Capture return value from python script into shell script

Question

I have a shell script inside of which I am calling a python script say new.py:

#!/usr/bin/ksh
python new.py

Now this new.py is something like -

if not os.path.exists('/tmp/filename'):
 print "file does not exist"
 sys.exit(0)

If the file does not exits then the python script returns but the shell script continue execution. I want the shell script also to stop at that point if the file does nt exits and my python script exits.

Please suggest how can I capture the return in shell script to stop its execution further.

Answer 1

You need to return something other than zero from the exit function.

if os.path.exists("/tmp/filename"):
 sys.exit(0)
else:
 sys.exit(1)

The error value is only 8 bits, so only the lower 8 bits of the integer is returned to the shell. If you supply a negative number, the lower 8 bits of the twos complement representation will be returned, which is probably not what you want. You usually don't return negative numbers.

• Kundan Kumar: I am returning sys.exit(-100) now but in shell echo $? gives 155 ...is it like this number is assigned for these kind of error and can I use it safely like if $? != 0 then exit from the shell
• Alex L: You could also do sys.exit(int(not os.path.exists("/tmp/filename"))), if you so desired

Answer 2

if ! python new.py
then
 echo "Script failed"
 exit
fi

This assumes that the Python script uses sys.exit(0) when your shell script should continue and sys.exit(1) (or some other non-zero value) when it should stop (it's customary to return a non-zero exit code when an error occurs).