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A2P(1) Perl Programmers Reference Guide A2P(1)
NAME
a2p - Awk to Perl translator
SYNOPSIS
a2p [options] filename
DESCRIPTION
A2p takes an awk script specified on the command line (or from standard input) and
produces a comparable perl script on the standard output.
Options
Options include:
-D<number>
sets debugging flags.
-F<character>
tells a2p that this awk script is always invoked with this -F switch.
-n<fieldlist>
specifies the names of the input fields if input does not have to be split
into an array. If you were translating an awk script that processes the pass-
word file, you might say:
a2p -7 -nlogin.password.uid.gid.gcos.shell.home
Any delimiter can be used to separate the field names.
-<number>
causes a2p to assume that input will always have that many fields.
-o tells a2p to use old awk behavior. The only current differences are:
* Old awk always has a line loop, even if there are no line actions,
whereas new awk does not.
* In old awk, sprintf is extremely greedy about its arguments. For exam-
ple, given the statement
print sprintf(some_args), extra_args;
old awk considers extra_args to be arguments to "sprintf"; new awk con-
siders them arguments to "print".
"Considerations"
A2p cannot do as good a job translating as a human would, but it usually does
pretty well. There are some areas where you may want to examine the perl script
produced and tweak it some. Here are some of them, in no particular order.
There is an awk idiom of putting int() around a string expression to force numeric
interpretation, even though the argument is always integer anyway. This is gener-
ally unneeded in perl, but a2p can’t tell if the argument is always going to be
integer, so it leaves it in. You may wish to remove it.
Perl differentiates numeric comparison from string comparison. Awk has one opera-
tor for both that decides at run time which comparison to do. A2p does not try to
do a complete job of awk emulation at this point. Instead it guesses which one you
want. It’s almost always right, but it can be spoofed. All such guesses are
marked with the comment ""#???"". You should go through and check them. You might
want to run at least once with the -w switch to perl, which will warn you if you
use == where you should have used eq.
Perl does not attempt to emulate the behavior of awk in which nonexistent array
elements spring into existence simply by being referenced. If somehow you are
relying on this mechanism to create null entries for a subsequent for...in, they
won’t be there in perl.
If a2p makes a split line that assigns to a list of variables that looks like
(Fld1, Fld2, Fld3...) you may want to rerun a2p using the -n option mentioned
above. This will let you name the fields throughout the script. If it splits to
an array instead, the script is probably referring to the number of fields some-
where.
The exit statement in awk doesn’t necessarily exit; it goes to the END block if
there is one. Awk scripts that do contortions within the END block to bypass the
block under such circumstances can be simplified by removing the conditional in the
END block and just exiting directly from the perl script.
Perl has two kinds of array, numerically-indexed and associative. Perl associative
arrays are called "hashes". Awk arrays are usually translated to hashes, but if
you happen to know that the index is always going to be numeric you could change
the {...} to [...]. Iteration over a hash is done using the keys() function, but
iteration over an array is NOT. You might need to modify any loop that iterates
over such an array.
Awk starts by assuming OFMT has the value %.6g. Perl starts by assuming its equiv-
alent, $#, to have the value %.20g. You’ll want to set $# explicitly if you use
the default value of OFMT.
Near the top of the line loop will be the split operation that is implicit in the
awk script. There are times when you can move this down past some conditionals
that test the entire record so that the split is not done as often.
For aesthetic reasons you may wish to change the array base $[ from 1 back to
perl’s default of 0, but remember to change all array subscripts AND all substr()
and index() operations to match.
Cute comments that say "# Here is a workaround because awk is dumb" are passed
through unmodified.
Awk scripts are often embedded in a shell script that pipes stuff into and out of
awk. Often the shell script wrapper can be incorporated into the perl script,
since perl can start up pipes into and out of itself, and can do other things that
awk can’t do by itself.
Scripts that refer to the special variables RSTART and RLENGTH can often be simpli-
fied by referring to the variables $‘, $& and $’, as long as they are within the
scope of the pattern match that sets them.
The produced perl script may have subroutines defined to deal with awk’s semantics
regarding getline and print. Since a2p usually picks correctness over efficiency.
it is almost always possible to rewrite such code to be more efficient by discard-
ing the semantic sugar.
For efficiency, you may wish to remove the keyword from any return statement that
is the last statement executed in a subroutine. A2p catches the most common case,
but doesn’t analyze embedded blocks for subtler cases.
ARGV[0] translates to $ARGV0, but ARGV[n] translates to $ARGV[$n]. A loop that
tries to iterate over ARGV[0] won’t find it.
ENVIRONMENT
A2p uses no environment variables.
AUTHOR
Larry Wall <larry AT wall.org>
FILES
SEE ALSO
perl The perl compiler/interpreter
s2p sed to perl translator
DIAGNOSTICS
BUGS
It would be possible to emulate awk’s behavior in selecting string versus numeric
operations at run time by inspection of the operands, but it would be gross and
inefficient. Besides, a2p almost always guesses right.
Storage for the awk syntax tree is currently static, and can run out.
perl v5.8.6 2001-02-28 A2P(1)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2009-01-08 05:14 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)