#!/bin/sh

# Check if first and second parameters exist
if [ ! -z "$2" ]; then
    STRING=$(cat $1)
    # Check if the supplied file exist
    if [ -e $2 ]; then
        sed -i -e "2i$STRING" $2
        echo "The string "$STRING" has been successfully inserted."
    else
        echo "The file does not exist."
    fi
else
   echo "Error: both parameters must be given."
fi

first_line
second_line

REAL_FIRST_LINE
REAL_SECOND_LINE

sed: -e expression #1, char 24: unterminated `s' command
The string "first_line
second_line" has been successfully inserted.

REAL_FIRST_LINE
first_line
second_line
REAL_SECOND_LINE

sed "1r $1" "$2"

cat "$1" | sed '2r /dev/stdin' "$2"

r filename
    As a GNU extension, this command accepts two addresses.

    Queue the contents of filename to be read and inserted into the output stream
    at the end of the current cycle, or when the next input line is read. Note that
    if filename cannot be read, it is treated as if it were an empty file, without
    any error indication.

    As a GNU sed extension, the special value /dev/stdin is supported for the file
    name, which reads the contents of the standard input.

$ sed '1r content.txt' example.txt
REAL_FIRST_LINE
first_line
second_line
REAL_SECOND_LINE