a2p - Awk to Perl translator
a2p [options] filename
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 [Toc] [Back]
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 password 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:
o Old awk always has a line loop, even if there are no line
actions, whereas new awk does not.
o In old awk, sprintf is extremely greedy about its arguments.
For example, given the statement
print sprintf(some_args), extra_args;
old awk considers extra_args to be arguments to "sprintf";
new awk considers 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 generally 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 operator 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 somewhere.
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 equivalent, $#, 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 simplified 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 discarding 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.
A2p uses no environment variables.
Larry Wall <[email protected]>
SEE ALSO
perl The perl compiler/interpreter
s2p sed to perl translator
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.
3rd Berkeley Distribution 2004-12-24 A2P(1)
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