On 27/06/2025 10:10, Pádraig Brady 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.
Thanks for flagging this.
Note the GNU info doc on tail says:
"GNU ‘tail’ can output any amount of data (some other versions of
‘tail’ cannot). It also has no ‘-r’ option (print in reverse), sincereversing
a file is really a different job from printing the end of a
file; BSD ‘tail’ (which is the one with ‘-r’) can only reverse files
that are at most as large as its buffer, which is typically 32 KiB. A
more reliable and versatile way to reverse files is the GNU ‘tac’
command.
"
BTW the POSIX tracker on this is:
https://www.austingroupbugs.net/view.php?id=877
cheers,
Padraig
