[Summary view] [Print] [Text view]

   1  =head1 NAME
   2  
   3  a2p - Awk to Perl translator
   4  
   5  =head1 SYNOPSIS
   6  
   7  B<a2p> [I<options>] [I<filename>]
   8  
   9  =head1 DESCRIPTION
  10  
  11  I<A2p> takes an awk script specified on the command line (or from
  12  standard input) and produces a comparable I<perl> script on the
  13  standard output.
  14  
  15  =head2 OPTIONS
  16  
  17  Options include:
  18  
  19  =over 5
  20  
  21  =item B<-DE<lt>numberE<gt>>
  22  
  23  sets debugging flags.
  24  
  25  =item B<-FE<lt>characterE<gt>>
  26  
  27  tells a2p that this awk script is always invoked with this B<-F>
  28  switch.
  29  
  30  =item B<-nE<lt>fieldlistE<gt>>
  31  
  32  specifies the names of the input fields if input does not have to be
  33  split into an array.  If you were translating an awk script that
  34  processes the password file, you might say:
  35  
  36      a2p -7 -nlogin.password.uid.gid.gcos.shell.home
  37  
  38  Any delimiter can be used to separate the field names.
  39  
  40  =item B<-E<lt>numberE<gt>>
  41  
  42  causes a2p to assume that input will always have that many fields.
  43  
  44  =item B<-o>
  45  
  46  tells a2p to use old awk behavior.  The only current differences are:
  47  
  48  =over 5
  49  
  50  =item *
  51  
  52  Old awk always has a line loop, even if there are no line
  53  actions, whereas new awk does not.
  54  
  55  =item *
  56  
  57  In old awk, sprintf is extremely greedy about its arguments.
  58  For example, given the statement
  59  
  60      print sprintf(some_args), extra_args;
  61  
  62  old awk considers I<extra_args> to be arguments to C<sprintf>; new awk
  63  considers them arguments to C<print>.
  64  
  65  =back
  66  
  67  =back
  68  
  69  =head2 "Considerations"
  70  
  71  A2p cannot do as good a job translating as a human would, but it
  72  usually does pretty well.  There are some areas where you may want to
  73  examine the perl script produced and tweak it some.  Here are some of
  74  them, in no particular order.
  75  
  76  There is an awk idiom of putting int() around a string expression to
  77  force numeric interpretation, even though the argument is always
  78  integer anyway.  This is generally unneeded in perl, but a2p can't
  79  tell if the argument is always going to be integer, so it leaves it
  80  in.  You may wish to remove it.
  81  
  82  Perl differentiates numeric comparison from string comparison.  Awk
  83  has one operator for both that decides at run time which comparison to
  84  do.  A2p does not try to do a complete job of awk emulation at this
  85  point.  Instead it guesses which one you want.  It's almost always
  86  right, but it can be spoofed.  All such guesses are marked with the
  87  comment "C<#???>".  You should go through and check them.  You might
  88  want to run at least once with the B<-w> switch to perl, which will
  89  warn you if you use == where you should have used eq.
  90  
  91  Perl does not attempt to emulate the behavior of awk in which
  92  nonexistent array elements spring into existence simply by being
  93  referenced.  If somehow you are relying on this mechanism to create
  94  null entries for a subsequent for...in, they won't be there in perl.
  95  
  96  If a2p makes a split line that assigns to a list of variables that
  97  looks like (Fld1, Fld2, Fld3...) you may want to rerun a2p using the
  98  B<-n> option mentioned above.  This will let you name the fields
  99  throughout the script.  If it splits to an array instead, the script
 100  is probably referring to the number of fields somewhere.
 101  
 102  The exit statement in awk doesn't necessarily exit; it goes to the END
 103  block if there is one.  Awk scripts that do contortions within the END
 104  block to bypass the block under such circumstances can be simplified
 105  by removing the conditional in the END block and just exiting directly
 106  from the perl script.
 107  
 108  Perl has two kinds of array, numerically-indexed and associative.
 109  Perl associative arrays are called "hashes".  Awk arrays are usually
 110  translated to hashes, but if you happen to know that the index is
 111  always going to be numeric you could change the {...} to [...].
 112  Iteration over a hash is done using the keys() function, but iteration
 113  over an array is NOT.  You might need to modify any loop that iterates
 114  over such an array.
 115  
 116  Awk starts by assuming OFMT has the value %.6g.  Perl starts by
 117  assuming its equivalent, $#, to have the value %.20g.  You'll want to
 118  set $# explicitly if you use the default value of OFMT.
 119  
 120  Near the top of the line loop will be the split operation that is
 121  implicit in the awk script.  There are times when you can move this
 122  down past some conditionals that test the entire record so that the
 123  split is not done as often.
 124  
 125  For aesthetic reasons you may wish to change the array base $[ from 1
 126  back to perl's default of 0, but remember to change all array
 127  subscripts AND all substr() and index() operations to match.
 128  
 129  Cute comments that say "# Here is a workaround because awk is dumb"
 130  are passed through unmodified.
 131  
 132  Awk scripts are often embedded in a shell script that pipes stuff into
 133  and out of awk.  Often the shell script wrapper can be incorporated
 134  into the perl script, since perl can start up pipes into and out of
 135  itself, and can do other things that awk can't do by itself.
 136  
 137  Scripts that refer to the special variables RSTART and RLENGTH can
 138  often be simplified by referring to the variables $`, $& and $', as
 139  long as they are within the scope of the pattern match that sets them.
 140  
 141  The produced perl script may have subroutines defined to deal with
 142  awk's semantics regarding getline and print.  Since a2p usually picks
 143  correctness over efficiency.  it is almost always possible to rewrite
 144  such code to be more efficient by discarding the semantic sugar.
 145  
 146  For efficiency, you may wish to remove the keyword from any return
 147  statement that is the last statement executed in a subroutine.  A2p
 148  catches the most common case, but doesn't analyze embedded blocks for
 149  subtler cases.
 150  
 151  ARGV[0] translates to $ARGV0, but ARGV[n] translates to $ARGV[$n].  A
 152  loop that tries to iterate over ARGV[0] won't find it.
 153  
 154  =head1 ENVIRONMENT
 155  
 156  A2p uses no environment variables.
 157  
 158  =head1 AUTHOR
 159  
 160  Larry Wall E<lt>F<larry@wall.org>E<gt>
 161  
 162  =head1 FILES
 163  
 164  =head1 SEE ALSO
 165  
 166   perl    The perl compiler/interpreter
 167   
 168   s2p    sed to perl translator
 169  
 170  =head1 DIAGNOSTICS
 171  
 172  =head1 BUGS
 173  
 174  It would be possible to emulate awk's behavior in selecting string
 175  versus numeric operations at run time by inspection of the operands,
 176  but it would be gross and inefficient.  Besides, a2p almost always
 177  guesses right.
 178  
 179  Storage for the awk syntax tree is currently static, and can run out.