How to use shell script within a python program?

Question

I want to use a C-shell script within a python program, which works with two arguments.

os.system("recall 20170121 ../E725.txt xy 1")

But I want to use it for a stack, so declared the variables like below, but when I call them within the script it gives an error, that the input file doesn't exist. How can I call the variables?

date_ID=(filename[17:25])
fullpath = '../%s' % (filename)
os.system("import_S1_TOPS_modified $date_ID $fullpath vv 1")

Answer 1

The shell doesn't know about the Python variables, as it's a completely different system. So you can't use the shell's variable substitution mechanism ( $date_ID ). Instead, you'll have to pass them as a Python string:

os.system("import_S1_TOPS_modified %s %s vv 1" % (date_ID, fullpath))

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Note that this code has a serious problem: what if someone gives ; rm -rf /; ; rm -rf /; as filename ? the command will now look like:

import_S1_TOPS_modified 20181021; rm -rf /; vv 1

Which will delete everything.

This is why it is a better idea to use subprocess , which will not use the shell at all, and isn't vulnerable to this sort of problem:

subprocess.call(['import_S1_TOPS_modified', date_ID, fullpath, 'vv', '1'])

If you must use the shell, then use shlex.quote() and add shell=True :

subprocess.call("import_S1_TOPS_modified %s %s vv 1" % (
    shlex.quote(date_ID), shlex.quote(fullpath)),
    shell=True)