IRIX Base Documentation 1998 November/IRIX 6.5.2 Base Documentation November 1998.img/usr/share/catman/u_man/cat1/a2p.z/a2p

Text File | 1998-10-30 | 8KB | 265 lines

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     a2p - Awk to Perl translator

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     _A_2_p takes an awk script specified on the command line (or from standard
     input) and produces a comparable _p_e_r_l script on the standard output.

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     Options include:

     ----DDDD<<<<nnnnuuuummmmbbbbeeeerrrr>>>>
          sets debugging flags.

     ----FFFF<<<<cccchhhhaaaarrrraaaacccctttteeeerrrr>>>>
          tells a2p that this awk script is always invoked with this ----FFFF
          switch.

     ----nnnn<<<<ffffiiiieeeellllddddlllliiiisssstttt>>>>
          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.

     ----<<<<nnnnuuuummmmbbbbeeeerrrr>>>>
          causes a2p to assume that input will always have that many fields.

     ----oooo   tells a2p to use old awk behavior.  For now, the only difference is
          that old awk always has a line loop, even if there are no line
          actions, whereas new awk does not.

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     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 _i_n_t() 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.






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     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 ----wwww 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 ----nnnn 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 _k_e_y_s() 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 _s_u_b_s_t_r() and _i_n_d_e_x() 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,



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     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.

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     A2p uses no environment variables.

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     Larry Wall <_l_a_r_r_y@_w_a_l_l._o_r_g>

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SSSSEEEEEEEE AAAALLLLSSSSOOOO
      perl   The perl compiler/interpreter

      s2p    sed to perl translator


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     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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