[Toybox] ps -t is cheating.

[Rich Felker]

On Tue, Oct 20, 2015 at 09:16:37AM -0500, Rob Landley wrote:
> On 10/19/2015 11:40 PM, Isaac Dunham wrote:
> > On Mon, Oct 19, 2015 at 12:09:17AM -0500, Rob Landley wrote:
> >> So posix says this:
> >>
> >> -t  termlist
> >>   Write information for processes associated with terminals given in
> >>   termlist. The application shall ensure that the termlist is a single
> >>   argument in the form of a <blank> or <comma>-separated list. Terminal
> >>   identifiers shall be given in an implementation-defined format. [XSI]
> >>   [Option Start]  On XSI-conformant systems, they shall be given in one
> >>   of two forms: the device's filename (for example, tty04) or, if the
> >>   device's filename starts with tty, just the identifier following the
> >>   characters tty (for example, "04" ). [Option End]
> >>
> >> And once again posix is somewhere back before the 1980's because
> >> pseudo-terminals do not start with "/dev/tty". In the case of linux,
> >> they've been /dev/pts/12 since, apparently, 1998.
> > 
> > VTs start with /dev/tty, as do all interfaces that are theoretically
> > serial-based. Note that 'if'.
> > 
> > /dev/pts is actually the result of another part of POSIX (posix_openpt & co.)
> 
> There's a kernel .config option for legacy "unix98" ptys...

"Unix98" is the kernel's name for the /dev/pts ptys. The old
/dev/tty[pqrs]N and /dev/pty[pqrs]N monstrosities are the legacy
option, sometimes also called "BSD ptys" from the old BSD tradition of
using suid binaries for everything and having a super-dumb kernel. :-)

> >> The ps man page says:
> >>
> >>   -t ttylist
> >>        Select by tty.  This selects the processes associated with the
> >>        terminals given in ttylist.  Terminals (ttys, or screens for
> >>        text output) can be specified in several forms: /dev/ttyS1,
> >>        ttyS1, S1.  A plain "-" may be used to select processes not
> >>        attached to any terminal.
> >>
> >> Which is, once again, outright lying, because "-t 41" matches pts/41 but
> >> -t 5 does _not_ match tty5. You have to say "-t tty5" to get the getty
> >> instance on there.
> >>
> >> Meanwhile procutils "ps -t pts/41" works as does "ps -t pts/../tty5"
> >> which is just _creepy_ and I'm not doing that bit. And -tty S0 is of
> >> course /dev/ttyS0 not /dev/pts/S0.
> >>
> >> I pine for a spec that means something,
> >>
> >> Rob
> >>
> >> P.S.  Once again, the hard part is writing help text so ps --help can
> >> explain the expected behavior succinctly. Yeah, I can do that, but how
> >> do I explain it in a way that makes it sound intentional: -t ## is a pts
> >> (unless maybe there isn't one? Does it fall back to tty5 if there's no
> >> pts5? Hard to test right now because I've got /dev/pts/68 but only
> >> /dev/tty63). Grrr. STOP TRYING TO BE CLEVER IN WAYS YOU DON'T EXPLAIN TO
> >> YOUR USERS IN THE MAN PAGE. Right special case "-t number" to be pts/
> >> instead of tty, accept pts/ to _not_ mean implicit tty prefix...
> > 
> > accept the following:
> > * filenames relative to /dev
> 
> ps -t urandom
> 
> Yup, it took it.
> 
> > * filenames relative to /dev/pts
> 
> Like /dev/pts/../zero?
> 
> Oddly it doesn't seem to like symlinks. ln -s /dev/urandom 1234 neither
> "ps -t 1234" or "ps -t ./1234" go through. But if I "cp -a /dev/zero"
> zero and then "ps -t ./zero"...
> 
> Ah. 'mv zero zzero" then "ps -t ./zzero" and it won't find it, the
> problem is it forces relative paths to be vs /dev (but doesn't stop you
> from .. back out of them, so "ps -t ../$HOME/zzero" finds it.
> 
> What a mess.

Indeed.

> > * fallback to "/dev/tty<arg>".
> > No need to special case numbers.
> 
> The reference question "What does busybox do?" has the answer "Not
> implement -t." (The option is in posix but not busybox...)
> 
> > This is *roughly* what procutils is doing, apart from that 'no need to
> > special-case numbers' I'd assume.
> 
> Except "ps -t S0" is /dev/ttyS0...

This doesn't look like it needs any special case. It's just the string
"S0" appended to "/dev/tty". From what I can tell, /dev/pts/N vs
/dev/ttyN is where numbers get nasty..

Rich

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