How do I execute any command editing its file (argument) "in place" using bash?

sort temp.txt -o temp.txt

A sort needs to see all input before it can start to output. For this reason, the sort program can easily offer an option to modify a file in-place:

sort temp.txt -o temp.txt

Specifically, the documentation of GNU sort says:

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.

While the documentation of BSD sort says:

If [the] output-file is one of the input files, sort copies it to a temporary file before sorting and writing the output to [the] output-file.

Commands such as uniq can start writing output before they finish reading the input. These commands typically do not support in-place editing (and it would be harder for them to support this feature).

You typically work around this with a temporary file, or if you absolutely want to avoid having an intermediate file, you could use a buffer to store the complete result before writing it out. For example, with perl:

uniq temp.txt | perl -e 'undef $/; $_ = <>; open(OUT,">temp.txt"); print OUT;'

Here, the perl part reads the complete output from uniq in variable $_ and then overwrites the original file with this data. You could do the same in the scripting language of your choice, perhaps even in Bash. But note that it will need enough memory to store the entire file, this is not advisable when working with large files.

Here's a more general approach, works with uniq, sort and whatnot.

{ rm file && uniq > file; } < file