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 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:
- •
- 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 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 index variables from being 1-based
(awk style) to 0-based (Perl style). Be sure to change all operations the
variable is involved in 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-1]. A loop that
tries to iterate over ARGV[0] won't find it.
ENVIRONMENT¶
A2p uses no environment variables.
AUTHOR¶
Larry Wall <
larry@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.