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