How to copy the first few lines of a giant file, and add a line of text at the end of it using some Linux commands?

The head command can get the first n lines. Variations are:

head -7 file
head -n 7 file
head -7l file

which will get the first 7 lines of the file called "file". The command to use depends on your version of head. Linux will work with the first one.

To append lines to the end of the same file, use:

echo 'first line to add' >> file
echo 'second line to add' >> file
echo 'third line to add' >> file

or:

echo 'first line to add
second line to add
third line to add' >> file

to do it in one hit.

So, tying these two ideas together, if you wanted to get the first 10 lines of the input.txt file to output.txt and append a line with five "=" characters, you could use something like:

( head -10 input.txt ; echo '=====' ) > output.txt

In this case, we do both operations in a sub-shell so as to consolidate the output streams into one, which is then used to create or overwrite the output file.