Why piping to the same file doesn't work on some platforms?

Question

In cygwin, the following code works fine

$ cat junk
bat
bat
bat

$ cat junk | sort -k1,1 |tr \'b\' \'z\' > junk

$ cat junk
zat
zat
zat

Answer 1

In general this can be expected to break. The processes in a pipeline are all started up in parallel, so the > junk at the end of the line will usually truncate your input file before the process at the head of the pipelining has finished (or even started) reading from it.

Even if bash under Cygwin let's you get away with this you shouldn't rely on it. The general solution is to redirect to a temporary file and then rename it when the pipeline is complete.