Stephane Chazelas <stephane.chaze...@gmail.com> wrote, on 30 Apr 2018:
>
> The head/tail specifications refer to line/byte offsets as
> offsets within *files* as opposed to *input*.
>
> Does it mean that:
>
> { head -n 1; head -n 1; } < file
> { tail -n 1; tail -n 1; } < file
>
> are required to print the first/last line of "file" twice
> (assuming "file" is seekable and is not modified between the two
> head/tail invocations)?
>
> In the case of "head", I can't find any implementation that
> does, they all return the first line of their *input* as opposed
> to the first line of whatever file may be open on stdin.
The intended behaviour of the head example is that the first head
writes the first line of "file" and the second head writes the second
line of "file". See XCU 1.4 under INPUT FILES.
However, I can see that text such as "The first <i>number</i> lines
of each input file shall be copied" for head -n is misleading in
this respect.
> However, in the case of "tail", for seekable stdin, traditional
> implementations used to seek to the end of the file open on
> stdin and look backward for the last line from there even if the
> initial position of stdin was past the start of that last line
> (it could even be past the end of the file).
The intention is certainly that when reading from standard input,
tail should not write anything that is before the initial offset of
standard input.
--
Geoff Clare <g.cl...@opengroup.org>
The Open Group, Apex Plaza, Forbury Road, Reading, RG1 1AX, England
