Sending input to an existing process by its Process ID (PID) can be achieved through various methods, depending on the operating system and the nature of the process.
On Linux/Unix-like Systems:

   Writing to /proc/<PID>/fd/0 (Standard Input):

This method involves writing directly to the process's standard input "file." However, simply writing to this file may not be sufficient to trigger the process to read the input. For interactive processes, you might need to use ioctl with TIOCSTI to simulate keystrokes, effectively placing bytes into the process's standard input queue.
Code

   # Example using TIOCSTI (requires specific tools or custom code)
   # This is a more complex method often requiring C, Perl, or Python scripts.

Using Named Pipes (FIFOs).
Create a named pipe and configure the target process to read from it. Then, from another shell or program, write to the named pipe.
Code

   # Create a named pipe
   mkfifo /tmp/my_pipe

   # In the target process (e.g., a C program)
   # ./my_program < /tmp/my_pipe

   # In another shell to send input
   echo "My input" > /tmp/my_pipe