How to use sed to remove the last n lines of a file

A funny & simple sed and tac solution :

n=4
tac file.txt | sed "1,$n{d}" | tac

NOTE

double quotes " are needed for the shell to evaluate the $n variable in sed command. In single quotes, no interpolate will be performed.
tac is a cat reversed, see man 1 tac
the {} in sed are there to separate $n & d (if not, the shell try to interpolate non existent $nd variable)

From the sed one-liners:

# delete the last 10 lines of a file
sed -e :a -e '$d;N;2,10ba' -e 'P;D'   # method 1
sed -n -e :a -e '1,10!{P;N;D;};N;ba'  # method 2

Seems to be what you are looing for.

I don't know about sed, but it can be done with head:

head -n -2 myfile.txt

If hardcoding n is an option, you can use sequential calls to sed. For instance, to delete the last three lines, delete the last one line thrice:

sed '$d' file | sed '$d' | sed '$d'

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