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I am working on a project with Python in which I am supposed to execute shell script with Python.

I have written a simple program from which I am able to execute shell script with my python code. But now I need to pass certain parameters from my Python code to the shell script and then print out those parameters by executing the shell script.

For the simplicity sake, Currently I am executing shell script which will print out Hello World but now I want to pass hostname and pp and sp values to my shell script and then print out those values from the shell script when it is getting execute by Python client.

#!/usr/bin/python

import subprocess
import json
import socket

hostname = socket.gethostname()

jsonData = '{"desc": "some information about the host", "pp": [0,3,5,7,9], "sp": [1,2,4,6,8]}'
jj = json.loads(jsonData)

print jj['pp'] # printing it from Python program for now
print jj['sp'] # printing it from Python program for now

print hostname # printing it from Python program for now

# pass the above values to my shell script
jsonStr = '{"script":"#!/bin/bash\\necho Hello World\\n"}'
j = json.loads(jsonStr)

print "start"
subprocess.call(j['script'], shell=True)
print "end"

In general, I want to pass, hostname, pp and sp values to my shell script as shown in jsonStr and then print out those values from the shell script itself when I run my Python code.

So it should print out like this whenever I execute my shell script in jsonStr-

start
Hello world
[0, 3, 5, 7, 9]
[1, 2, 4, 6, 8]
myhostname
end

Is this possible to do it in Python?

share|improve this question
 
Either encode / decode it as one argument (dataset) or iterate the JSON output (native dict will work too) into the shell call as named parameter and call those from the args globals. –  Allendar Dec 23 at 23:46
 
Why? I'm not sure whether this is an XY problem or just a WTF problem, but I can't imagine why you'd want to build a script dynamically to echo some variables just so you can figure out how to pass those variables to the script. –  abarnert Dec 24 at 0:04
 
It's an example to echo it, I am writing a shell script which will use those parameters in the shell script to perform certain task. –  TrekkieTechie T-T Dec 24 at 0:07
 
@TrekkieTechieT-T: And you're going to build that shell script dynamically as one big string in Python to pass as a command line? –  abarnert Dec 24 at 0:07
 
that shell script won't be too long because I just need to scp the files from one machine to another machine using the input being passed. –  TrekkieTechie T-T Dec 24 at 0:08
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1 Answer

up vote 1 down vote accepted

There are two ways to pass variables to a script: as arguments, or through the environment.

Since you're trying to execute a script as if it were a giant command line (which won't actually work—especially if you're escaping your newlines as \\n so the shell sees the whole thing as one line—but let's pretend it would), you can't pass arguments, so you will need to pass an environment.

This is trivial:

env = {}
env.update(os.environ)
env.update(jj)
subprocess.call('echo ${pp}', shell=True, env=env)

This will print out whatever was in jj[pp], and return 0.

Why? Well, in bash, ${pp} means "whatever is in the environment variable pp". And we copied every key-value pair from jj into the env environment, so the environment variable pp has whatever value was in jj[pp]. (In some cases you might want to quote things, e.g. "${pp}", but for echo there's no reason to do that, and without knowing what you're going to do in your real-life code I can't guess what you might need.)

If you actually had a script that you wanted to call, stored in a file, then of course you could pass it arguments the same way you do with any other program you run via subprocess, and inside the script you could reference them as $1, etc.

However, I don't see why you need to pass arguments to a script you're building on the fly. Just build the values into the script. While that's usually a bad idea, that's mainly because building a script on the fly is a bad idea, and you've already committed to that part for some reason. Format your strings in Python, where you have access to the full power of str.format or % (as you prefer), and the entire Python stdlib. For example:

script = 'echo {}'.format(shlex.quote(jj['pp']))
subprocess.call(script, shell=True)

Now the script doesn't have to do anything to access the value; it's hard-coded into the script.

share|improve this answer
 
Thanks abarnert for the suggestion. In this case how my shell script will look like? –  TrekkieTechie T-T Dec 24 at 0:09
 
@TrekkieTechieT-T: Like the example in the answer. What part do you not understand? –  abarnert Dec 24 at 0:11
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