Bash: Inserting one file's content into another file after the pattern

Question

I'm trying to write a bash script, which will do the following:

1. reads the content from the first file (as a first argument)
2. reads the content from the second file (as a second argument)
3. finds the line in the second file with the given pattern (as a third argument)
4. inserts text from the first file to the second file after the line of the pattern.
5. prints final file on the screen.

For example:

first_file.txt:

111111
1111
11
1

second_file.txt:

122221
2222
22
2

pattern:

2222

output:

122221
111111
1111
11
1
2222
111111
1111
11
1
22
2

What should I use to realize this functionality on BASH?

I wrote the code, but it doesn't work properly (why?):

    #!/bin/bash

    first_filename="$1"
    second_filename="$2"
    pattern="$3"

    while read -r line
    do
    if [[ $line=˜$pattern ]]; then
            while read -r line2
            do
                    echo $line2
            done < $second_filename
    fi
    echo $line
    done < $first_filename

Answer 1

sed can do that without loops. Use its r command:

sed -e '/pattern/rFILE1' FILE2

Test session:

$ cd -- "$(mktemp -d)" 
$ printf '%s\n' 'nuts' 'bolts' > first_file.txt
$ printf '%s\n' 'foo' 'bar' 'baz' > second_file.txt
$ sed -e '/bar/r./first_file.txt' second_file.txt
foo
bar
nuts
bolts
baz