Taking input from a file and writing to it in the same command

Question

My encoder is a program that encodes a piece of text and prints the encoded text to the standard output. But when I do:

cat file.txt | encoder > file.txt

what happens is that all the text in file.txt is completely erased and there remains nothing in it.

Why does this happen? Is there something wrong with the command?

Note: There is nothing wrong with encoder. It works perfectly. I can achieve what I want with

cat file.txt | encoder > file2.txt

Answer 1

You want sponge ; it's in the moreutils package.

cat file.txt | encoder | sponge file.txt

Answer 2

You can use Vim in Ex mode:

ex -sc '%!encoder' -cx file.txt

1. % select all lines

2. ! run command

3. x save and close