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1 | =head1 NAME |
2 | ||
3 | a2p - Awk to Perl translator | |
4 | ||
5 | =head1 SYNOPSIS | |
6 | ||
7 | B<a2p> [I<options>] [I<filename>] | |
8 | ||
9 | =head1 DESCRIPTION | |
10 | ||
11 | I<A2p> takes an awk script specified on the command line (or from | |
12 | standard input) and produces a comparable I<perl> script on the | |
13 | standard output. | |
14 | ||
15 | =head2 OPTIONS | |
16 | ||
17 | Options include: | |
18 | ||
19 | =over 5 | |
20 | ||
21 | =item B<-DE<lt>numberE<gt>> | |
22 | ||
23 | sets debugging flags. | |
24 | ||
25 | =item B<-FE<lt>characterE<gt>> | |
26 | ||
27 | tells a2p that this awk script is always invoked with this B<-F> | |
28 | switch. | |
29 | ||
30 | =item B<-nE<lt>fieldlistE<gt>> | |
31 | ||
32 | specifies the names of the input fields if input does not have to be | |
33 | split into an array. If you were translating an awk script that | |
34 | processes the password file, you might say: | |
35 | ||
36 | a2p -7 -nlogin.password.uid.gid.gcos.shell.home | |
37 | ||
38 | Any delimiter can be used to separate the field names. | |
39 | ||
40 | =item B<-E<lt>numberE<gt>> | |
41 | ||
42 | causes a2p to assume that input will always have that many fields. | |
43 | ||
44 | =item B<-o> | |
45 | ||
46 | tells a2p to use old awk behavior. The only current differences are: | |
47 | ||
48 | =over 5 | |
49 | ||
50 | =item * | |
51 | ||
52 | Old awk always has a line loop, even if there are no line | |
53 | actions, whereas new awk does not. | |
54 | ||
55 | =item * | |
56 | ||
57 | In old awk, sprintf is extremely greedy about its arguments. | |
58 | For example, given the statement | |
59 | ||
60 | print sprintf(some_args), extra_args; | |
61 | ||
62 | old awk considers I<extra_args> to be arguments to C<sprintf>; new awk | |
63 | considers them arguments to C<print>. | |
64 | ||
65 | =back | |
66 | ||
67 | =back | |
68 | ||
69 | =head2 "Considerations" | |
70 | ||
71 | A2p cannot do as good a job translating as a human would, but it | |
72 | usually does pretty well. There are some areas where you may want to | |
73 | examine the perl script produced and tweak it some. Here are some of | |
74 | them, in no particular order. | |
75 | ||
76 | There is an awk idiom of putting int() around a string expression to | |
77 | force numeric interpretation, even though the argument is always | |
78 | integer anyway. This is generally unneeded in perl, but a2p can't | |
79 | tell if the argument is always going to be integer, so it leaves it | |
80 | in. You may wish to remove it. | |
81 | ||
82 | Perl differentiates numeric comparison from string comparison. Awk | |
83 | has one operator for both that decides at run time which comparison to | |
84 | do. A2p does not try to do a complete job of awk emulation at this | |
85 | point. Instead it guesses which one you want. It's almost always | |
86 | right, but it can be spoofed. All such guesses are marked with the | |
87 | comment "C<#???>". You should go through and check them. You might | |
88 | want to run at least once with the B<-w> switch to perl, which will | |
89 | warn you if you use == where you should have used eq. | |
90 | ||
91 | Perl does not attempt to emulate the behavior of awk in which | |
92 | nonexistent array elements spring into existence simply by being | |
93 | referenced. If somehow you are relying on this mechanism to create | |
94 | null entries for a subsequent for...in, they won't be there in perl. | |
95 | ||
96 | If a2p makes a split line that assigns to a list of variables that | |
97 | looks like (Fld1, Fld2, Fld3...) you may want to rerun a2p using the | |
98 | B<-n> option mentioned above. This will let you name the fields | |
99 | throughout the script. If it splits to an array instead, the script | |
100 | is probably referring to the number of fields somewhere. | |
101 | ||
102 | The exit statement in awk doesn't necessarily exit; it goes to the END | |
103 | block if there is one. Awk scripts that do contortions within the END | |
104 | block to bypass the block under such circumstances can be simplified | |
105 | by removing the conditional in the END block and just exiting directly | |
106 | from the perl script. | |
107 | ||
108 | Perl has two kinds of array, numerically-indexed and associative. | |
109 | Perl associative arrays are called "hashes". Awk arrays are usually | |
110 | translated to hashes, but if you happen to know that the index is | |
111 | always going to be numeric you could change the {...} to [...]. | |
112 | Iteration over a hash is done using the keys() function, but iteration | |
113 | over an array is NOT. You might need to modify any loop that iterates | |
114 | over such an array. | |
115 | ||
116 | Awk starts by assuming OFMT has the value %.6g. Perl starts by | |
117 | assuming its equivalent, $#, to have the value %.20g. You'll want to | |
118 | set $# explicitly if you use the default value of OFMT. | |
119 | ||
120 | Near the top of the line loop will be the split operation that is | |
121 | implicit in the awk script. There are times when you can move this | |
122 | down past some conditionals that test the entire record so that the | |
123 | split is not done as often. | |
124 | ||
125 | For aesthetic reasons you may wish to change the array base $[ from 1 | |
126 | back to perl's default of 0, but remember to change all array | |
127 | subscripts AND all substr() and index() operations to match. | |
128 | ||
129 | Cute comments that say "# Here is a workaround because awk is dumb" | |
130 | are passed through unmodified. | |
131 | ||
132 | Awk scripts are often embedded in a shell script that pipes stuff into | |
133 | and out of awk. Often the shell script wrapper can be incorporated | |
134 | into the perl script, since perl can start up pipes into and out of | |
135 | itself, and can do other things that awk can't do by itself. | |
136 | ||
137 | Scripts that refer to the special variables RSTART and RLENGTH can | |
138 | often be simplified by referring to the variables $`, $& and $', as | |
139 | long as they are within the scope of the pattern match that sets them. | |
140 | ||
141 | The produced perl script may have subroutines defined to deal with | |
142 | awk's semantics regarding getline and print. Since a2p usually picks | |
143 | correctness over efficiency. it is almost always possible to rewrite | |
144 | such code to be more efficient by discarding the semantic sugar. | |
145 | ||
146 | For efficiency, you may wish to remove the keyword from any return | |
147 | statement that is the last statement executed in a subroutine. A2p | |
148 | catches the most common case, but doesn't analyze embedded blocks for | |
149 | subtler cases. | |
150 | ||
151 | ARGV[0] translates to $ARGV0, but ARGV[n] translates to $ARGV[$n]. A | |
152 | loop that tries to iterate over ARGV[0] won't find it. | |
153 | ||
154 | =head1 ENVIRONMENT | |
155 | ||
156 | A2p uses no environment variables. | |
157 | ||
158 | =head1 AUTHOR | |
159 | ||
160 | Larry Wall E<lt>F<larry@wall.org>E<gt> | |
161 | ||
162 | =head1 FILES | |
163 | ||
164 | =head1 SEE ALSO | |
165 | ||
166 | perl The perl compiler/interpreter | |
167 | ||
168 | s2p sed to perl translator | |
169 | ||
170 | =head1 DIAGNOSTICS | |
171 | ||
172 | =head1 BUGS | |
173 | ||
174 | It would be possible to emulate awk's behavior in selecting string | |
175 | versus numeric operations at run time by inspection of the operands, | |
176 | but it would be gross and inefficient. Besides, a2p almost always | |
177 | guesses right. | |
178 | ||
179 | Storage for the awk syntax tree is currently static, and can run out. |