"?kshdb> " args; do
        case $cmd in
            \*bp ) _setbp $args ;;  # set breakpoint at line num or string.

            \*bc ) _setbc $args ;;  # set break condition.

            \*cb ) _clearbp ;;      # clear all breakpoints.

            \*g  ) return ;;        # start/resume execution

            \*s  ) let _steps=${args:-1} # single-step N times (default 1) return ;;

            \*x  ) _xtrace ;;        # toggle execution trace

            \*\? | /*h ) _menu ;;    # print command menu        

            \*q  ) exit ;;           # quit

            \**  ) _msg "Invalid command: $cmd" ;; 

            *  ) eval $cmd $args ;;  # otherwise, run shell command

        esac
    done
}

At each iteration, cmdloop prints a prompt, reads a command, and processes it. We use read -s so that the user can take advantage of command-line editing within kshdb. All kshdb commands start with * to prevent confusion with shell commands. Anything that isn't a kshdb command (and doesn't start with *) is passed off to the shell for execution. Table 9.3 summarizes the debugger commmands.

Before we look at the individual commands, it is important that you understand how control passes through _steptrap, the command loop, and the guinea pig.

_steptrap runs after every statement in the guinea pig as a result of the trap ... DEBUG statement in the preamble. If a breakpoint has been reached or the user previously typed in a step command (*s), _steptrap calls the command loop. In doing so, it effectively "interrupts" the shell that is running the guinea pig to hand control over to the user. [6]

[6] In fact, low-level systems programmers can think of the entire trap mechanism as quite similar to an interrupt-handling scheme.

The user can invoke debugger commands as well as shell commands that run in the same shell as the guinea pig. This means that you can use shell commands to check values of variables, signal traps, and any other information local to the script being debugged.

The command loop runs, and the user stays in control, until the user types *g, *s, or *q. Let's look in detail at what happens in each of these cases.

*g has the effect of running the guinea pig uninterrupted until it finishes or hits a breakpoint. But actually, it simply exits the command loop and returns to _steptrap, which exits as well. The shell takes control back; it runs the next statement in the guinea pig script and calls _steptrap again. Assuming there is no breakpoint, this time _steptrap will just exit again, and the process will repeat until there is a breakpoint or the guinea pig is done.

9.2.3.2 Stepping

When the user types *s, the command loop code sets the variable _steps to the number of steps the user wants to execute, i.e., to the argument given. Assume at first that the user omits the argument, meaning that _steps is set to 1. Then the command loop exits and returns control to _steptrap, which (as above) exits and hands control back to the shell. The shell runs the next statement and returns to _steptrap, which sees that _steps is 1 and decrements it to 0. Then the second elif conditional sees that _steps is 0, so it prints a "stopped" message and calls the command loop.

Now assume that the user supplies an argument to *s, say 3. _steps is set to 3. Then the following happens:

1. After the next statement runs, _steptrap is called again. It enters the first if clause, since _steps is greater than 0. _steptrap decrements _steps to 2 and exits, returning control to the shell.

2. This process repeats, another step in the guinea pig is run, and _steps becomes 1.

3. A third statement is run and we're back in _steptrap. _steps is decremented to 0, the second elif clause is run, and _steptrap breaks out to the command loop again.

The overall effect is that three steps run and then the debugger takes over again.

Finally, the *q command calls the function _cleanup, which just erases the temp file and exits the entire program.

All other debugger commands (*bp, *bc, *cb, *x and shell commands) cause the shell to stay in the command loop, meaning that the user prolongs the "interruption" of the shell.

9.2.3.3 Breakpoints

Now we'll examine the breakpoint-related commands and the breakpoint mechanism in general. The *bp command calls the function _setbp, which can set two kinds of breakpoints, depending on the type of argument given. If it is a number, it's treated as a line number; otherwise it's interpreted as a string that the breakpoint line should contain.

For example, the command *bp 15 sets a breakpoint at line 15, and *bp grep sets a breakpoint at the next line that contains the string grep-whatever number that turns out to be. Although you can always look at a numbered listing of a file, [7] string arguments to *bp can make that unnecessary.

[7] pr -n filename prints a numbered listing to standard output on System V-derived versions of UNIX. Some older BSD-derived systems don't support it. If this doesn't work on your system, try cat -n filename, or if that doesn't work, create a shell script with this single line:

awk '{ print NR, "\t", $0 }' $1

Here is the code for _setbp:

# Set breakpoint(s) at given line numbers and/or strings
# by appending lines to breakpoint file
function _setbp {
    if [[ -z $1 ]]; then
        _listbp
    elif [[ $1 = +([0-9]) ]]; then  # number, set bp at that line
        _linebp="${_linebp}$1|"
        _msg "Breakpoint at line " $1
    else                            # string, set bp at next line w/string
        _stringbp="${_stringbp}$@|"
        _msg "Breakpoint at next line containing $@."
    fi
}

_setbp sets the breakpoints by storing them in the variables _linebp (line number breakpoints) and _stringbp (string breakpoints). Both have breakpoints separated by pipe character delimiters, for reasons that will become clear shortly. This implies that breakpoints are cumulative; setting new breakpoints does not erase the old ones.

The only way to remove breakpoints is with the command *cb, which (in function _clearbp) clears all of them at once by simply resetting the two variables to null. If you don't remember what breakpoints you have set, the command *bp without arguments lists them.

The functions _at_linenumbp and _at_stringbp are called by _steptrap after every statement; they check whether the shell has arrived at a line number or string breakpoint, respectively.

Here is _at_linenumbp:

# See if next line no. is a breakpoint.
function _at_linenumbp {
    [[ $_curline = @(${_linebp%\|}) ]]
}

_at_linenumbp takes advantage of the pipe character as the separator between line numbers: it constructs a regular expression of the form @(N1|N2|...) by taking the list of line numbers _linebp, removing the trailing |, and surrounding it with @( and ). For example, if $_linebp is 3|15|19|, then the resulting expression is @(3|15|19).

If the current line is any of these numbers, then the conditional becomes true, and _at_linenumbp also returns a "true" (0) exit status.

The check for a string breakpoint works on the same principle, but it's slightly more complicated; here is _at_stringbp:

# Search string breakpoints to see if next line in script matches.
function _at_stringbp {
    [[ -n $_stringbp && ${_lines[$_curline]} = *@(${_stringbp%\|})* ]]
}

The conditional first checks if $_stringbp is non-null (meaning that string breakpoints have been defined). If not, the conditional evaluates to false, but if so, its value depends on the pattern match after the &&-which tests the current line to see if it contains any of the breakpoint strings.

The expression on the right side of the equal sign is similar to the one in _at_linenumbp above, except that it has * before and after it. This gives expressions of the form *@(S1|S2|...)*, where the Ss are the string breakpoints. This expression matches any line that contains any one of the possibilities in the parenthesis.

The left side of the equal sign is the text of the current line in the guinea pig. So, if this text matches the regular expression, then we've reached a string breakpoint; accordingly, the conditional expression and _at_stringbp return exit status 0.

_steptrap uses the || ("or") construct in its if statement, which evaluates to true if either type of breakpoint occurred. If so, it calls the main command loop.

9.2.3.4 Break conditions

kshdb has another feature related to breakpoints: the break condition. This is a string that the user can specify that is evaluated as a command; if it is true (i.e., returns exit status 0), the debugger enters the command loop. Since the break condition can be any line of shell code, there's lots of flexibility in what can be tested. For example, you can break when a variable reaches a certain value (e.g., (( $x < 0 ))) or when a particular piece of text has been written to a file (grep string file). You will probably think of all kinds of uses for this feature. [8] To set a break condition, type *bc string. To remove it, type *bc without arguments-this installs the null string, which is ignored. _steptrap evaluates the break condition $_brcond only if it's non-null. If the break condition evaluates to 0, then the if clause is true and, once again, _steptrap calls the command loop.

[8] Bear in mind that if your break condition produces any standard output (or standard error), you will see it after every statement. Also, make sure your break condition doesn't take a long time to run; otherwise your script will run very, very slowly.

9.2.3.5 Execution tracing

The final feature is execution tracing, available through the *x command. This feature is meant to overcome the fact that a kshdb user can't use set -o xtrace while debugging (by entering it as a shell command), because its scope is limited to the _cmdloop function.

The function _xtrace "toggles" execution tracing by simply assigning to the variable _trace the logical "not" of its current value, so that it alternates between 0 (off) and 1 (on). The preamble initializes it to 0.

9.2.3.6 Limitations

kshdb was not designed to push the state of the debugger art forward or to have an overabundance of features. It has the most useful basic features, its implementation is compact and (we hope) comprehensible, and it does have some important limitations. The ones we know of are described in the list that follows.

1. The shell should really have the ability to trap before each statement, not after. This is the way most commercial source-code debuggers work. [9] At the very least, the shell should provide a variable that contains the number of the line about to run instead of (or in addition to) the number of the line that just ran.

   [9] This kind of functionality is expected to be added in the next Korn shell release.

2. String breakpoints cannot begin with digits or contain pipe characters (|) unless they are properly escaped.

3. You can only set breakpoints-whether line number or string-on lines in the guinea pig that contain what the shell's documentation calls simple commands, i.e., actual UNIX commands, shell built-ins, function calls, or aliases. If you set a breakpoint on a line that contains only whitespace or a comment, the shell will always skip over that breakpoint. More importantly, control keywords like while, if, for, do, done, and even conditionals ([[...]] and ((...))) won't work either, unless a simple command is on the same line.

4. kshdb will not "step down" into shell scripts that are called from the guinea pig. To do this, you have to edit your guinea pig and change a call to scriptname to kshdb scriptname.

5. Similarly, nested subshells are treated as one gigantic statement; you cannot step down into them at all.

6. The guinea pig should not trap on the fake signals DEBUG or EXIT; otherwise the debugger won't work.

7. Variables that are typeset (see Chapter 4, Basic Shell Programming) are not accessible in break conditions. However, you can use the shell command print to check their values.

8. Command error handling is weak. For example, a non-numeric argument to *s will cause it to bomb.

Many of these are not insurmountable; see the exercises.