appending variable to end of file with sed

I have dates on the last line in files like this:

20230220

and I want to add hours like this:

2023022000

The hours are continuously assigned to a variable i in a loop.

How to use sed inside the loop to append that variable to the end of files.

I tried with:

sed '$a|$i|>end>' file.txt

But couldn't append to file.

You can do it with sed like so:

sed '$a'"$i"'' file.txt

Notice: you need to break the single quotes ' around your shell parameter/variable so that it is expanded to its value or otherwise, it will be treated as literal $i.

You might, however not need sed at all ... Since you're already using a shell loop, you can append it using shell redirection with the append operator >> like so:

echo "$i" >> file.txt

That will put the value from the expansion of $i on a new line after the last line of the file.

sed, however might be needed if you want to put that value on the same last line at the end next to what's already in that line ... e.g. if the file has an existing last line that already has 20230220 at its end and you want to add 00 right next to it so it becomes 2023022000, then you can use:

sed '$s/$/'"$i"'/' file.txt

Notice: for sed to edit the files(instead of only printing to terminal), you need to set the -i flag like so:

sed -i '$a'"$i"'' file.txt