vi 편집기

In vi do

:1,$d

to delete all lines.

The : introduces a command (and moves the cursor to the bottom).
The 1,$ is an indication of which lines the following command (d) should work on. In this case the range from line one to the last line (indicated by $, so you don't need to know the number of lines in the document).
The final d stands for delete the indicated lines.

There is a shorter form (:%d) but I find myself never using it. The :1,$d can be more easily "adapted" to e.g. :4,$-2d leaving only the first 3 and last 2 lines, deleting the rest.