piping python variable value to bash script (inside python script)

Question

After years of bash scripting I've been given the task of modifying a python script to have it call a shell script when certain conditions are met. That part wasn't too bad, but now I'm trying to also send this shell script a variable from within the python script and I'm getting lost.

The shell script takes stdin, which is where I'm trying to place the value of the python variable, which is just an interger.

my code (so far) looks like this:

if VAR > NUMBER: import os bashCommand = "echo something | /path/to/script --args" os.system(bashCommand) 

and it works just fine. But what I am looking to do is make os.system(bashCommand) an equivalent of:

echo $VAR | /path/to/script --args 

or even

echo 'some text' $VAR | /path/to/script --args 

given the way os.system appears to work though it seems I'm approaching this completely wrong.

So I guess my question is how can I pass the value of VAR to bashCommand, preferably as stdin?

Answer 1

Don't use os.system(), it is deprecated in favor of subprocess module.

In your case you can use subprocess module directly and its better to not use the shell=True parameter as this is not safe to run commands directly into the shell. All the functionalities you need here like pipe can be emulated by the subprocess module.

Lets take this example :

import subprocess var = 'foobar' first = subprocess.Popen(['/bin/echo', var], stdout=subprocess.PIPE) second = subprocess.Popen(['bash', '/path/to/script', '--args'], stdin=first.stdout) 

we have defined a varible var, then we have used the subprocess.Popen class to get the value of var using /bin/echo and then sent this value to the pipe. The first is an object of subprocess.Popen class.

Then we have used the STDOUT of first as STDIN of second via pipe and executed the bash script accordingly.

If you want to save the output of a command as a string use subprocess.check_output function instead e.g. :

second = subprocess.check_output(['bash', '/path/to/script', '--args'], stdin=first.stdout) 

Read the offical documentation of subprocess module to get more idea.

Answer 2

Python doesn't expand variables in strings in the same way as bash. If you have VAR in python and want to pass that to bash you could do

subprocess.call('echo {} | /path/to/script --args'.format(VAR), shell=True) 

if VAR in python holds the name of a bash variable you want to expand you could do similar:

subprocess.call('echo "${}" | /path/to/script --args'.format(VAR), shell=True) 

Answer 3

Calling os.system() is deprecated, though still valid. That functionality may be going away in the future (probably will be), so you might want to give serious consideration to refactoring those calls with the subprocess module, which has an very easy way to send data to a subprocess's STDIN and read from its STDOUT. https://docs.python.org/2/library/subprocess.html

Answer 4

I think the good practice is:

var = subprocess.Popen(['/bin/echo', var], stdout=subprocess.PIPE) second = subprocess.Popen(['bash', '/path/to/script', '--args'],stdin=var.stdout, stdout=subprocess.PIPE) var.stdout.close() output = second.communicate()[0] var.wait()