head+tail with perl

head-and-tail.bash

# One-line version of head-and-tail.plx (w/o --no-overlap and w/o
# proper handling of tail less than number of lines):

seq 1 100000 | HEAD=5 TAIL=2 SEP=$'...\n' perl -e 'my($h,$t,$s,$l)=($ENV{"HEAD"}||10,$ENV{"TAIL"}||10,$ENV{"SEP"}||"",0);my @b=();while(<>){print if $.<=$h;$b[$.%$t]=$_;$l=$.};print $s;for(my $i=$t-1;$i>=0;--$i){print $b[($l-$i)%$t]}'

# For files (not streams), this seems to work just fine too (but it
# will not overlap):

( head -n 5; echo ...; tail -n 2 ) < some-file

# Also: all 3 will always print the ... even if head and tail are
# contiguous.

head-and-tail.plx

#!/usr/bin/perl -w

# Example:
#   $ seq 1 100000 | head-and-tail.plx -h 3 -t 2 --sep $'...\n'
#   1
#   2
#   3
#   ...
#   99999
#   100000

use strict; use Getopt::Long;

my ($head, $tail, $sep, $overlap) = (10, 10, '', 1);
my $last = 0; my @buf = ();

GetOptions (
  'head=i' => \$head, 'tail=i' => \$tail, 'sep=s' => \$sep,
  'overlap!' => \$overlap
) or die ('Error in command line arguments.');

while (<>)
{
  print if $. <= $head; $buf[$. % $tail] = $_; $last = $.;
}

print $sep;

my $t1 = $tail > $last ? $last : $tail;
my $t2 = !$overlap && $last - $head < $t1 ? $last - $head : $t1;

for (my $i = $t2 - 1; $i >= 0; --$i)
{
  print $buf[($last - $i) % $tail];
}