Re sponge: the shell will open the output file for writing before invoking the command so in the example joey provides, /etc/passwd will be an empty file by the time sed opens it.

I think this is a good question as moreutils doesn’t do a very clear job of explaining it.

Sponge lets you do a series of piped transformations on 1 or more files and overwrite the result back to the original files.

Using > things will just break.

I’m guessing this also means you can’t pipe something bigger than what your memory can hold.

> Why not use > file?

This, I learned the hard way, and I am not the only one. So in order for you not to make that mistake: the first thing that happens is that "file" is truncated, that happens before any command is run, so

  sed s/foo/bar/ file > file 

will always result in an empty "file", because it will be truncated before "sed" is run. I lost a couple of hours of work like that, once, and then I learned.

From the manpage of sponge

  sponge reads standard input and writes it out to the specified file. Unlike a shell redirect, sponge soaks up all its input before opening the output file. This allows constricting pipelines that read from and write to the same file.

So, the command

  sed s/foo/bar/ file | sponge file 

will do what you expect

Great question.

There's a good Stack Overflow answer here: https://unix.stackexchange.com/questions/207919/sponge-from-...

> Why not use > file?

I'm assuming sponge buffers everything in memory so you don't get concurrent modification bugs.