Shell Programming and Scripting

$ cat abc.txt
vivek is a good boy
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Replace character position 7-8 with "ok" in 1st line of abc.txt, here is the way using sed:

$ sed -r "1 ~ s/^(.{6})(.{2})/\1ok/" abc.txt > abc.txt.tmp; mv abc.txt.tmp abc.txt

$ cat abc.txt
vivek ok a good boy
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shell programming forum

file=$1
line=$2
char=$3
len=$4
val=$5
nawk -v l="$line" -v c="$char" -v len="$len" -v v="$val" '{
if(NR==l)
	print substr($0,1,c-1) v substr($0,c+len)
else
	print $0
}' $file > ${file}.bak
rm $file
mv ${file}.bak $file

awk 'BEGIN{
   printf "Enter line number: "
   getline linenum < "-"
   printf "Character position eg 2-3: "
   getline chpos < "-"
   printf "Substituion  value eg :" 
   getline subval < "-"
   if ( chpos ~ /-/  ) {
    n=split( chpos, charpos,"-")
    startpos = charpos[1]
    endpos=charpos[2]
   }else {
    startpos=chpos
    endpos=""
   }
}
NR==linenum{
   sub( substr($0,startpos,endpos) , subval)   
}
1' "file"