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