Writing to a text file in Python called via PHP using shell_exec

Question

So first off, sorry if this question has already been answered elsewhere, but I couldn't find the answer myself.

I've got a LAMP server running in Ubuntu 16.04 and I'm trying to use Python 2.7.12 to write to a text file from PHP.

I've got the following two files (note, these two files are in the same directory):

index.php

<?php
    $command = escapeshellcmd('python test.py a b c');
    $output = shell_exec($command);
    echo $output;
?>

test.py

import sys

print "argv1: ", sys.argv[1]
print "argv2: ", sys.argv[2]
print "argv3: ", sys.argv[3]

f = open("test-" + sys.argv[1] + ".txt","w+")
f.write("test")
f.close()

print "Text added."

When I navigate to the index.php file in my browser, I get the following output:

argv1: a argv2: b argv3: c

However, when I execute this code in the terminal the file test-a.txt is created and I get the following output:

layer8@alpha:/var/www/html/$ python test.py a b c
argv1:  a
argv2:  b
argv3:  c
Text added.

I can't seem to understand and work out why the Python file isn't executing any code past the third print argv3 statement?

I've done a lot of research into this and I've been unable to find a solution. If you need any further information from me, please do not hesitate to ask; any responses are appreciated.

Thanks.

bruno desthuilliers · Accepted Answer · 2018-07-16 12:16:21Z

1

shell_exec will only return what get's printed to stdout. If an exception arises in the Python script, the whole exception message and traceback are printed on stderr, so they won't be part of the output. Note that theoretically, shell_exec should return NULL if an error happens but well, this is PHP so it wouldn't be much fun if the builtin funcs behaved as documented...

Anyway: the problem is very probably with opening the file, and is very probably a permission issue.

First, you're using a relative path, and relative path are resolved against the current working directory, not against the directory where your script is located, so if you expect a sane and predictable behaviour always use an absolute path (either pass it as argument or build it against the script's current location).

Also your Apache server may not (this is an understatement) run with the same env and user as you so it might just not be allowed to open a file for wrting in whatever-his-own-current-working-directory is at that time.

And finally, even it's PYTHONPATH might be different than yours so it might even be executing another (leftover) test.py OR a stale test.pyc file.

To make a long story short: it's impossible to tell for sure what's wrong without having access to your system, but at least now you know where to start debugging.

The first thing I'd personnaly do would be to make sure any error in the python script is printed to stdout:

import sys

print "argv1: ", sys.argv[1]
print "argv2: ", sys.argv[2]
print "argv3: ", sys.argv[3]

try:
    fname = "test-%s.txt" % sys.argv[1]
    with open(fname,"w+") as f:
        f.write("test")

    print "Text added."
except Exception as e:
    print "oops : got %s" % e
    raise

answered Jul 16, 2018 at 12:16

bruno desthuilliers

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Layer 8 Over a year ago

Thanks for your response. After adding the code you've written above, I get the error message "Permission Denied". I've looked into this and a working solution is to create a new directory for the outputs and assign www-data as the owner using chown (rather than giving www-data access to everything). I'm sure there are better practices and solutions for this; but this is one work-around :)

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