The Preamble

Now we'll look at the code that gets prepended to the guinea pig script; we call this the preamble. It's kept in the file bashdb.pre and looks like this:

# bashdb preamble
# This file gets prepended to the shell script being debugged.
# Arguments:
# $1 = the name of the original guinea pig script
# $2 = the directory where temporary files are stored
# $3 = the directory where bashdb.pre and bashdb.fns are stored
    
_debugfile=$0
_guineapig=$1
    
_tmpdir=$2
_libdir=$3
    
shift 3
    
source $_libdir/bashdb.fns
_linebp=
let _trace=0
let _i=1
    
while read; do
  _lines[$_i]=$REPLY
  let _i=$_i+1
done < $_guineapig
    
trap _cleanup EXIT
let _steps=1
trap '_steptrap $(( $LINENO -29 ))' DEBUG

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The first few lines save the three fixed arguments in variables and shift them out of the way, so that the positional parameters (if any) are those that the user supplied on the command line as arguments to the guinea pig. Then, the preamble reads in another file, bashdb.fns, which contains all of the functions necessary for the operation of the debugger itself. We put this code in a separate file to minimize the size of the temporary file. We'll examine bashdb.fns shortly.

Next, bashdb.pre initializes a breakpoint array to empty and execution tracing to off (see the following discussion), then reads the original guinea pig script into an array of lines. We need the source lines from the original script for two reasons: to allow the debugger to print out the script showing where the breakpoints are, and to print out the lines of code as they execute if tracing is turned on. You'll notice that we assign the script lines to _lines from the environment variable $REPLY rather than reading them into the array directly. This is because $REPLY preserves any leading whitespace in the lines, i.e., it preserves the indentation and layout of the original script.

The last five lines of code set up the conditions necessary for the debugger to begin working. The first trap command sets up a clean-up routine that runs when the fake signal EXIT occurs. The clean-up routine, normally called when the debugger and guinea pig script finish, just erases the temporary file. The next line sets the variable _steps to 1 so that when the debugger is first entered, it will stop after the first line.

The next line sets up the routine _steptrap to run when the fake signal DEBUG occurs.

The built-in variable LINENO, which we saw earlier in the chapter, is used to provide line numbers in the debugger. However, if we just used LINENO as is, we'd get line numbers above 30 because LINENO would be including the lines in the preamble. To get around this, we can pass LINENO minus the number of lines in the preamble to the trap.[13]