Shell Programming and Scripting

#!/bin/sh

if [ -z $1 ]; then                                 
        echo "Usage: ./script <filename>"           
        exit 1
elif [ -f "$1" ]; then
        fname="$1"
fi

output="output.txt"                               
i=0                                                

while read a;                                                        
do                                                

  i=$((i+1))
                                                                                                    
     num=$a                                      
   
    
    echo "$a" | sed 's/$/_'$num'/' >> "$output"    

done < "$fname"