sort inplace - like sed --in-place - exists?

Question

Am I blind or is there no option like --in-place for sort?

In order to save results to the input file, sed uses -i (--in-place).

Redirecting the output of sort to the input file

sort < f > f

results in making it empty. If there is no --in-place option - maybe there is some trick how to do this in handy way?

(The only thing that cames to my mind:

sort < f > /tmp/f$$ ; cat /tmp/f$$ f ; rm /tmp/f$$

Moving is not right choice, cause file permissions might be changed. That's why I overwrite it with contents of tmp which I then remove.)

Answer 1

sort has the -o, --output option that take a filename as argument. If it is the same as the input files, it write the result to a temporary file, then overwrite the original input file (somewhat as sed do).

From GNU sort info page:

`-o OUTPUT-FILE'
`--output=OUTPUT-FILE'
      Write output to OUTPUT-FILE instead of standard output.  Normally,
      `sort' reads all input before opening OUTPUT-FILE, so you can
      safely sort a file in place by using commands like `sort -o F F'
      and `cat F | sort -o F'.  However, `sort' with `--merge' (`-m')
      can open the output file before reading all input, so a command
      like `cat F | sort -m -o F - G' is not safe as `sort' might start
      writing `F' before `cat' is done reading it.

      On newer systems, `-o' cannot appear after an input file if
      `POSIXLY_CORRECT' is set, e.g., `sort F -o F'.  Portable scripts
      should specify `-o OUTPUT-FILE' before any input files.

and from The Open Group Base Specifications Issue 7:

-o  output
    Specify the name of an output file to be used instead of the standard 
    output. This file can be the same as one of the input files.

Answer 2

It's dangerous to overwrite the input file with the output file, because if the program or the system crashes while the file is being written, you've lost both.

A few programs (mostly GNU versions) have an in-place option (e.g. -i on perl and GNU sed; -o on GNU sort). They work by putting the data in a temporary file and then moving it into place. For programs that have no such option, Colin Watson's sponge utility (included in Joey Hess's moreutils) does the job safely for any program (examples: Can I make cut change a file in place?; How can I make iconv replace the input file with the converted output?).

Only in those rare cases where you can't recreate the original file with the same permissions do I recommend overwriting the file in place. In this case, you'd better save the original input somewhere. And then you can simply process the copy of the input and send it into the original file.

cp -p f ~/f.backup
sort <~/f.backup >|f
rm ~/f.backup # optional