foo="Hello"
foo="${foo} World"
echo "${foo}"
> Hello World

In general to concatenate two variables you can just write them one after another:

a='Hello'
b='World'
c="${a} ${b}"
echo "${c}"
> Hello World

for finding a specific line in a file, have you considered using grep?

grep "thing I'm looking for" /path/to/my.file

this will output the lines that match the thing you're looking for. Moreover this can be piped to xargs as in your question.

If you need to look at a particularly numbered line of a file, consider using the head and tail commands (which can also be piped to grep).

cat /path/to/my.file | head -n5 | tail -n1 | grep "thing I'm looking for"

These commands take the first lines specified (in this case, 5 and 1 respectively) and only prints those out. Hopefully this will help you accomplish your task.

Happy coding! Leave a comment if you have any questions.

Use sed

Usage

$ cat file
Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9
Line 10

To print one line (5)

$ sed -n 5p file
Line 5

To print multiple lines (5 & 8)

$ sed -n -e 5p -e 8p file
Line 5
Line 8

To print specific range (5 - 8)

$ sed -n 5,8p file
Line 5
Line 6
Line 7
Line 8

To print range with other specific line (5 - 8 & 10)

$ sed -n -e 5,8p -e 10p file
Line 5
Line 6
Line 7
Line 8
Line 10