Is it safe to pipe the output of several parallel processes to one file using >>?

No. It is not guaranteed that lines will remain intact. They can become intermingled.

From searching based on liori’s answer I found this:

Write requests of {PIPE_BUF} bytes or less shall not be interleaved with data from other processes doing writes on the same pipe. Writes of greater than {PIPE_BUF} bytes may have data interleaved, on arbitrary boundaries, with writes by other processes, whether or not the O_NONBLOCK flag of the file status flags is set.

So lines longer than {PIPE_BUF} bytes are not guaranteed to remain intact.

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