On Fri, Jun 27, 2025 at 2:16 AM Pádraig Brady <p...@draigbrady.com> wrote: > On 27/06/2025 06:36, Collin Funk wrote: > > Creating this bug report since I have not been able to create a working > > implementation yet, and perhaps my looking into this will inspire others > > thoughts... > > > > POSIX-1.2024 adds the following SYNOPSIS for 'tail' [1]: > > > > tail [-f] [-c number|-n number] [file] > > tail -r [-n number] [file] > > > > Where the description for '-r' is: > > > > Copy the lines in reverse order (last line first). If -n is > > specified, that many lines of the file, starting with the last line, > > shall be copied. If -n is not specified, every line of the input > > file shall be copied. > > > > In a simple implementation of 'tail', one that allocates an array to > > store each line in memory, this would be easy to implement. However, > > the Coreutils version reads the file in BUFSIZE/getpagesize() chunks > > and then prints, repeating until completed. Therefore, we cannot just > > modify the existing code to swap lines in memory. > > > > Collin > > > > [1] https://pubs.opengroup.org/onlinepubs/9799919799/utilities/tail.html > > tail -r comes from the BSDs. > Also the BSDs don't have tac(1) which overlaps in functionality quite a bit. > I'm a bit surprised -r was added by POSIX, but fair enough.
"Surprised" is putting it lightly. I am disappointed and am tempted to push back and to delay encumbering GNU tail with -r. That is an option no GNU system needs, since they've all had tac since before 1992-era textutils. I've Cc'd Eric Blake, in case someone wants to propose adding tac to POSIX in spite of the fact that the BSDs still lack it.
