On: moreutils

[cgag]

If you do something like:

  echo "hello world" >> dummyfile
  cat dummyfile | sed 's/hello/goodbye/' > dummyfile

dummyfile will be blank afterwards

if you do:

  cat dummyfile | sed 's/hello/goodbye/' | sponge dummyfile

It'll work as you'd expect (or at least as I'd expect). Otherwise `> dummyfile` will truncate dummyfile before it's read and inputed into sed.

[StillBored → cgag]

For the sed example, what you actually want is -i which modifies the given file.

AKA

sed -i 's/hello/goodby/' dummyfile

[phamilton → StillBored]

That's only in gnu coreutils. BSD coreutils don't support -i

[cheiVia0 → phamilton]

didn't

https://www.freebsd.org/cgi/man.cgi?query=sed

http://man.openbsd.org/OpenBSD-current/man1/sed.1

http://netbsd.gw.com/cgi-bin/man-cgi?sed++NetBSD-current

[bluejekyll → cgag]

Ah, yes. I like it. I've definitely made the exact mistake you point out. Thanks!

[cgag → bluejekyll]

I actually think I learned about sponge after I burned myself with this same mistake.

[mfontani → cgag]

Isn't that what `tee` does, minus the "also displaying on STDOUT" bit?

[FreeFull → mfontani]

tee doesn't store all the input before writing as far as I know, so it wouldn't work either in this case.

[Chris2048 → cgag]

Is this true? I though the issue was that the write might happen while the file is still being read?

Does the '>' operator blanks the file first, before the read happens?

[ferrari8608 → Chris2048]

> erases the file first and >> appends without erasing

This isn't the issue.