| Category: |
Utility Scripts |
| Author/Contact Info |
Ed Halley <ed@halley.cc>
This program is free software; you can redistribute it and/or modify it under the same terms as Perl |
| Description: |
The historically common head(1) and tail(1) commands are for keeping the head or tail of stream input, usually by a count of lines. This pair of scripts differ in three respects:
- these scripts don't show the head or tail, they show everything except the head or tail,
- these scripts don't count lines but instead work on a single regex (regular expression) to find a matching "cut here" point in the text,
- these scripts can actually modify/edit text files in place if filenames are given.
Check out the pod output for a complete man-page.
|
#!/usr/bin/perl -w
#---------------------------------------------------------------------
+-------
# Copyright (C) 2001-2003 Ed Halley
#---------------------------------------------------------------------
+-------
=head1 NAME
unhead - remove the lines before a match from the input stream
untail - remove the lines following a match from the input stream
=cut
# Both 'unhead' and 'untail' are identical; they determine the task fr
+om
# the $0 perl variable. The scripts can be linked or symlinked or sim
+ply
# copied, and will still work as long as they have the correct filenam
+es.
=head1 SYNOPSIS
unhead '--BEGIN MESSAGE--' *.txt
untail '--END MESSAGE--' *.txt
=head1 DESCRIPTION
The historically common C<head>(1) and C<tail>(1) commands are for
keeping the head or tail of stream input, usually by a count of lines.
This pair of scripts differ in two respects: these scripts work on a
single regex (regular expression) to find a matching "cut here" point
+in
the text, and these scripts edit text files in place.
By default, the first argument should be a regular expression that sho
+uld
match at least once in each subsequent file. If no additional argumen
+ts
are given, or if the filename is a hyphen (-), then the standard input
and standard output streams are assumed. For filenames, each file is
processed in turn, creating a backup file with a tilde (~) appended to
the original name.
The C<unhead> variant will remove the header above the given match, an
+d
write out the tail (including the first matching line). The C<untail>
variant will remove the trailing below the given match, and only write
out the head (not including the first matching line).
=cut
#---------------------------------------------------------------------
+-------
use warnings;
use strict;
my $want = ($0 =~ /untail/)? 'head' : 'tail';
my $suffix = '~';
my $pattern = shift(@ARGV);
die "First argument should be a regular expression string" if not $pat
+tern;
$pattern = qr/$pattern/;
@ARGV = ('-') if not @ARGV;
exit(main(@ARGV));
#---------------------------------------------------------------------
+-------
sub main
{
while (@_)
{
my $inp = *STDIN;
my $outp = *STDOUT;
my $file = shift;
if ($file ne '-')
{
die if not -w $file;
$inp = *FIN; $inp = *FIN; # hush hush warning warning
$outp = *FOUT; $outp = *FOUT;
unlink($file.$suffix) if -f $file.$suffix;
rename($file, $file.$suffix);
open($inp, $file.$suffix)
or die "Cannot open $file";
open($outp, '>'.$file)
or die "Cannot open $file";
}
my $have = 'head';
while (<$inp>)
{
$have = 'tail' if m{$pattern};
print $outp $_ if $want eq $have;
}
if ($file ne '-')
{
close($outp);
close($inp);
}
}
}
__END__
#---------------------------------------------------------------------
+-------
=head1 IDEAS
# chop off email routing header info (first empty line)
cat email.msg | unhead '^$'
# just students from "Danziger, Jane" to "Funicello, Thomas"
query-students | sort | unhead '^D' | untail '^G'
# keep the center section of a YACC grammar
unhead '^%%$' *.y ; untail '^%%$' *.y
=head1 BUGS
The C<unhead> variety will discard everything and leave an empty file
+if
no lines match the given pattern. This is not a bug, but worth a
warning. In the case of actual files, you still have the tilde backup
+.
The script for both C<unhead> and C<untail> are identical, and one can
+ be
a link to the other. The script determines which part to keep based o
+n
the script's name.
=head1 LICENSE
Copyright (C) 2001-2003 Ed Halley <ed@halley.cc>
This program is free software; you can redistribute it and/or modify i
+t
under the same terms as Perl itself. For details on the Perl Artistic
License, read the F<http://www.perl.com/language/misc/Artistic.html>
page.
=cut
|
|
Re: unhead and untail
by dmitri (Priest) on Apr 15, 2003 at 19:54 UTC
|
A couple of thoughts:
The cutting up of input stream as you describe can be much done much easier with sed, for instance, instead of
query-students | sort | unhead '^D' | untail '^G'
do
query-students | sort | sed '/^D/,/^G/p; d'
Also, neither head nor tail modify files they operate on. I understand, you were probably solving a specific problem, but this script can be generalized even more.
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