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

It doesn’t work because ‘>’ redirection implies truncation, and to avoid keeping the whole output of sort in the memory before re-directing to the file, bash truncates and redirects output before running sort. Thus, contents of the file1 file will be truncated before sort will have a chance to read it.