Shell Programming and Scripting

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$ cat infile
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4

$ cat script.sed
2 i\
2

$ sed -f script.sed infile
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2                                                                                                                                                                                                                 
3                                                                                                                                                                                                                 
4

#!/usr/bin/env ksh
awk '
        {
                if( last )
                        for( i = last + 1; i < $1+0; i++ )
                                printf( "%d\n", i );
                last = $1+0;
                printf( "%s\n", $0 );
        }
' <input-file >output-file

#!/usr/bin/env ksh

last=0
while read x junk
do
        for (( i=$(( $last + 1 )); i < $x; i++ ))
        do
                printf "%d\n" $i
        done

        last=$x
        print $x $junk
done <input >output

awk 'NR==2{$0="2" RS $0}1' infile

sed '2s/^/2\n/' infile

#!/bin/ksh
while read -r line; do
  if (( ++i == 2 )); then
    echo 2
  fi
  printf "%s\n" "$line"
done < infile