When is STTY ignored?

When is STTY ignored?

[Stephane Chazelas]

Hello,

It's not very clear to me when one may use STTY.

STTY=-echo line
zsh -c 'STTY=-echo line'
STTY=-echo command line
f() line; STTY=-echo f
f() STTY=-echo line; f

work (at reading a line with tty echo disabled)

But:

STTY=-echo read line
STTY=-echo eval line
zsh -c 'f() line; STTY=-echo f'
zsh -c 'f() STTY=-echo line; f'

don't (stty not run).

See also:

$ f() STTY=-onlcr line; STTY=-echo f; stty
asd
   asd
      speed 38400 baud; line = 0;
                                 -brkint -imaxbel iutf8
                                                       -onlcr

-echo not applied, -onlcr not restored.

-- 
Stephane

Re: When is STTY ignored?

[Bart Schaefer]

On May 29,  9:36pm, Stephane Chazelas wrote:
}
} It's not very clear to me when one may use STTY.

It applies only to external commands, not those executed within zsh
(functions, loops, other multiline constructs, builtins).

If it's in the environment of a function et al., it'll apply to all
external commands run from within that construct.

Re: When is STTY ignored?

[Stephane Chazelas]

2017-05-29 17:02:09 -0700, Bart Schaefer:
> On May 29,  9:36pm, Stephane Chazelas wrote:
> }
> } It's not very clear to me when one may use STTY.
> 
> It applies only to external commands, not those executed within zsh
> (functions, loops, other multiline constructs, builtins).
> 
> If it's in the environment of a function et al., it'll apply to all
> external commands run from within that construct.

Thanks. That doesn't seem to be the whole story though.

See the examples I gave

"line" is an external command. "eval" and "command" are builtin
commands.

STTY=-echo line
STTY=-echo command line
f() line; STTY=-echo f

Do work, but not

STTY=-echo eval line

About zsh -c, it looks like it "works" as long as there's only one
command:

zsh -c 'STTY=-echo line'

works (well, it doesn't seem to restore the settings afterwards)
but not

zsh -c 'STTY=-echo line; exit'

It seems it never works in scripts even if they contain a single
command. With scripts source from interactive shells, they seem
to "work" but don't restore the tty settings.

See also:

line | STTY=-echo wc -c
STTY=-echo head -n1 /dev/tty < /dev/null

which don't "work" (even if they called stty, stty would fail
because stdin is not a terminal anyway, but that would still be
worth documenting).

In:

(STTY=-echo line; STTY=-echo line); line

the "-echo" is only applied to the 2nd and 3rd "line"
invocations (instead of 1st and 2nd).

And the issue when several are stacked mentioned before:

$ f() STTY=-onlcr line; STTY=-echo f; stty
asd
   asd
      speed 38400 baud; line = 0;
                                 -brkint -imaxbel iutf8
                                                       -onlcr

-echo not applied, -onlcr not restored.

It would be worth documenting that it can only be used in
interactive shells and only for the simplest case of one simple
external command at the prompt, not in subshell, and without
stdin redirection

-- 
Stephane

Re: When is STTY ignored?

[Bart Schaefer]

On May 30,  7:53am, Stephane Chazelas wrote:
}
} In:
} 
} (STTY=-echo line; STTY=-echo line); line
} 
} the "-echo" is only applied to the 2nd and 3rd "line"
} invocations (instead of 1st and 2nd).

So, this is a little odd.

The "stty" command is run only when isatty(0) and the process is the
group leader, i.e., the foreground job.

In the construct
    (command1; command2)
the group leader is the subshell, so STTY is ignored for command1;
but then command2 is optimized to an "exec" so it assumes the group
leadership and "stty" is run.  However, that exec means the subshell
is gone and there is never an opportunity to undo the setting change,
so it remains in effect after command2 exits, and that's why it
appears to have been applied to the 3rd command in your example.

This means we ought to treat having STTY in the environment as a
reason to skip the optimization.  However, I'm not sure we look for
STTY early enough in the call chain to handle this.  For example,
jobtab[thisjob].stty_in_env is only assigned after the decision is
made and the child has been forked.