Peng Yu wrote:
> `sort input.txt -o input.txt` overwrites the input file. My
> understanding is that sort reads everything and then write the output.
> So it is OK to overwrite the original file. But I want to be sure. Can
> anyone confirm if this is the case? Thanks.
Yes. Using -o is how you sort a file in place. This is the
traditional Unix sort behavior. The GNU sort documentation says:
-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'.
And so -o is the correct option to sort in place. However note it also
says:
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.
Do not use with -m!
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.
For maximum safety you should always set -o before any input files.
As in the documented example. Your example had options last. Instead
for maximum safety and portability you should place the option first.
sort -o file.txt file.txt
Bob
