Why doesn’t “sort file1 > file1” work?

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As other people explained, the problem is that the I/O redirection is done before the sort command is executed, so the file is truncated before sort gets a chance to read it. If you think for a bit, the reason why is obvious – the shell handles the I/O redirection, and must do that before running the command.

The sort command has ‘always’ (since at least Version 7 UNIX) supported a -o option to make it safe to output to one of the input files:

sort -o file1 file1 file2 file3

The trick with tee depends on timing and luck (and probably a small data file). If you had a megabyte or larger file, I expect it would be clobbered, at least in part, by the tee command. That is, if the file is large enough, the tee command would open the file for output and truncate it before sort finished reading it.

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