Let's say file.txt contains the following text.

Hello
Hello
World
World
How
How
are
are
you?
you?

Delete a specific line

The following command will remove a specific line. In this example, line 5 is removed.

~]# sed -i 5d /path/to/file.txt

The result of the command will be:

Hello
Hello
World
World
How
are
are
you?
you?

Remove every other line

The following sed command will remove every other line of text from the file.

~]# sed -i '1~3d' /path/to/file.txt

The result of the command will be:

Hello
World
How
are
you
today?