mavlink_shell.py: allow scripted use

E.g. echo free|./Tools/mavlink_shell.py

Or creating a virtual nsh console:
socat pty,rawer,link=/tmp/fmu_nsh_pts exec:"mavlink_shell.py 0.0.0.0\\:14550,pty,rawer"

Tools/mavlink_shell.py

@@ -12,6 +12,7 @@ import sys, select
 import termios
 from timeit import default_timer as timer
 from argparse import ArgumentParser
+import os
 
 try:
     from pymavlink import mavutil
@@ -135,15 +136,20 @@ def main():
 
     mav_serialport.write('\n') # make sure the shell is started
 
-    # setup the console, so we can read one char at a time
+    # disable echo & avoid buffering on stdin
     fd_in = sys.stdin.fileno()
-    old_attr = termios.tcgetattr(fd_in)
-    new_attr = termios.tcgetattr(fd_in)
-    new_attr[3] = new_attr[3] & ~termios.ECHO # lflags
-    new_attr[3] = new_attr[3] & ~termios.ICANON
-
     try:
+        old_attr = termios.tcgetattr(fd_in)
+        new_attr = termios.tcgetattr(fd_in)
+        new_attr[3] = new_attr[3] & ~termios.ECHO # lflags
+        new_attr[3] = new_attr[3] & ~termios.ICANON
         termios.tcsetattr(fd_in, termios.TCSANOW, new_attr)
+    except termios.error:
+        # tcgetattr can fail if stdin is not a tty
+        old_attr = None
+    ubuf_stdin = os.fdopen(fd_in, 'rb', buffering=0)
+
+    try:
         cur_line = ''
         command_history = []
         cur_history_index = 0
@@ -156,11 +162,19 @@ def main():
 
         next_heartbeat_time = timer()
 
-        while True:
+        quit_time = None
+        while quit_time is None or quit_time > timer():
             while True:
-                i, o, e = select.select([sys.stdin], [], [], 0)
+                i, o, e = select.select([ubuf_stdin], [], [], 0)
                 if not i: break
-                ch = sys.stdin.read(1)
+                ch = ubuf_stdin.read(1).decode('utf8')
+
+                if len(ch) == 0: # EOF
+                    if quit_time is None:
+                        # run a bit longer to read the response (we could also
+                        # read until we get a prompt)
+                        quit_time = timer() + 1
+                    break
 
                 # provide a simple shell with command history
                 if ch == '\n':
@@ -182,8 +196,8 @@ def main():
                         cur_line = cur_line[:-1]
                         sys.stdout.write(ch)
                 elif ord(ch) == 27:
-                    ch = sys.stdin.read(1) # skip one
-                    ch = sys.stdin.read(1)
+                    ch = ubuf_stdin.read(1).decode('utf8') # skip one
+                    ch = ubuf_stdin.read(1).decode('utf8')
                     if ch == 'A': # arrow up
                         if cur_history_index > 0:
                             cur_history_index -= 1
@@ -223,7 +237,8 @@ def main():
         mav_serialport.close()
 
     finally:
-        termios.tcsetattr(fd_in, termios.TCSADRAIN, old_attr)
+        if old_attr:
+            termios.tcsetattr(fd_in, termios.TCSADRAIN, old_attr)
 
 
 if __name__ == '__main__':