Re: [PATCH] test: Fix printf error when debug is enabled

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 30 Sep 2025 10:23:39 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Sep 30, 2025 at 12:24:17AM +0200, Stefano Brivio wrote:
> > On Mon, 29 Sep 2025 14:30:14 +0800
> > Yumei Huang <yuhuang@redhat.com> wrote:
> >   
> > > Running test pasta/tcp with debug enabled would get stuck with
> > > below error:
> > > 
> > >   + printf 'DEBUG: ns     socat -u OPEN:__BASEPATH__/big.bin TCP6:[__GW6__%__IFNAME__]:10003\n'
> > >   lib/term: line 38: printf: `_': invalid format character
> > > 
> > > The error occurs because printf interprets the % character as the
> > > start of a format specifier, and the following '_' isn't one of
> > > them.
> > > 
> > > Fix it by replacing 'printf "${*}\n"' with 'printf "%s\n" "$*"'.  
> > 
> > I'm not sure why, but this breaks colour highlights for me. Instead of
> > seeing a part of this message in blue, now I get:
> > 
> >   Test layout: \033[1;34msingle passt instance with guest\033[0m.\n
> > 
> > I can look into it if it only happens for me, or if needed.  
> 
> Oh, that's interesting.  I believe the colouring has been broken
> forever on Fedora, but I never got around to looking into it.
> Figuring this out might reveal why.  Maybe.
> 
> I realised I think I know why: those colour codes rely on turning
> "\033" into a terminal escape.  printf(1) will do that in the format
> string, but not in %s parameters
> 
> 	$ printf "a\tb%sc\n " "\t"
> 	a	b\tc
> 
> We can fix this either by moving the escape codes back into the format
> string, or using %b instead of %s, which explicitly interprets string
> escape codes.

Oh, I didn't know about %b, nice. It's also in POSIX.1-2024, see point
7. here:

  https://pubs.opengroup.org/onlinepubs/9799919799/utilities/printf.html#tag_20_96_13

...side note for Yumei: the whole test suite _should_ be POSIX shell,
that is, it doesn't rely on specific shell (e.g. Bash) implementations.

For example, on Debian, Ubuntu, and others, the default shell for
non-interactive usage (not what you have on terminal, but what executes
scripts) is dash:

  https://wiki.ubuntu.com/DashAsBinSh
  https://wiki.archlinux.org/title/Dash

So, if it's described in the POSIX specification (the one I just
linked) it will almost certainly work everywhere. If not, it might
break (but not necessarily).

-- 
Stefano