Running bash script from python

Question

I'm encountering the following problem:

I have this simple script, called test.sh:

#!/bin/bash

function hello() {
    echo "hello world"
}
hello

when I run it from shell, I got the expected result:

$ ./test2.sh
hello world

However, when I try to run it from Python (2.7.?) I get the following:

>>> import commands
>>> cmd="./test2.sh"
>>> commands.getoutput(cmd)
'./test2.sh: 3: ./test2.sh: Syntax error: "(" unexpected'

I believe it somehow runs the script from "sh" rather than bash. I think so because when I run it with sh I get the same error message:

$ sh ./test2.sh
./test2.sh: 3: ./test2.sh: Syntax error: "(" unexpected

In addition, when I run the command with preceding "bash" from python, it works:

>>> cmd="bash ./test2.sh"
>>> commands.getoutput(cmd)
'hello world'

My question is: Why does python choose to run the script with sh instead of bash though I added the #!/bin/bash line at the beginning of the script? How can I make it right (I don't want to use preceding 'bash' in python since my script is being run from python by distant machines which I cant control).

Thanks!

Uhm, your shebang looks all wrong. #~/bin/bash, this isn't even a shebang but a simple comment. It should be #!/bin/bash. — Octavian Helm
– Octavian Helm, Commented Mar 25, 2013 at 8:46
It works unaltered on my Ubuntu 12.10 32-bit. May be something with your default environment that is causing sh to be used instead of bash. — TAS
– TAS, Commented Mar 25, 2013 at 9:03
Oh I see, that's very strange then. What distribution are you using there? — Octavian Helm
– Octavian Helm, Commented Mar 25, 2013 at 9:12

viraptor · Accepted Answer · 2013-03-25 23:07:12Z

There seems to be some other problem - the shbang and commands.getoutput should work properly as you show here. Change the shell script to just:

#!/bin/bash
sleep 100

and run the app again. Check with ps f what's the actual process tree. It's true that getoutput calls sh -c ..., but this shouldn't change which shell executes the script itself.

From a minimal test as described in the question, I see the following process tree:

11500 pts/5    Ss     0:00 zsh
15983 pts/5    S+     0:00  \_ python2 ./c.py
15984 pts/5    S+     0:00      \_ sh -c { ./c.sh; } 2>&1
15985 pts/5    S+     0:00          \_ /bin/bash ./c.sh
15986 pts/5    S+     0:00              \_ sleep 100

So in isolation, this works as expected - python calls sh -c { ./c.sh; } which is executed by the shell specified in the first line (bash).

Make sure you're executing the right script - since you're using ./test2.sh, double-check you're in the right directory and executing the right file. (Does print open('./test2.sh').read() return what you expect?)

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