In that situation, since head's standard input is not interactive, head's stdin stream is allowed to be fully buffered. If that is the case, it may consume more than just a single line.
I had this happen to me a few years ago. I thought I had found a bug, but upon further investigation learned that there was a hole in my understanding.
Just a head's up.
This is what POSIX has to say:
Quote:
At program start-up, three streams are predefined and need not be opened explicitly: standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). When opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device.
An demonstration of the issue (old debian system, but I first encountered a similar issue on a BSD system):
Note that due to full buffering, line boundaries are not respected. The first line of the second head's output is empty even though seq does not generate any empty lines.
The following is safe against a buffering "head" command (designed by performance junkies who don't trust the OS buffering mechanisms):
NB this time I use { grouping; } not ( subshell ).
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