In subprocess, some commands work but not others from Mac terminal

Question

I am trying to create a python script that runs a perl script on the Mac terminal. The popular 3D printer slicing engine, Slic3r, has the ability to use command line usage, which is written in Perl. I want to write a python script to automate some processes, which is the language I know best. If I type the commands I want to use directly into the terminal, it works as it should, however, if I try to use python's subprocess, it works for some commands but not others.

For example if I use my script to fetch the Slic3r version using the syntax outlined in the docs, it works correctly. This script works:

import os
import subprocess

os.system("cd Slic3r")

perl = "perl"
perl_script = "/Users/path/to/Slic3r/slic3r.pl"
params = "--version"

pl_script = subprocess.Popen([perl, perl_script, params], stdout=sys.stdout)
pl_script.communicate()

print 'done'

This returns:

1.3.0-dev
done

If I use a command such as --info (see Slic3r docs under repairing models for more info) using the same script I get:

In:

import os
import subprocess

os.system("cd Slic3r")

perl = "perl"
perl_script = "/Users/path/to/Slic3r/slic3r.pl"
params = "--info /Users/path/to/Desktop/STL_Files/GEAR.stl"

pl_script = subprocess.Popen([perl, perl_script, params], stdout=sys.stdout)
pl_script.communicate()

print 'done'

Out:

Unknown option: info /Users/path/to/Desktop/STL_Files/GEAR.stl
Slic3r 1.3.0-dev is a STL-to-GCODE translator for RepRap 3D printers
written by Alessandro Ranellucci <aar@cpan.org> - http://slic3r.org/

Usage: slic3r.pl [ OPTIONS ] [ file.stl ] [ file2.stl ] ...

From what I have researched, I suspect that there is some issue with the whitespace of a string being used as a argument. I have never used subprocess until attempting this project, so a simple syntax error could be likely.

I know that the Slic3r syntax is correct because it works perfectly if I type it directly into the terminal. Can anybody see what I am doing wrong?

You're doing the equivalent of the shell command `'perl' 'slic3r.pl' '--info GEAR.stl'` instead of `'perl' 'slic3r.pl' '--info' 'GEAR.stl'` — ikegami, Dec 29 '17 at 08:00

PerlDuck · Accepted Answer · 2017-12-28T16:27:17.657

subprocess.Popen accepts args as the first parameter. This can be either a string with the complete command (including parameters):

args = "perl /Users/path/to/Slic3r/slic3r.pl --info /Users/path/to/Desktop/STL_Files/GEAR.stl"
pl_script = subprocess.Popen(args, stdout=sys.stdout)

or a list consisting of the actual command and all its parameters (the actual command in your case is perl):

args = ["perl", 
        "/Users/path/to/Slic3r/slic3r.pl", 
        "--info", 
        "/Users/path/to/Desktop/STL_Files/GEAR.stl"]
pl_script = subprocess.Popen(args, stdout=sys.stdout)

The latter is preferred because it bypasses the shell and directly executes perl. From the docs:

args should be a sequence of program arguments or else a single string. By default, the program to execute is the first item in args if args is a sequence. If args is a string, the interpretation is platform-dependent and described below. See the shell and executable arguments for additional differences from the default behavior. Unless otherwise stated, it is recommended to pass args as a sequence.

^{(emphasis mine)}

The args list may of course be built with Python's standard list operations:

base_args = ["perl", 
             "/Users/path/to/Slic3r/slic3r.pl"]
options   = ["--info", 
             "/Users/path/to/Desktop/STL_Files/GEAR.stl"]
args = base_args + options
args.append("--verbose")
pl_script = subprocess.Popen(args, stdout=sys.stdout)

Sidenote: You wrote os.system("cd Slic3r"). This will open a shell, change the directory in that shell, and then exit. Your Python script will still operate in the original working directory. To change it, use os.chdir("Slic3r") instead. (See here.)

This worked perfectly! Thank you! An additional question I have being this worked: Since args is a list, if I wanted to add, modify, etc. can standard list modifiers such as append be used without it messing up the format? — paperstsoap, Dec 28 '17 at 16:15
@paperstsoap Sure. `args` is just a list. You can modify it with standard list operations. Just make sure the first element is `perl` and the second is your script. I've amended my answer. — PerlDuck, Dec 28 '17 at 16:28

score 0 · Answer 2 · answered Dec 29 '17 at 07:26

you can also use shlex to break down the complex arguments expecially in mac or unix

more information here https://docs.python.org/2/library/shlex.html#shlex.split

e.g.

import shlex, subprocess
args = "perl /Users/path/to/Slic3r/slic3r.pl --info /Users/path/to/Desktop/STL_Files/GEAR.stl"

#using shlex to break down the arguments 

mac_arg=shlex.split(args)

#shlex.split will return all the arguments in a list

output

['perl', '/Users/path/to/Slic3r/slic3r.pl', '--info', '/Users/path/to/Desktop/STL_Files/GEAR.stl']

This can then further be used with Popen

p1=Popen(mac_arg)

Shlex main adavantage is that you dont need to worry about the commands , it will always split them in a manner accepted by Popen

In subprocess, some commands work but not others from Mac terminal

2 Answers2