replace "line_number" line to "repace_string" in "file_name"

[bash] replace.sh

#!/bin/bash

# function replace (file_name, line_number, replace_string)
replace () {
    file=$1
    number=$2
    string=$3

    n=1
    cat $file | while read line
    do
        # empty file
        if [[ $n == 1 ]]; then
            > $file
        fi

        # write file
        if [[ $number == $n ]]; then
            echo $string >> $file
        else
            echo $line >> $file
        fi

        # increase counter
        n=$(( $n + 1 ))
    done
}

# test
replace file.txt 1 "hello1"
replace file.txt 2 "hello2"
replace file.txt 3 "hello3"
replace file.txt 4 "hello4"
cat file.txt

file.txt

1
2
3
4
5
6
7