using os.system() in a python cgi script [duplicate]

Question

I'm building a cgi script with python on my web host and using wget to test it. The shebang #!/home/me/virtualenv/public_html/chartex/3.6/bin/python3.6 points it at the right version of python to use so that print(sys.version, sys.executable) in the cgi correctly returns:

3.6.15 (default, Jan 14 2022, 12:16:54) 
[GCC 4.8.5 20150623 (Red Hat 4.8.5-44)] /home/me/virtualenv/public_html/chartex/3.6/bin/python3.6_bin

Moreover print(os.path.abspath('.')) in the cgi returns the correct path to where the script resides.

However, if I want the cgi to report the version of an existing binary on the system print(os.system('twopi -V')) returns only 0. The same thing happens doing print(os.system('ls')).

If I run the same python installed in this virtualenv (calling it by its full pathname on the server) at the command line, and there call print(os.system('twopi -V')), it correctly tells me: twopi - graphviz version 4.0.0 (20220529.0937) and then on the next line: 0. Likewise if I do print(os.system('ls'))

Is this kind of system command impossible for a cgi script? Is there a way I can coerce the script to report the result of such a system command?

The return value of os.system() is the exit status of the program. 0 means that the program terminated without an error. Why do you think that it's a problem if it returns 0? — Barmar
– Barmar, Commented Jun 9, 2022 at 23:47
I don't. The problem is that when the cgi calls os.system('twopi -V'), it doesn't also return twopi - graphviz version 4.0.0 (20220529.0937) — jjon
– jjon, Commented Jun 10, 2022 at 0:08
you would have to run one of the functions in module subprocess to catch displayed text twopi - graphviz version 4.0.0 (20220529.0937) — furas
– furas, Commented Jun 10, 2022 at 0:42
when you run os.system() then it runs system shell which runs program and shell displays its result - not Python. Python displays only exit code - 0. — furas
– furas, Commented Jun 10, 2022 at 0:45
see ie, subprocess,run() or subprocess,check_output(). It runs program and it can catch text displayed in shell. And when you get this text then you can use print() to display it on page. — furas
– furas, Commented Jun 10, 2022 at 0:54

jjon · Accepted Answer · 2022-06-14 19:40:26Z

0

In case anyone is similarly confused about subprocess:

Thanks to the observations of @furas and a great deal of messing about with subprocess I finally figured out an incantation that would compel my python2.7 cgi script to give me the shell output of the command twopi -V. A comment from @tripleee in this thread was the key. For Python 3.7.9 and up: "capture_output is a shorthand for the option combination stdout=supprocess.PIPE, stderr=subprocess.PIPE

p = subprocess.Popen(['twopi', '-V'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
print("stderr:  " + p.stderr.read())
print("stdout:  " + p.stdout.read())
# with the result:
stderr:  twopi - graphviz version 4.0.0 (20220529.0937)
stdout:

The actual shell output was going to stderr. The subprocess module is byzantine! It's a little better under python3, but not much.

answered Jun 14, 2022 at 19:40

jjon

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