Appending a line to a file in the cat command?

I can do cat file.txt to get the contents of a file but I also want to tack on a final line of my own choosing.

I tried piping (cat file.txt ; echo "My final line") | but only the final line is getting passed through the pipe. How can I join the cat and final line?

Edit for a point of clarification: I do not wish to modify the file itself. I am aware that if this were the case I could do echo "My final line" >> file.txt or echo "My final line" | tee -a file.txt but I am only trying to do the append within the context of this particular command so I can pipe in the concatenation of file.txt and "My final line".

You can leverage cat's ability to read from stdin combined with it's ability to read multiple files to achieve this.

~$ cat file.txt
Hello from file.txt

~$ echo "My final line" | cat file.txt -
Hello from file.txt
My final line

You can also prepend a line, as such:

~$ echo "My final line" | cat - file.txt
My final line
Hello from file.txt

Note that you are not limited to a single line. cat will read from stdin until it reaches EOF. You can pass the output from curl, for example, to prepend or append to the output of cat.

~$ curl -s http://perdu.com | cat file.txt -
Hello from file.txt
<html><head><title>Vous Etes Perdu ?</title></head><body><h1>Perdu sur l'Internet ?</h1><h2>Pas de panique, on va vous aider</h2><strong><pre>    * <----- vous &ecirc;tes ici</pre></strong></body></html>

For appending a line to a file, you can just use shell append redirection operator, >> (the file will be open(2)-ed with O_APPEND flag):

echo 'My final line' >>file.txt

Now, if you want just to view the content of the file with a final line appended, i would use cat with two arguments:

First, your file obviously, let's say file.txt
Second argument would be the string of your choice, and to pass the string as a filename (as cat only deals with files) you can leverage process substitution, <(), which would return a file descriptor (/proc/self/fd/<fd_number>).

Putting these together:

cat file.txt <(echo 'My final line')

If you want the output to be paged, assuming less is your favorite pager:

less file.txt <(echo 'My final line')

sed -e '$aMy final line' file.txt

From man sed the option -e

-e script, --expression=script
    add the script to the commands to be executed

$ matches the last line and a appends the string.

If you wanted to permanently append the line to the file, use -i

-i[SUFFIX], --in-place[=SUFFIX]
    edit files in place (makes backup if SUFFIX supplied)

Changing the command to

sed -i -e '$aMy final line' file.txt

Appending a line to a file in the cat command?

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