Why does redirecting the output of a file to itself produce a blank file?

When you use >, the file is opened in truncation mode so its contents are removed before the command attempts to read it.

When you use >>, the file is opened in append mode so the existing data is preserved. It is however still pretty risky to use the same file as input and output in this case. If the file is large enough not to fit the read input buffer size, its size might grow indefinitely until the file system is full (or your disk quota is reached).

Should you want to use a file both as input and output with a command that doesn't support in place modification, you can use a couple of workarounds:

• Use an intermediary file and overwrite the original one when done and only if no error occurred while running the utility (this is the safest and more common way).

  fold foo.txt > fold.txt.$$ && mv fold.txt.$$ foo.txt
  

• Avoid the intermediary file at the expense of a potential partial or complete data loss should an error or interruption happen. In this example, the contents of foo.txt are passed as input to a subshell (inside the parentheses) before the file is deleted. The previous inode stays alive as the subshell is keeping it open while reading data. The file written by the inner utility (here fold) while having the same name (foo.txt) points to a different inode because the old directory entry has been removed so technically, there are two different "files" with the same name during the process. When the subshell ends, the old inode is released and its data is lost. Beware to make sure you have enough space to temporarily store both the old file and the new one at the same time otherwise you'll lose data.

  (rm foo.txt; fold > foo.txt) < foo.txt
  

The file is opened for writing by the shell before the application has a chance to read it. Opening the file for writing truncates it.

In bash, the stream redirection operator ... > foo.txt empties foo.txt before evaluating the left operand.

One might use command substitution and print its result as a workaround. This solution takes less additional characters than in other answers:

printf '%s\n' "$(less foo.txt)" > foo.txt

Beware: This command does not preserve any trailling newline(s) in foo.txt. Have a look in the comment section below for more information

Here, the command substitution $(...) is evaluated before the stream redirection operator >, hence the preservation of information.