Have you ever had a script or program running in the background, with no TTY/console, asking for input to continue running? How do you tell it ‘y’ so it will keep going? By faking input using TIOCSTI! I had this issue when running a long chain of tasks in an ansible playbook, and a command in one of those tasks wanted to know if I was “sure”. I identified the PID that was waiting for input, and wrote this little python script:
import fcntl
import sys
import termios
with open('/proc/<PID GOES HERE>/fd/0', 'w') as fd:
for char in "y\n":
fcntl.ioctl(fd, termios.TIOCSTI, char)When you run this, it will send the letter ‘y‘, followed by newline to the PID, and the process continues to run. What is TIOCSTI? From the man page of ioctl_tty:
IOCTL_TTY(2) Linux Programmer's Manual IOCTL_TTY(2)
NAME
ioctl_tty - ioctls for terminals and serial lines
SYNOPSIS
#include <termios.h>
int ioctl(int fd, int cmd, ...);
DESCRIPTION
The ioctl(2) call for terminals and serial ports accepts many possible
command arguments. Most require a third argument, of varying type,
here called argp or arg.
Use of ioctl makes for nonportable programs. Use the POSIX interface
described in termios(3) whenever possible.
[...]
Faking input
TIOCSTI const char *argp
Insert the given byte in the input queue.
[...]
Linux 2020-06-09 IOCTL_TTY(2)When I googled around for a way to solve this, this Stackexchange Q&A was pretty much spot on: https://unix.stackexchange.com/questions/48103/construct-a-command-by-putting-a-string-into-a-tty and this answer in particular for the Python script: https://unix.stackexchange.com/a/345572/315097