cat abc.txt > abc.txt
cat abc.txt | sed "any sed arguments" > abc.txt
sed "any sed arguments" abc.txt > abc.txt

#!/bin/bash
IAm=${0##*/}

file="abc.txt"
tmpfile="$IAm".$$

trap 'rm -rf "$tmpfile"' EXIT

line_number=1
current_value=MOHIT
new_value=Ankit

# Function to convert a string to a BRE that will match a case-insensitive
# version of that string.
# Usage: string2CIBRE "string"
string2CIBRE() {
	awk -v s="$1" '
		BEGIN {	for(i = 1; i <= length(s); i++)
			printf("%s", ((L = substr(s, i, 1)) ~ /[[:alpha:]]/) ? \
			    "[" tolower(L) toupper(L) "]" : L)
		}
	'
}

printf 'Contents of "%s" before sed:\n' "$file"
cat "$file"
sed "${line_number}s/$(string2CIBRE "$current_value")/$new_value/g" "$file" > "$tmpfile"  &&
	cp "$tmpfile" "$file"
printf '**********\nContents of "%s" after sed:\n' "$file"
cat "$file"

Hello , My name is Mohit, all lowercase: mohit, all uppercase: MOHIT
I am learning shell script
Hello , My name is Mohit, all lowercase: mohit, all uppercase: MOHIT
I am learning shell script

Contents of "abc.txt" before sed:
Hello , My name is Mohit, all lowercase: mohit, all uppercase: MOHIT
I am learning shell script
Hello , My name is Mohit, all lowercase: mohit, all uppercase: MOHIT
I am learning shell script
**********
Contents of "abc.txt" after sed:
Hello , My name is Ankit, all lowercase: Ankit, all uppercase: Ankit
I am learning shell script
Hello , My name is Mohit, all lowercase: mohit, all uppercase: MOHIT
I am learning shell script

Hello , My name is Ankit, all lowercase: Ankit, all uppercase: Ankit
I am learning shell script
Hello , My name is Mohit, all lowercase: mohit, all uppercase: MOHIT
I am learning shell script