1 | =head1 NAME
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2 |
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3 | perldebtut - Perl debugging tutorial
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4 |
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5 | =head1 DESCRIPTION
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6 |
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7 | A (very) lightweight introduction in the use of the perl debugger, and a
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8 | pointer to existing, deeper sources of information on the subject of debugging
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9 | perl programs.
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10 |
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11 | There's an extraordinary number of people out there who don't appear to know
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12 | anything about using the perl debugger, though they use the language every
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13 | day.
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14 | This is for them.
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15 |
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16 |
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17 | =head1 use strict
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18 |
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19 | First of all, there's a few things you can do to make your life a lot more
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20 | straightforward when it comes to debugging perl programs, without using the
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21 | debugger at all. To demonstrate, here's a simple script, named "hello", with
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22 | a problem:
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23 |
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24 | #!/usr/bin/perl
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25 |
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26 | $var1 = 'Hello World'; # always wanted to do that :-)
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27 | $var2 = "$varl\n";
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28 |
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29 | print $var2;
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30 | exit;
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31 |
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32 | While this compiles and runs happily, it probably won't do what's expected,
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33 | namely it doesn't print "Hello World\n" at all; It will on the other hand do
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34 | exactly what it was told to do, computers being a bit that way inclined. That
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35 | is, it will print out a newline character, and you'll get what looks like a
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36 | blank line. It looks like there's 2 variables when (because of the typo)
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37 | there's really 3:
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38 |
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39 | $var1 = 'Hello World';
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40 | $varl = undef;
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41 | $var2 = "\n";
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42 |
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43 | To catch this kind of problem, we can force each variable to be declared
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44 | before use by pulling in the strict module, by putting 'use strict;' after the
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45 | first line of the script.
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46 |
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47 | Now when you run it, perl complains about the 3 undeclared variables and we
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48 | get four error messages because one variable is referenced twice:
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49 |
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50 | Global symbol "$var1" requires explicit package name at ./t1 line 4.
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51 | Global symbol "$var2" requires explicit package name at ./t1 line 5.
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52 | Global symbol "$varl" requires explicit package name at ./t1 line 5.
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53 | Global symbol "$var2" requires explicit package name at ./t1 line 7.
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54 | Execution of ./hello aborted due to compilation errors.
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55 |
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56 | Luvverly! and to fix this we declare all variables explicitly and now our
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57 | script looks like this:
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58 |
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59 | #!/usr/bin/perl
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60 | use strict;
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61 |
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62 | my $var1 = 'Hello World';
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63 | my $varl = undef;
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64 | my $var2 = "$varl\n";
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65 |
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66 | print $var2;
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67 | exit;
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68 |
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69 | We then do (always a good idea) a syntax check before we try to run it again:
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70 |
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71 | > perl -c hello
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72 | hello syntax OK
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73 |
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74 | And now when we run it, we get "\n" still, but at least we know why. Just
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75 | getting this script to compile has exposed the '$varl' (with the letter 'l')
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76 | variable, and simply changing $varl to $var1 solves the problem.
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77 |
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78 |
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79 | =head1 Looking at data and -w and v
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80 |
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81 | Ok, but how about when you want to really see your data, what's in that
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82 | dynamic variable, just before using it?
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83 |
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84 | #!/usr/bin/perl
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85 | use strict;
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86 |
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87 | my $key = 'welcome';
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88 | my %data = (
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89 | 'this' => qw(that),
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90 | 'tom' => qw(and jerry),
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91 | 'welcome' => q(Hello World),
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92 | 'zip' => q(welcome),
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93 | );
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94 | my @data = keys %data;
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95 |
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96 | print "$data{$key}\n";
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97 | exit;
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98 |
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99 | Looks OK, after it's been through the syntax check (perl -c scriptname), we
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100 | run it and all we get is a blank line again! Hmmmm.
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101 |
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102 | One common debugging approach here, would be to liberally sprinkle a few print
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103 | statements, to add a check just before we print out our data, and another just
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104 | after:
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105 |
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106 | print "All OK\n" if grep($key, keys %data);
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107 | print "$data{$key}\n";
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108 | print "done: '$data{$key}'\n";
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109 |
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110 | And try again:
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111 |
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112 | > perl data
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113 | All OK
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114 |
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115 | done: ''
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116 |
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117 | After much staring at the same piece of code and not seeing the wood for the
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118 | trees for some time, we get a cup of coffee and try another approach. That
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119 | is, we bring in the cavalry by giving perl the 'B<-d>' switch on the command
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120 | line:
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121 |
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122 | > perl -d data
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123 | Default die handler restored.
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124 |
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125 | Loading DB routines from perl5db.pl version 1.07
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126 | Editor support available.
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127 |
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128 | Enter h or `h h' for help, or `man perldebug' for more help.
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129 |
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130 | main::(./data:4): my $key = 'welcome';
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131 |
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132 | Now, what we've done here is to launch the built-in perl debugger on our
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133 | script. It's stopped at the first line of executable code and is waiting for
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134 | input.
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135 |
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136 | Before we go any further, you'll want to know how to quit the debugger: use
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137 | just the letter 'B<q>', not the words 'quit' or 'exit':
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138 |
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139 | DB<1> q
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140 | >
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141 |
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142 | That's it, you're back on home turf again.
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143 |
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144 |
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145 | =head1 help
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146 |
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147 | Fire the debugger up again on your script and we'll look at the help menu.
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148 | There's a couple of ways of calling help: a simple 'B<h>' will get the summary
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149 | help list, 'B<|h>' (pipe-h) will pipe the help through your pager (which is
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150 | (probably 'more' or 'less'), and finally, 'B<h h>' (h-space-h) will give you
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151 | the entire help screen. Here is the summary page:
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152 |
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153 | DB<1>h
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154 |
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155 | List/search source lines: Control script execution:
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156 | l [ln|sub] List source code T Stack trace
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157 | - or . List previous/current line s [expr] Single step [in expr]
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158 | v [line] View around line n [expr] Next, steps over subs
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159 | f filename View source in file <CR/Enter> Repeat last n or s
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160 | /pattern/ ?patt? Search forw/backw r Return from subroutine
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161 | M Show module versions c [ln|sub] Continue until position
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162 | Debugger controls: L List break/watch/actions
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163 | o [...] Set debugger options t [expr] Toggle trace [trace expr]
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164 | <[<]|{[{]|>[>] [cmd] Do pre/post-prompt b [ln|event|sub] [cnd] Set breakpoint
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165 | ! [N|pat] Redo a previous command B ln|* Delete a/all breakpoints
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166 | H [-num] Display last num commands a [ln] cmd Do cmd before line
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167 | = [a val] Define/list an alias A ln|* Delete a/all actions
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168 | h [db_cmd] Get help on command w expr Add a watch expression
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169 | h h Complete help page W expr|* Delete a/all watch exprs
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170 | |[|]db_cmd Send output to pager ![!] syscmd Run cmd in a subprocess
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171 | q or ^D Quit R Attempt a restart
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172 | Data Examination: expr Execute perl code, also see: s,n,t expr
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173 | x|m expr Evals expr in list context, dumps the result or lists methods.
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174 | p expr Print expression (uses script's current package).
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175 | S [[!]pat] List subroutine names [not] matching pattern
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176 | V [Pk [Vars]] List Variables in Package. Vars can be ~pattern or !pattern.
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177 | X [Vars] Same as "V current_package [Vars]".
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178 | y [n [Vars]] List lexicals in higher scope <n>. Vars same as V.
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179 | For more help, type h cmd_letter, or run man perldebug for all docs.
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180 |
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181 | More confusing options than you can shake a big stick at! It's not as bad as
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182 | it looks and it's very useful to know more about all of it, and fun too!
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183 |
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184 | There's a couple of useful ones to know about straight away. You wouldn't
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185 | think we're using any libraries at all at the moment, but 'B<M>' will show
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186 | which modules are currently loaded, and their version number, while 'B<m>'
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187 | will show the methods, and 'B<S>' shows all subroutines (by pattern) as
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188 | shown below. 'B<V>' and 'B<X>' show variables in the program by package
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189 | scope and can be constrained by pattern.
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190 |
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191 | DB<2>S str
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192 | dumpvar::stringify
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193 | strict::bits
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194 | strict::import
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195 | strict::unimport
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196 |
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197 | Using 'X' and cousins requires you not to use the type identifiers ($@%), just
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198 | the 'name':
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199 |
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200 | DM<3>X ~err
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201 | FileHandle(stderr) => fileno(2)
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202 |
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203 | Remember we're in our tiny program with a problem, we should have a look at
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204 | where we are, and what our data looks like. First of all let's view some code
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205 | at our present position (the first line of code in this case), via 'B<v>':
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206 |
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207 | DB<4> v
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208 | 1 #!/usr/bin/perl
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209 | 2: use strict;
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210 | 3
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211 | 4==> my $key = 'welcome';
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212 | 5: my %data = (
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213 | 6 'this' => qw(that),
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214 | 7 'tom' => qw(and jerry),
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215 | 8 'welcome' => q(Hello World),
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216 | 9 'zip' => q(welcome),
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217 | 10 );
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218 |
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219 | At line number 4 is a helpful pointer, that tells you where you are now. To
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220 | see more code, type 'v' again:
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221 |
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222 | DB<4> v
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223 | 8 'welcome' => q(Hello World),
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224 | 9 'zip' => q(welcome),
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225 | 10 );
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226 | 11: my @data = keys %data;
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227 | 12: print "All OK\n" if grep($key, keys %data);
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228 | 13: print "$data{$key}\n";
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229 | 14: print "done: '$data{$key}'\n";
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230 | 15: exit;
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231 |
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232 | And if you wanted to list line 5 again, type 'l 5', (note the space):
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233 |
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234 | DB<4> l 5
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235 | 5: my %data = (
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236 |
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237 | In this case, there's not much to see, but of course normally there's pages of
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238 | stuff to wade through, and 'l' can be very useful. To reset your view to the
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239 | line we're about to execute, type a lone period '.':
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240 |
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241 | DB<5> .
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242 | main::(./data_a:4): my $key = 'welcome';
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243 |
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244 | The line shown is the one that is about to be executed B<next>, it hasn't
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245 | happened yet. So while we can print a variable with the letter 'B<p>', at
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246 | this point all we'd get is an empty (undefined) value back. What we need to
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247 | do is to step through the next executable statement with an 'B<s>':
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248 |
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249 | DB<6> s
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250 | main::(./data_a:5): my %data = (
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251 | main::(./data_a:6): 'this' => qw(that),
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252 | main::(./data_a:7): 'tom' => qw(and jerry),
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253 | main::(./data_a:8): 'welcome' => q(Hello World),
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254 | main::(./data_a:9): 'zip' => q(welcome),
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255 | main::(./data_a:10): );
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256 |
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257 | Now we can have a look at that first ($key) variable:
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258 |
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259 | DB<7> p $key
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260 | welcome
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261 |
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262 | line 13 is where the action is, so let's continue down to there via the letter
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263 | 'B<c>', which by the way, inserts a 'one-time-only' breakpoint at the given
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264 | line or sub routine:
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265 |
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266 | DB<8> c 13
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267 | All OK
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268 | main::(./data_a:13): print "$data{$key}\n";
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269 |
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270 | We've gone past our check (where 'All OK' was printed) and have stopped just
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271 | before the meat of our task. We could try to print out a couple of variables
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272 | to see what is happening:
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273 |
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274 | DB<9> p $data{$key}
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275 |
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276 | Not much in there, lets have a look at our hash:
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277 |
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278 | DB<10> p %data
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279 | Hello Worldziptomandwelcomejerrywelcomethisthat
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280 |
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281 | DB<11> p keys %data
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282 | Hello Worldtomwelcomejerrythis
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283 |
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284 | Well, this isn't very easy to read, and using the helpful manual (B<h h>), the
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285 | 'B<x>' command looks promising:
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286 |
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287 | DB<12> x %data
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288 | 0 'Hello World'
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289 | 1 'zip'
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290 | 2 'tom'
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291 | 3 'and'
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292 | 4 'welcome'
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293 | 5 undef
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294 | 6 'jerry'
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295 | 7 'welcome'
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296 | 8 'this'
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297 | 9 'that'
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298 |
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299 | That's not much help, a couple of welcomes in there, but no indication of
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300 | which are keys, and which are values, it's just a listed array dump and, in
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301 | this case, not particularly helpful. The trick here, is to use a B<reference>
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302 | to the data structure:
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303 |
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304 | DB<13> x \%data
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305 | 0 HASH(0x8194bc4)
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306 | 'Hello World' => 'zip'
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307 | 'jerry' => 'welcome'
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308 | 'this' => 'that'
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309 | 'tom' => 'and'
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310 | 'welcome' => undef
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311 |
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312 | The reference is truly dumped and we can finally see what we're dealing with.
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313 | Our quoting was perfectly valid but wrong for our purposes, with 'and jerry'
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314 | being treated as 2 separate words rather than a phrase, thus throwing the
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315 | evenly paired hash structure out of alignment.
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316 |
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317 | The 'B<-w>' switch would have told us about this, had we used it at the start,
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318 | and saved us a lot of trouble:
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319 |
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320 | > perl -w data
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321 | Odd number of elements in hash assignment at ./data line 5.
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322 |
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323 | We fix our quoting: 'tom' => q(and jerry), and run it again, this time we get
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324 | our expected output:
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325 |
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326 | > perl -w data
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327 | Hello World
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328 |
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329 |
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330 | While we're here, take a closer look at the 'B<x>' command, it's really useful
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331 | and will merrily dump out nested references, complete objects, partial objects
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332 | - just about whatever you throw at it:
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333 |
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334 | Let's make a quick object and x-plode it, first we'll start the debugger:
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335 | it wants some form of input from STDIN, so we give it something non-committal,
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336 | a zero:
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337 |
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338 | > perl -de 0
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339 | Default die handler restored.
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340 |
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341 | Loading DB routines from perl5db.pl version 1.07
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342 | Editor support available.
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343 |
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344 | Enter h or `h h' for help, or `man perldebug' for more help.
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345 |
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346 | main::(-e:1): 0
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347 |
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348 | Now build an on-the-fly object over a couple of lines (note the backslash):
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349 |
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350 | DB<1> $obj = bless({'unique_id'=>'123', 'attr'=> \
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351 | cont: {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')
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352 |
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353 | And let's have a look at it:
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354 |
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355 | DB<2> x $obj
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356 | 0 MY_class=HASH(0x828ad98)
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357 | 'attr' => HASH(0x828ad68)
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358 | 'col' => 'black'
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359 | 'things' => ARRAY(0x828abb8)
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360 | 0 'this'
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361 | 1 'that'
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362 | 2 'etc'
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363 | 'unique_id' => 123
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364 | DB<3>
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365 |
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366 | Useful, huh? You can eval nearly anything in there, and experiment with bits
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367 | of code or regexes until the cows come home:
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368 |
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369 | DB<3> @data = qw(this that the other atheism leather theory scythe)
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370 |
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371 | DB<4> p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
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372 | atheism
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373 | leather
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374 | other
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375 | scythe
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376 | the
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377 | theory
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378 | saw -> 6
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379 |
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380 | If you want to see the command History, type an 'B<H>':
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381 |
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382 | DB<5> H
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383 | 4: p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
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384 | 3: @data = qw(this that the other atheism leather theory scythe)
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385 | 2: x $obj
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386 | 1: $obj = bless({'unique_id'=>'123', 'attr'=>
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387 | {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')
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388 | DB<5>
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389 |
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390 | And if you want to repeat any previous command, use the exclamation: 'B<!>':
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391 |
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392 | DB<5> !4
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393 | p 'saw -> '.($cnt += map { print "$_\n" } grep(/the/, sort @data))
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394 | atheism
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395 | leather
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396 | other
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397 | scythe
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398 | the
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399 | theory
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400 | saw -> 12
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401 |
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402 | For more on references see L<perlref> and L<perlreftut>
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403 |
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404 |
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405 | =head1 Stepping through code
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406 |
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407 | Here's a simple program which converts between Celsius and Fahrenheit, it too
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408 | has a problem:
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409 |
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410 | #!/usr/bin/perl -w
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411 | use strict;
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412 |
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413 | my $arg = $ARGV[0] || '-c20';
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414 |
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415 | if ($arg =~ /^\-(c|f)((\-|\+)*\d+(\.\d+)*)$/) {
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416 | my ($deg, $num) = ($1, $2);
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417 | my ($in, $out) = ($num, $num);
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418 | if ($deg eq 'c') {
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419 | $deg = 'f';
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420 | $out = &c2f($num);
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421 | } else {
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422 | $deg = 'c';
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423 | $out = &f2c($num);
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424 | }
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425 | $out = sprintf('%0.2f', $out);
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426 | $out =~ s/^((\-|\+)*\d+)\.0+$/$1/;
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427 | print "$out $deg\n";
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428 | } else {
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429 | print "Usage: $0 -[c|f] num\n";
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430 | }
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431 | exit;
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432 |
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433 | sub f2c {
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434 | my $f = shift;
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435 | my $c = 5 * $f - 32 / 9;
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436 | return $c;
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437 | }
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438 |
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439 | sub c2f {
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440 | my $c = shift;
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441 | my $f = 9 * $c / 5 + 32;
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442 | return $f;
|
---|
443 | }
|
---|
444 |
|
---|
445 |
|
---|
446 | For some reason, the Fahrenheit to Celsius conversion fails to return the
|
---|
447 | expected output. This is what it does:
|
---|
448 |
|
---|
449 | > temp -c0.72
|
---|
450 | 33.30 f
|
---|
451 |
|
---|
452 | > temp -f33.3
|
---|
453 | 162.94 c
|
---|
454 |
|
---|
455 | Not very consistent! We'll set a breakpoint in the code manually and run it
|
---|
456 | under the debugger to see what's going on. A breakpoint is a flag, to which
|
---|
457 | the debugger will run without interruption, when it reaches the breakpoint, it
|
---|
458 | will stop execution and offer a prompt for further interaction. In normal
|
---|
459 | use, these debugger commands are completely ignored, and they are safe - if a
|
---|
460 | little messy, to leave in production code.
|
---|
461 |
|
---|
462 | my ($in, $out) = ($num, $num);
|
---|
463 | $DB::single=2; # insert at line 9!
|
---|
464 | if ($deg eq 'c')
|
---|
465 | ...
|
---|
466 |
|
---|
467 | > perl -d temp -f33.3
|
---|
468 | Default die handler restored.
|
---|
469 |
|
---|
470 | Loading DB routines from perl5db.pl version 1.07
|
---|
471 | Editor support available.
|
---|
472 |
|
---|
473 | Enter h or `h h' for help, or `man perldebug' for more help.
|
---|
474 |
|
---|
475 | main::(temp:4): my $arg = $ARGV[0] || '-c100';
|
---|
476 |
|
---|
477 | We'll simply continue down to our pre-set breakpoint with a 'B<c>':
|
---|
478 |
|
---|
479 | DB<1> c
|
---|
480 | main::(temp:10): if ($deg eq 'c') {
|
---|
481 |
|
---|
482 | Followed by a view command to see where we are:
|
---|
483 |
|
---|
484 | DB<1> v
|
---|
485 | 7: my ($deg, $num) = ($1, $2);
|
---|
486 | 8: my ($in, $out) = ($num, $num);
|
---|
487 | 9: $DB::single=2;
|
---|
488 | 10==> if ($deg eq 'c') {
|
---|
489 | 11: $deg = 'f';
|
---|
490 | 12: $out = &c2f($num);
|
---|
491 | 13 } else {
|
---|
492 | 14: $deg = 'c';
|
---|
493 | 15: $out = &f2c($num);
|
---|
494 | 16 }
|
---|
495 |
|
---|
496 | And a print to show what values we're currently using:
|
---|
497 |
|
---|
498 | DB<1> p $deg, $num
|
---|
499 | f33.3
|
---|
500 |
|
---|
501 | We can put another break point on any line beginning with a colon, we'll use
|
---|
502 | line 17 as that's just as we come out of the subroutine, and we'd like to
|
---|
503 | pause there later on:
|
---|
504 |
|
---|
505 | DB<2> b 17
|
---|
506 |
|
---|
507 | There's no feedback from this, but you can see what breakpoints are set by
|
---|
508 | using the list 'L' command:
|
---|
509 |
|
---|
510 | DB<3> L
|
---|
511 | temp:
|
---|
512 | 17: print "$out $deg\n";
|
---|
513 | break if (1)
|
---|
514 |
|
---|
515 | Note that to delete a breakpoint you use 'd' or 'D'.
|
---|
516 |
|
---|
517 | Now we'll continue down into our subroutine, this time rather than by line
|
---|
518 | number, we'll use the subroutine name, followed by the now familiar 'v':
|
---|
519 |
|
---|
520 | DB<3> c f2c
|
---|
521 | main::f2c(temp:30): my $f = shift;
|
---|
522 |
|
---|
523 | DB<4> v
|
---|
524 | 24: exit;
|
---|
525 | 25
|
---|
526 | 26 sub f2c {
|
---|
527 | 27==> my $f = shift;
|
---|
528 | 28: my $c = 5 * $f - 32 / 9;
|
---|
529 | 29: return $c;
|
---|
530 | 30 }
|
---|
531 | 31
|
---|
532 | 32 sub c2f {
|
---|
533 | 33: my $c = shift;
|
---|
534 |
|
---|
535 |
|
---|
536 | Note that if there was a subroutine call between us and line 29, and we wanted
|
---|
537 | to B<single-step> through it, we could use the 'B<s>' command, and to step
|
---|
538 | over it we would use 'B<n>' which would execute the sub, but not descend into
|
---|
539 | it for inspection. In this case though, we simply continue down to line 29:
|
---|
540 |
|
---|
541 | DB<4> c 29
|
---|
542 | main::f2c(temp:29): return $c;
|
---|
543 |
|
---|
544 | And have a look at the return value:
|
---|
545 |
|
---|
546 | DB<5> p $c
|
---|
547 | 162.944444444444
|
---|
548 |
|
---|
549 | This is not the right answer at all, but the sum looks correct. I wonder if
|
---|
550 | it's anything to do with operator precedence? We'll try a couple of other
|
---|
551 | possibilities with our sum:
|
---|
552 |
|
---|
553 | DB<6> p (5 * $f - 32 / 9)
|
---|
554 | 162.944444444444
|
---|
555 |
|
---|
556 | DB<7> p 5 * $f - (32 / 9)
|
---|
557 | 162.944444444444
|
---|
558 |
|
---|
559 | DB<8> p (5 * $f) - 32 / 9
|
---|
560 | 162.944444444444
|
---|
561 |
|
---|
562 | DB<9> p 5 * ($f - 32) / 9
|
---|
563 | 0.722222222222221
|
---|
564 |
|
---|
565 | :-) that's more like it! Ok, now we can set our return variable and we'll
|
---|
566 | return out of the sub with an 'r':
|
---|
567 |
|
---|
568 | DB<10> $c = 5 * ($f - 32) / 9
|
---|
569 |
|
---|
570 | DB<11> r
|
---|
571 | scalar context return from main::f2c: 0.722222222222221
|
---|
572 |
|
---|
573 | Looks good, let's just continue off the end of the script:
|
---|
574 |
|
---|
575 | DB<12> c
|
---|
576 | 0.72 c
|
---|
577 | Debugged program terminated. Use q to quit or R to restart,
|
---|
578 | use O inhibit_exit to avoid stopping after program termination,
|
---|
579 | h q, h R or h O to get additional info.
|
---|
580 |
|
---|
581 | A quick fix to the offending line (insert the missing parentheses) in the
|
---|
582 | actual program and we're finished.
|
---|
583 |
|
---|
584 |
|
---|
585 | =head1 Placeholder for a, w, t, T
|
---|
586 |
|
---|
587 | Actions, watch variables, stack traces etc.: on the TODO list.
|
---|
588 |
|
---|
589 | a
|
---|
590 |
|
---|
591 | w
|
---|
592 |
|
---|
593 | t
|
---|
594 |
|
---|
595 | T
|
---|
596 |
|
---|
597 |
|
---|
598 | =head1 REGULAR EXPRESSIONS
|
---|
599 |
|
---|
600 | Ever wanted to know what a regex looked like? You'll need perl compiled with
|
---|
601 | the DEBUGGING flag for this one:
|
---|
602 |
|
---|
603 | > perl -Dr -e '/^pe(a)*rl$/i'
|
---|
604 | Compiling REx `^pe(a)*rl$'
|
---|
605 | size 17 first at 2
|
---|
606 | rarest char
|
---|
607 | at 0
|
---|
608 | 1: BOL(2)
|
---|
609 | 2: EXACTF <pe>(4)
|
---|
610 | 4: CURLYN[1] {0,32767}(14)
|
---|
611 | 6: NOTHING(8)
|
---|
612 | 8: EXACTF <a>(0)
|
---|
613 | 12: WHILEM(0)
|
---|
614 | 13: NOTHING(14)
|
---|
615 | 14: EXACTF <rl>(16)
|
---|
616 | 16: EOL(17)
|
---|
617 | 17: END(0)
|
---|
618 | floating `'$ at 4..2147483647 (checking floating) stclass `EXACTF <pe>'
|
---|
619 | anchored(BOL) minlen 4
|
---|
620 | Omitting $` $& $' support.
|
---|
621 |
|
---|
622 | EXECUTING...
|
---|
623 |
|
---|
624 | Freeing REx: `^pe(a)*rl$'
|
---|
625 |
|
---|
626 | Did you really want to know? :-)
|
---|
627 | For more gory details on getting regular expressions to work, have a look at
|
---|
628 | L<perlre>, L<perlretut>, and to decode the mysterious labels (BOL and CURLYN,
|
---|
629 | etc. above), see L<perldebguts>.
|
---|
630 |
|
---|
631 |
|
---|
632 | =head1 OUTPUT TIPS
|
---|
633 |
|
---|
634 | To get all the output from your error log, and not miss any messages via
|
---|
635 | helpful operating system buffering, insert a line like this, at the start of
|
---|
636 | your script:
|
---|
637 |
|
---|
638 | $|=1;
|
---|
639 |
|
---|
640 | To watch the tail of a dynamically growing logfile, (from the command line):
|
---|
641 |
|
---|
642 | tail -f $error_log
|
---|
643 |
|
---|
644 | Wrapping all die calls in a handler routine can be useful to see how, and from
|
---|
645 | where, they're being called, L<perlvar> has more information:
|
---|
646 |
|
---|
647 | BEGIN { $SIG{__DIE__} = sub { require Carp; Carp::confess(@_) } }
|
---|
648 |
|
---|
649 | Various useful techniques for the redirection of STDOUT and STDERR filehandles
|
---|
650 | are explained in L<perlopentut> and L<perlfaq8>.
|
---|
651 |
|
---|
652 |
|
---|
653 | =head1 CGI
|
---|
654 |
|
---|
655 | Just a quick hint here for all those CGI programmers who can't figure out how
|
---|
656 | on earth to get past that 'waiting for input' prompt, when running their CGI
|
---|
657 | script from the command-line, try something like this:
|
---|
658 |
|
---|
659 | > perl -d my_cgi.pl -nodebug
|
---|
660 |
|
---|
661 | Of course L<CGI> and L<perlfaq9> will tell you more.
|
---|
662 |
|
---|
663 |
|
---|
664 | =head1 GUIs
|
---|
665 |
|
---|
666 | The command line interface is tightly integrated with an B<emacs> extension
|
---|
667 | and there's a B<vi> interface too.
|
---|
668 |
|
---|
669 | You don't have to do this all on the command line, though, there are a few GUI
|
---|
670 | options out there. The nice thing about these is you can wave a mouse over a
|
---|
671 | variable and a dump of its data will appear in an appropriate window, or in a
|
---|
672 | popup balloon, no more tiresome typing of 'x $varname' :-)
|
---|
673 |
|
---|
674 | In particular have a hunt around for the following:
|
---|
675 |
|
---|
676 | B<ptkdb> perlTK based wrapper for the built-in debugger
|
---|
677 |
|
---|
678 | B<ddd> data display debugger
|
---|
679 |
|
---|
680 | B<PerlDevKit> and B<PerlBuilder> are NT specific
|
---|
681 |
|
---|
682 | NB. (more info on these and others would be appreciated).
|
---|
683 |
|
---|
684 |
|
---|
685 | =head1 SUMMARY
|
---|
686 |
|
---|
687 | We've seen how to encourage good coding practices with B<use strict> and
|
---|
688 | B<-w>. We can run the perl debugger B<perl -d scriptname> to inspect your
|
---|
689 | data from within the perl debugger with the B<p> and B<x> commands. You can
|
---|
690 | walk through your code, set breakpoints with B<b> and step through that code
|
---|
691 | with B<s> or B<n>, continue with B<c> and return from a sub with B<r>. Fairly
|
---|
692 | intuitive stuff when you get down to it.
|
---|
693 |
|
---|
694 | There is of course lots more to find out about, this has just scratched the
|
---|
695 | surface. The best way to learn more is to use perldoc to find out more about
|
---|
696 | the language, to read the on-line help (L<perldebug> is probably the next
|
---|
697 | place to go), and of course, experiment.
|
---|
698 |
|
---|
699 |
|
---|
700 | =head1 SEE ALSO
|
---|
701 |
|
---|
702 | L<perldebug>,
|
---|
703 | L<perldebguts>,
|
---|
704 | L<perldiag>,
|
---|
705 | L<dprofpp>,
|
---|
706 | L<perlrun>
|
---|
707 |
|
---|
708 |
|
---|
709 | =head1 AUTHOR
|
---|
710 |
|
---|
711 | Richard Foley <[email protected]> Copyright (c) 2000
|
---|
712 |
|
---|
713 |
|
---|
714 | =head1 CONTRIBUTORS
|
---|
715 |
|
---|
716 | Various people have made helpful suggestions and contributions, in particular:
|
---|
717 |
|
---|
718 | Ronald J Kimball <[email protected]>
|
---|
719 |
|
---|
720 | Hugo van der Sanden <[email protected]>
|
---|
721 |
|
---|
722 | Peter Scott <[email protected]>
|
---|
723 |
|
---|