Explanation
The problem with the command
sed -e "s/a/b/" /tmp/file >/tmp/file
is that it tries to read and write from the same file, which results in overwriting the file before the command can process it. The shell sets up the redirection by opening the file /tmp/file for writing and truncating it to zero length. Then it executes the sed command, which tries to read from the same file, but finds it empty.
Therefore, the output is also empty and the file remains empty. A possible solution is to use a temporary file for the output and then rename it to the original file name. For example:
sed -e "s/a/b/" /tmp/file >/tmp/file.tmp && mv /tmp/file.tmp /tmp/file
This way, the original file is not overwritten until the sed command finishes successfully. The other options are either incorrect or not applicable. The file order is correct, the sed command does match something in the file, and the > character is valid for redirection. The | character is used for piping, not redirection. References:
* LPIC-1 Exam 101 Objectives, Topic 103: GNU and Unix Commands, 103.4 Use streams, pipes and redirects
* LPIC-1 Linux Administrator 101-500 Exam FAQ, LPIC-1 Exam 101 Objectives, GNU and Unix
* Commands (Total Weight: 25)