Outputting less into a file

Question

while on bash, i tried to delete the 3 last columns of a file using:

head -n -3 file > file

but the file comes out empty. I can walkaround it but I would like to know what causes that.

Thanks a bunch!

Answer 1

Let's break down what that command does.

1. > file

   This opens file for writing, and truncates it.

2. head -n -3 file

   This reads all but the last 3 lines of file. Yes, the same file you truncated in step 1.

Answer 2

What you're trying to do is inline replacement, which isn't really possible without workarounds. In your command, the original file is overwritten because it's already opened and cleared when you send off the command.

Here are a couple of ideas:

• Use a temporary file

  head -n -3 file > tmp
  mv tmp file
  

• Use the sponge tool from moreutils, which soaks up all input and then writes to the file:

  head -n -3 file | sponge file