178 lines
5.9 KiB
Plaintext
178 lines
5.9 KiB
Plaintext
=head1 NAME
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a2p - Awk to Perl translator
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=head1 SYNOPSIS
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B<a2p [options] filename>
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=head1 DESCRIPTION
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I<A2p> takes an awk script specified on the command line (or from
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standard input) and produces a comparable I<perl> script on the
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standard output.
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=head2 Options
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Options include:
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=over 5
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=item B<-DE<lt>numberE<gt>>
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sets debugging flags.
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=item B<-FE<lt>characterE<gt>>
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tells a2p that this awk script is always invoked with this B<-F>
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switch.
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=item B<-nE<lt>fieldlistE<gt>>
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specifies the names of the input fields if input does not have to be
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split into an array. If you were translating an awk script that
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processes the password file, you might say:
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a2p -7 -nlogin.password.uid.gid.gcos.shell.home
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Any delimiter can be used to separate the field names.
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=item B<-E<lt>numberE<gt>>
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causes a2p to assume that input will always have that many fields.
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=item B<-o>
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tells a2p to use old awk behavior. The only current differences are:
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=over 5
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=item
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Old awk always has a line loop, even if there are no line
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actions, whereas new awk does not.
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=item
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In old awk, sprintf is extremely greedy about its arguments.
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For example, given the statement
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print sprintf(some_args), extra_args;
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old awk considers I<extra_args> to be arguments to C<sprintf>; new awk
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considers them arguments to C<print>.
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=back
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=head2 "Considerations"
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A2p cannot do as good a job translating as a human would, but it
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usually does pretty well. There are some areas where you may want to
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examine the perl script produced and tweak it some. Here are some of
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them, in no particular order.
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There is an awk idiom of putting int() around a string expression to
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force numeric interpretation, even though the argument is always
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integer anyway. This is generally unneeded in perl, but a2p can't
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tell if the argument is always going to be integer, so it leaves it
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in. You may wish to remove it.
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Perl differentiates numeric comparison from string comparison. Awk
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has one operator for both that decides at run time which comparison to
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do. A2p does not try to do a complete job of awk emulation at this
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point. Instead it guesses which one you want. It's almost always
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right, but it can be spoofed. All such guesses are marked with the
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comment "C<#???>". You should go through and check them. You might
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want to run at least once with the B<-w> switch to perl, which will
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warn you if you use == where you should have used eq.
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Perl does not attempt to emulate the behavior of awk in which
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nonexistent array elements spring into existence simply by being
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referenced. If somehow you are relying on this mechanism to create
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null entries for a subsequent for...in, they won't be there in perl.
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If a2p makes a split line that assigns to a list of variables that
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looks like (Fld1, Fld2, Fld3...) you may want to rerun a2p using the
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B<-n> option mentioned above. This will let you name the fields
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throughout the script. If it splits to an array instead, the script
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is probably referring to the number of fields somewhere.
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The exit statement in awk doesn't necessarily exit; it goes to the END
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block if there is one. Awk scripts that do contortions within the END
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block to bypass the block under such circumstances can be simplified
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by removing the conditional in the END block and just exiting directly
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from the perl script.
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Perl has two kinds of array, numerically-indexed and associative.
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Perl associative arrays are called "hashes". Awk arrays are usually
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translated to hashes, but if you happen to know that the index is
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always going to be numeric you could change the {...} to [...].
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Iteration over a hash is done using the keys() function, but iteration
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over an array is NOT. You might need to modify any loop that iterates
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over such an array.
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Awk starts by assuming OFMT has the value %.6g. Perl starts by
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assuming its equivalent, $#, to have the value %.20g. You'll want to
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set $# explicitly if you use the default value of OFMT.
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Near the top of the line loop will be the split operation that is
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implicit in the awk script. There are times when you can move this
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down past some conditionals that test the entire record so that the
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split is not done as often.
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For aesthetic reasons you may wish to change the array base $[ from 1
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back to perl's default of 0, but remember to change all array
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subscripts AND all substr() and index() operations to match.
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Cute comments that say "# Here is a workaround because awk is dumb"
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are passed through unmodified.
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Awk scripts are often embedded in a shell script that pipes stuff into
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and out of awk. Often the shell script wrapper can be incorporated
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into the perl script, since perl can start up pipes into and out of
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itself, and can do other things that awk can't do by itself.
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Scripts that refer to the special variables RSTART and RLENGTH can
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often be simplified by referring to the variables $`, $& and $', as
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long as they are within the scope of the pattern match that sets them.
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The produced perl script may have subroutines defined to deal with
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awk's semantics regarding getline and print. Since a2p usually picks
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correctness over efficiency. it is almost always possible to rewrite
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such code to be more efficient by discarding the semantic sugar.
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For efficiency, you may wish to remove the keyword from any return
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statement that is the last statement executed in a subroutine. A2p
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catches the most common case, but doesn't analyze embedded blocks for
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subtler cases.
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ARGV[0] translates to $ARGV0, but ARGV[n] translates to $ARGV[$n]. A
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loop that tries to iterate over ARGV[0] won't find it.
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=head1 ENVIRONMENT
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A2p uses no environment variables.
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=head1 AUTHOR
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Larry Wall E<lt>F<larry@wall.org>E<gt>
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=head1 FILES
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=head1 SEE ALSO
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perl The perl compiler/interpreter
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s2p sed to perl translator
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=head1 DIAGNOSTICS
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=head1 BUGS
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It would be possible to emulate awk's behavior in selecting string
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versus numeric operations at run time by inspection of the operands,
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but it would be gross and inefficient. Besides, a2p almost always
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guesses right.
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Storage for the awk syntax tree is currently static, and can run out.
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