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Re: VT2 for Xorg?




On Mon, Jul 09, 2018 at 09:08:17AM -0400, Greg Wooledge wrote:
> On Sun, Jul 08, 2018 at 01:08:59PM +1000, Zenaan Harkness wrote:
> > With a bash fn like:
> >   ttyGetNum () { return $(tty | grep -oE '[0-9]+$') ; }
> > 
> > Then something like this will id your current tty:
> >   ttyGetNum;  export Xorg_vt=vt$?
> 
> Ewwwww!

:D


> The return status of a shell function is NOT intended to be used this way.
> It's analogous to the exit status of a shell command.  It's a single
> byte, unsigned, so you can only return 0 to 255.  It's intended to convey
> success or failure, not to carry an actual data payload.

It's a byte, and I have slightly less than 255 Linux VTs, so it's
adequate to this particular purpose ;)


> I recommend that you switch to using a regular variable for this purpose.
> I happen to like "r".  With a regular variable, you're not restricted to
> integers from 0 to 255, and you retain the ability to convey succss/failure
> through the return code.
> 
> As an aside, you can save the call to GNU grep by simply stripping the
> leading "/dev/tty" from the output of tty using a shell parameter
> expansion:
> 
>   ttyGetNum() {
>     local t
>     t=$(tty) || return 1
>     [[ $t = /dev/tty+([0-9]) ]] || return 1
>     r=${t#/dev/tty}
>   }
> 
>   if ttyGetNum; then export Xorg_vt=vt"$r"; fi

I have been on a bit of a "support dash/ posix shell" and therefore
removed various Bash-isms. There was at least one bash-ism (that I
was using) that I was not able to remove. At least not easily.

I don't remember what that was at the moment - haven't been hacking
on this for some months.