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  84                   <p>The Perl debugger is one of those valuable tools that surprisingly it seems few Perl coders know well. Here are some quick tips on using the Perl debugger. First a few explanations about commands I tend to use:
  85
  86 <blockquote>
  87 <dl>
  88   <dt>s</dt>
  89
  90   <dd>Single step. Step to the next statement stepping into any subroutines (where the source file is known and accessible).</dd>
  91
  92   <dt>n</dl>
  93
  94   <dd>Step over - if the line contains a call to a subroutine then this will step over that subroutine.</dd>
  95
  96   <dt>r</dt>
  97
  98   <dd>Return from subroutine - if, say you accidentally stepped into a subroutine or if you just want to return,</dd>
  99
 100   <dt>R</dt>
 101
 102   <dd>Rerun - start your Perl script again in the debugger with all the parms you started with.</dd>
 103
 104   <dt>q</dt>
 105
 106   <dd>quit</dd>
 107
 108   <dt>p &lt;variable or expression&gt;</dt>
 109
 110   <dd>Will print the contents of a variable or expression. Expressions can be Perl expressions including calls to subroutines. You can, for example, do "p 'There are " . scalar @foo . ' lines in foo';</dd>
 111
 112   <dt>x &lt;variable or expression&gt;</dt>
 113
 114   <dd>Like p above however p will simply print out HASH for hashes whereas x will format them out. Also x will print out "undef" for things that are undefined yet p will print nothing for them.</dd>
 115
 116   <dt>l (ell)</dt>
 117
 118   <dd>List the next windowSize lines (see below). Use "l &lt;n&gt;"  where &lt;n&gt; = a line number to list that line.</dd>
 119
 120   <dt>v &lt;n&gt;</dt>
 121
 122   <dd>View lines around &lt;n&gt;</dd>
 123
 124   <dt>V &lt;package&gt;</dt>
 125
 126   <dd>List exported subroutines and variables for &lt;package&gt; (e.g. V MyModule will is all stuff exported from MyModule).</dd>
 127
 128   <dt>f &lt;filename&gt;</dt>
 129
 130    <dd>File - switch to another file. (e.g. f MyModule) and the debugger switches to viewing MyModule.pm.</dd>
 131
 132   <dt>c &lt;n&gt;</dt>
 133
 134   <dd>Continue to line &lt;n&gt;. If n is not specified then just continue until the next break point or the end of the script. Continue is like setting a temporary break point that disappears when you hit the line.</dd>
 135
 136   <dt>b &lt;n&gt; &lt;condition&gt;</dt>
 137
 138   <dd>Breakpoint - set a break point (or b &lt;n&gt; $name eq "Donna" which will break at line &lt;n&gt; iff $name is "Donna" (evaluated when the debugger gets to line &lt;n&gt;))
 139 </dl>
 140 </blockquote>
 141
 142 <p>Also, at the Perl db prompt you can type in any Perl. So, for example, I often work out regex's that way. I'll be debugging a Perl script and stepping up to something like:</p>
 143
 144 <div class=code><pre>
 145      10==> if (/(\d*).*\s+/) {
 146      11      print "match!\n";
 147      12      $x = $1;
 148      13    }
 149 </pre></div>
 150
 151 <p>Then I'll type in stuff like:</p>
 152
 153 <div class=code><pre>
 154      DB<10> if (/(\d*).*\s+/) { print "1 = $1\n"; } else { print "No
 155      match!\n"; }
 156      No match!
 157      DB<11>
 158 </pre></div>
 159
 160 <p>Then I can use the command history (with set -o emacs at the shell before executing perl db emacs key bindings work for me) to edit and enter that perl if statement changing the regex until it works correctly. This way I know I got the right regex. Copy and paste the new, tested, regex from the debugging session into my code then "R" to reload the debugger.</p>
 161
 162 <p>Or you can say call an arbitrary subroutine in your script:</p>
 163
 164 <div class=code><pre>
 165        DB<2> b Rexec::ssh
 166        DB<3> p Rexec::ssh
 167 Rexec::ssh(/view/cmdt_x0062320/vobs/cmtools/src/misc/GPDB/bin/../../../../lib/perl/Rexec.pm:60):
 168      60:         my $self = shift;
 169        DB<<4>>
 170 </pre></div>
 171
 172 <p>The "p Rexec::ssh" says to print the results of the following expression. The expression is a function call in to the Rexec module for the subroutine ssh. Since we just set a break point there in the previous debug command we break at the start of that subroutine and can then debug it. Note you don't want to "c Rexec::ssh" because that would continue the actual execution of your script and only stop at Rexec::ssh if that routine was actually called. Viola, you just forcefully caused the Perl interpreter to branch to this routine!</p>
 173
 174 <p>Another thing I'll frequently do is set or change variables to see how the code would proceed <b>if</b> the variables were correct (or perhaps incorrect to test error conditions). So let's say a forced execution of the subroutine Log like the above:
 175
 176 <div class=code><pre>
 177 42      sub Log {
 178 43:==>    my $msg = shift;
 179 44        print "$msg\n";
 180
 181   DB<23> s
 182 main::Log(EvilTwin.pl:45):       print "$msg\n";
 183   DB<24>$msg = "Now I set msg to something I want it to be"
 184   DB<25>s
 185 Now I set msg to something I want it to be
 186 main::Log(EvilTwin.pl:47):              return;
 187   DB<25>
 188 </pre></div>
 189
 190 <p>There are all sorts of good reasons to examine (p $variable) and set ($variable = "new value") variables during debugging.</p>
 191
 192 <p>Finally put the following into ~/.perldb:</p>
 193
 194 <div class=code><pre>
 195      parse_options ("windowSize=23");
 196 </pre></div>
 197
 198 <p>This sets the window size to 23 so that 'l" lists the next 23 lines.</p>
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 547       <option value="http://defaria.com/blogs/Status/2005/11/">November 2005</option>
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 556
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 563       <option value="http://defaria.com/blogs/Status/2005/07/">July 2005</option>
 564
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 568
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 571       <option value="http://defaria.com/blogs/Status/2005/05/">May 2005</option>
 572
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 579       <option value="http://defaria.com/blogs/Status/2005/03/">March 2005</option>
 580
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 583       <option value="http://defaria.com/blogs/Status/2005/02/">February 2005</option>
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 587       <option value="http://defaria.com/blogs/Status/2005/01/">January 2005</option>
 588
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 591       <option value="http://defaria.com/blogs/Status/2004/12/">December 2004</option>
 592
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 595       <option value="http://defaria.com/blogs/Status/2004/09/">September 2004</option>
 596
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 599       <option value="http://defaria.com/blogs/Status/2004/08/">August 2004</option>
 600
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 603       <option value="http://defaria.com/blogs/Status/2004/07/">July 2004</option>
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 607       <option value="http://defaria.com/blogs/Status/2004/06/">June 2004</option>
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 636
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