python - Program output that is neither STDOUT or STDERR? -

i'm getting strange behavior when trying capture output of django's manager.py

> ./manage.py runserver 0.0.0.0:2869 validating models...  0 errors found django version 1.3.1, using settings 'polling.settings' development server running @ http://0.0.0.0:2869/ quit server control-c. error: port in use.  > ./manage.py runserver 0.0.0.0:2869 >stdout 2>stderr  > cat stderr error: port in use.  > cat stdout  > 

why getting empty string when try capture output on second run?

any program detect if stdout and/or stderr connected terminal or not: man isatty(3). python has such functionality: sys.stdout.isatty().

probably python script or logging subsystem prints lines missing in second run when running on terminal i.e. in interactive mode. it's common , proper practice. example, there no sense print "quit server control-c" if output redirected log file.

below simple illustration:

#!/usr/bin/python  import sys  print "print always." if sys.stdout.isatty():   print "print on tty!" 

here go:

$ ./test.py  print always. print on tty! $ ./test.py > log $ cat log print always. $