MirOS Manual: a2p(1)


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 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".

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     "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 com-
     parison.  Awk has one operator for both that decides at run
     time which comparison to do.  A2p does not try to do a com-
     plete 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 sim-
     ply 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 vari-
     ables 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 con-
     tortions 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 associ-
     ative. 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

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     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 vari-
     ables $`, $& 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 effi-
     cient 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.

ENVIRONMENT

     A2p uses no environment variables.

AUTHOR

     Larry Wall <larry@wall.org>

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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.

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