why sed replace + redirection deletes my file?

Question

I am using sed to search and replace two strings in a file in bash (GNU sed)

This is the file after

-rw-r--r-- 1 websync www-data 4156 mar 27 12:56 /home/websync/tmp/sitio-oficial/sitios/wp-config.php

here is the command I run

sed 's/www-test/www/g' /home/websync/tmp/sitio-oficial/sitios/wp-config.php > /home/websync/tmp/sitio-oficial/sitios/wp-config.php

and the result

-rw-r--r-- 1 websync www-data 0 mar 27 13:05 /home/websync/tmp/sitio-oficial/sitios/wp-config.php

EDIT: If I don't redirect sed's output, then I got the correct output. If I redirect to a new file, all works ok.

Answer 1

This is normal. you can't read and write to the same file in a pipeline like this. (this will fail with other utilities than sed).

Use the in-place flag -i instead:

sed -i 's/www-test/www/g' /home/websync/tmp/sitio-oficial/sitios/wp-config.php

Answer 2

sed reads your files in as a stream and outputs a stream as well. As soon as you perform the redirection into your file the contents are overwritten, and since that file is being read as a stream, it hasn't even started being read by sed yet. When sed does start reading the file it is empty so it finishes immediately with no output.

Use -i, to do an in-place edit, instead:

sed 's/www-test/www/g' -i /home/websync/tmp/sitio-oficial/sitios/wp-config.php

Answer 3

The redirection opens the file for output, truncating it. This happens simultaneously to sed opening it for reading, so sed sees the truncated version. You should redirect your output to a different file to avoid clobbering your input, or use sed's in-place editing mode instead of using redirection:

sed 's/www-test/www/g' -i /home/websync/tmp/sitio-oficial/sitios/wp-config.php