How to mix bash with python

Question

I enjoy using unix commands very much, but I came to the point, where I would find embedded python parts useful. This is my code:

#!/bin/bash -
echo "hello!";

exec python <<END_OF_PYTHON
#!/usr/bin/env python

import sys

print ("xyzzy")

sys.exit(0)
END_OF_PYTHON

echo "goodbye!";

However, only "hello" gets printed.

$ ./script.sh 
hello!
xyzzy

How can I modify the bash script to fully embedd python? And would it then be possible to pass values from python variables into bash variables? Thanks a lot.

Answer 1

On the exec python ... line, you're exec()ing the Python interpreter on your PATH, so the python image will replace the bash image, and there is absolutely no hope of the echo "goodbye!" ever being executed. If that's what you want, that's fine, but otherwise, just omit the exec.

The shebang (“#!”) line in the python code is completely unnecessary. When you try to run an ordinary file, the kernel sees the “#!”, runs whatever follows it (/usr/bin/env python), and feeds the rest of the file to the stdin of whatever has been run. This is a general facility used to invoke interpreters. Since you are invoking the python interpreter yourself, not asking the kernel to do it, this is neither needed nor useful.

The sys.exit(0) is also unnecessary, since the Python interpreter will naturally exit when it gets to the end of its input (at END_OF_PYTHON) anyway. This means that the import sys is also unnecessary.

In summary, the following is what I would write to achieve what you appear to want to achieve:

#!/bin/bash
echo "hello!";

python <<END_OF_PYTHON
print ("xyzzy")
END_OF_PYTHON

echo "goodbye!";

Answer 2

Don't use exec. That replaces the shell process with the program you're running, so the rest of the script doesn't execute.

#!/bin/bash -
echo "hello!";

python <<END_OF_PYTHON
#!/usr/bin/env python

import sys

print ("xyzzy")

sys.exit(0)
END_OF_PYTHON

echo "goodbye!";