GNU bug report logs - #65659
RFC: changing printf(1) behavior on %b

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From: Stephane Chazelas <stephane <at> chazelas.org>
To: Stephane Chazelas via austin-group-l at The Open Group <austin-group-l <at> opengroup.org>,  Eric Blake <eblake <at> redhat.com>, Phi Debian <phi.debian <at> gmail.com>, chet.ramey <at> case.edu, 65659 <at> debbugs.gnu.org, bug-bash <at> gnu.org, Steffen Nurpmeso <steffen <at> sdaoden.eu>
Subject: bug#65659: RFC: changing printf(1) behavior on %b
Date: Sat, 2 Sep 2023 09:49:12 +0100

2023-09-01 23:28:50 +0200, Steffen Nurpmeso via austin-group-l at The Open Group:
[...]
>  |FWIW, a "printf %b" github shell code search returns ~ 29k
>  |entries
>  |(https://github.com/search?q=printf+%25b+language%3AShell&type=code&l=Sh\
>  |ell)
>  |
>  |That likely returns only a small subset of the code that uses
>  |printf with %b inside the format and probably a few false
>  |positives, but that gives many examples of how printf %b is used
>  |in practice.
> 
> Actually this returns a huge amount of false positives where
> printf(1) and %b are not on the same line, let alone the same
> command, if you just scroll down a bit it starts like neovim match
[...]

You're right, I only looked at the first few results and saw
that already gave interesting ones.

Apparently, we can also search with regexps and searching for
printf.*%b
(https://github.com/search?q=%2Fprintf.*%25b%2F+language%3AShell&type=code)
It's probably a lot more accurate. It returns ~ 19k.

(still FWIW, that's still just a sample of random code on the
internet)

[...]
> Furthermore it shows a huge amount of false use cases like
> 
>  printf >&2 "%b\n" "The following warnings and non-fatal errors were encountered during the installation process:"
[...]

Yes, I also see a lot of echo -e stuff that should have been
echo -E stuff (or echo alone in those (many) implementations
that don't expand by default or use the more reliable printf
with %s (not %b)).

> It seems people think you need this to get colours mostly, which
> then, it has to be said, is also practically mislead.  (To the
> best of *my* knowledge that is.)
[...]

Incidentally, ANSI terminal colour escape sequences are somewhat
connecting those two %b's as they are RGB (well BGR) in binary
(white is 7 = 0b111, red 0b001, green 0b010, blue 0b100), with:

R=0 G=1 B=1
printf '%bcyan%b\n' "\033[3$(( 2#$B$G$R ))m" '\033[m'

(with Korn-like shells, also $(( 0b$B$G$R )) in zsh though zsh
has builtin colour output support including RGB-based).

Speaking of stackexchange, on the June data dump of
unix.stackexchange.com:

stackexchange/unix.stackexchange.com$ xml2 < Posts.xml | grep -c 'printf.*%b'
494

(FWIW)

Compared with %d (though that will have entries for printf(3) as well):

stackexchange/unix.stackexchange.com$ xml2 < Posts.xml | grep -c 'printf.*%d'
3444

-- 
Stephane

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