Why doesn't "sort file1 > file1" work?

Question

When I am trying to sort a file and save the sorted output in itself, like this

sort file1 > file1;

the contents of the file1 is getting erased altogether, whereas when i am trying to do the same with 'tee' command like this

sort file1 | tee file1;

it works fine [ed: "works fine" only for small files with lucky timing, will cause lost data on large ones or with unhelpful process scheduling], i.e it is overwriting the sorted output of file1 in itself and also showing it on standard output.

Can someone explain why the first case is not working?

Answer 1

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.