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<[email protected]>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.
|
---|