Re: [Help-bash] How to redirect tty to /dev/null?

[Stephane Chazelas]

2019-09-03 11:06:52 -0500, Eric Blake:
[...]
> All processes have a notion of a controlling terminal.  If you want a
> child process' interaction with its controlling terminal to behave
> differently than interacting with YOUR controlling terminal, then you
> need to open up a pty and become the master of that device prior to
> spawning your child process, at which point your child process's tty is
> now the slave end of the pty you just opened.

Note that that child process must be made the leader of a new
session and open that slave pty (without O_NOCTTY) for it to
attach the new tty.

> At that point, as master
> of the pty you can do whatever you want with the child's interactions
> with its controlling terminal (including blindly ignoring whatever the
> child wants to output to its tty, for the effect of that output going to
> /dev/null).  Look at the 'expect' program if you want a demonstration of
> creating your own pty for managing terminal interactions of child
> processes.

Or you could create a session detached from any terminal,
to cause unzip to fail when it tries to open /dev/tty.

You'd probably want unzip *not* to be the leader of that session
as it could end up attaching a tty device to the session if it
were to open one without O_NOCTTY.

util-linux have a setsid command to start a new session:

$ setsid -w sh -c 'ps -jp "$$"; exit'
  PID  PGID   SID TTY          TIME CMD
 6685  6685  6685 ?        00:00:00 sh

(sh is the leader of a session with no controlling terminal, and
ps is running in a child process).


$ setsid -w sh -c 'unzip a.zip; exit'
Archive:  a.zip
   skipping: a                       unable to get password

unzip fails to open /dev/tty

Beside expect, socat is another command that can let you create
ptys.

$ (sleep 1; printf '\r') | socat  - 'system:ps -j; ls -l /proc/self/fd; unzip 
a.zip; exit,setsid,pty,ctty,fdout=3'
  PID  PGID   SID TTY          TIME CMD
 7505  7505  7505 pts/10   00:00:00 socat
 7506  7505  7505 pts/10   00:00:00 sh
 7507  7505  7505 pts/10   00:00:00 ps
total 0
lrwx------ 1 chazelas chazelas 64 Sep  4 08:10 0 -> /dev/pts/10
lrwx------ 1 chazelas chazelas 64 Sep  4 08:10 1 -> /dev/pts/9
lrwx------ 1 chazelas chazelas 64 Sep  4 08:10 2 -> /dev/pts/9
lrwx------ 1 chazelas chazelas 64 Sep  4 08:10 3 -> /dev/pts/10
lrwx------ 1 chazelas chazelas 64 Sep  4 08:10 4 -> 'socket:[71040]'
lr-x------ 1 chazelas chazelas 64 Sep  4 08:10 5 -> /proc/7508/fd
Archive:  a.zip
[a.zip] a password:
   skipping: a                       incorrect password
2019/09/04 08:10:45 socat[7503] E waitpid(): child 7505 exited with status 82

In that case, we have stdin (and fd 3) on a new pty in a new
session. We simulate pressing enter (\r) after 1 second because
unzip drains the input before issuing the prompt.

Or:

$ socat -u file:/dev/null 'system:ps -jp "$$"; ls -l /proc/self/fd; unzip 
a.zip; exit,setsid'
  PID  PGID   SID TTY          TIME CMD
 7548  7547  7547 ?        00:00:00 sh
total 0
lrwx------ 1 chazelas chazelas 64 Sep  4 08:13 0 -> 'socket:[72978]'
lrwx------ 1 chazelas chazelas 64 Sep  4 08:13 1 -> /dev/pts/10
lrwx------ 1 chazelas chazelas 64 Sep  4 08:13 2 -> /dev/pts/10
lrwx------ 1 chazelas chazelas 64 Sep  4 08:13 3 -> 'socket:[72975]'
lrwx------ 1 chazelas chazelas 64 Sep  4 08:13 4 -> 'socket:[72976]'
lr-x------ 1 chazelas chazelas 64 Sep  4 08:13 5 -> /proc/7550/fd
Archive:  a.zip
   skipping: a                       unable to get password

New session, no controlling terminal.

> But that starts to get into the scope of pty management, and
> less about bash, so it becomes questionable that you'll get any better
> help on this topic from this list.

Yes, zsh has builtin support for creating ptys and sessions (in
the zsh/zpty module), I don't think bash does (there's no
mention of setsid in the source code AFAICT).

-- 
Stephane