#!/usr/bin/env bash

# @(#) s1       Demonstrate repeating string insertion, insert_every

PROGRAM="./insert_every"
# Utility functions: print-as-echo, print-line-with-visual-space, debug.
# export PATH="/usr/local/bin:/usr/bin:/bin"
LC_ALL=C ; LANG=C ; export LC_ALL LANG
pe() { for _i;do printf "%s" "$_i";done; printf "\n"; }
pl() { pe;pe "-----" ;pe "$*"; }
db() { ( printf " db, ";for _i;do printf "%s" "$_i";done;printf "\n" ) >&2 ; }
db() { : ; }
C=$HOME/bin/context && [ -f $C ] && $C 

FILE=${1-data1}

pl " Input data file $FILE:"
cat $FILE

pl " Results:"
$PROGRAM --n=3 "Hello" < $FILE

pl " Text of perl code, $PROGRAM:"
cat $PROGRAM

exit 0

$  ./s1

Environment: LC_ALL = C, LANG = C
(Versions displayed with local utility "version")
OS, ker|rel, machine: Linux, 3.16.0-4-amd64, x86_64
Distribution        : Debian 8.3 (jessie) 
bash GNU bash 4.3.30

-----
 Input data file data1:
1
2
3
4
5
6
7
8
9
10

-----
 Results:
1
2
3
Hello
4
5
6
Hello
7
8
9
Hello
10

-----
 Text of perl code, ./insert_every:
#!/usr/bin/env perl

# @(#) insert_every     Write first-parameter-string after --n=x lines.
# $Id: insert_every,v 1.2 2015/03/18 12:38:41 drl Exp drl $

# The Missing Textutils, Ondrej Bojar, obo@cuni.cz
# http://www.cuni.cz/~obo/textutils
#
# 'insert_every' processes stdin to stdout, writing ARG1 after every n
# lines

use strict;
use warnings;
use Getopt::Long;

sub usage {
  print STDERR "insert_every text_to_insert <stdin >stdout
Options:
  --n=X   ... insert after n lines
";
  exit 1;
}

my $n     = 10;
my $usage = 0;
GetOptions(
  "help" => \$usage,
  "n=i"  => \$n,
);
usage() if $usage;

my $extra = shift;

my $nr = 0;
while (<>) {
  $nr++;
  print;
  print $extra. "\n" if $nr % $n == 0;
}