How do I execute a program or call a system command?

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#python #shell #terminal #subprocess #command

Solution 1

Use subprocess.run:

import subprocess

subprocess.run([“ls”, “-l”])

Another common way is os.system but you shouldn't use it because it is unsafe if any parts of the command come from outside your program or can contain spaces or other special characters, also subprocess.run is generally more flexible (you can get the stdout, stderr, the "real" status code, better error handling, etc.). Even the documentation for os.system recommends using subprocess instead.

On Python 3.4 and earlier, use subprocess.call instead of .run:

subprocess.call(["ls", "-l"])

Solution 2

Here is a summary of ways to call external programs, including their advantages and disadvantages:

  1. os.system passes the command and arguments to your system's shell. This is nice because you can actually run multiple commands at once in this manner and set up pipes and input/output redirection. For example:

    os.system("some_command < input_file | another_command > output_file")

    However, while this is convenient, you have to manually handle the escaping of shell characters such as spaces, et cetera. On the other hand, this also lets you run commands which are simply shell commands and not actually external programs.

  2. os.popen will do the same thing as os.system except that it gives you a file-like object that you can use to access standard input/output for that process. There are 3 other variants of popen that all handle the i/o slightly differently. If you pass everything as a string, then your command is passed to the shell; if you pass them as a list then you don't need to worry about escaping anything. Example:

    print(os.popen("ls -l").read())

  3. subprocess.Popen. This is intended as a replacement for os.popen, but has the downside of being slightly more complicated by virtue of being so comprehensive. For example, you'd say:

    print subprocess.Popen("echo Hello World", shell=True, stdout=subprocess.PIPE).stdout.read()

    instead of

    print os.popen("echo Hello World").read()

    but it is nice to have all of the options there in one unified class instead of 4 different popen functions. See the documentation.

  4. subprocess.call. This is basically just like the Popen class and takes all of the same arguments, but it simply waits until the command completes and gives you the return code. For example:

    return_code = subprocess.call("echo Hello World", shell=True)

  5. subprocess.run. Python 3.5+ only. Similar to the above but even more flexible and returns a CompletedProcess object when the command finishes executing.

  6. os.fork, os.exec, os.spawn are similar to their C language counterparts, but I don't recommend using them directly.

The subprocess module should probably be what you use.

Finally, please be aware that for all methods where you pass the final command to be executed by the shell as a string and you are responsible for escaping it. There are serious security implications if any part of the string that you pass can not be fully trusted. For example, if a user is entering some/any part of the string. If you are unsure, only use these methods with constants. To give you a hint of the implications consider this code:

print subprocess.Popen("echo %s " % user_input, stdout=PIPE).stdout.read()

and imagine that the user enters something "my mama didnt love me && rm -rf /" which could erase the whole filesystem.

Solution 3

Typical implementation:

import subprocess

p = subprocess.Popen(‘ls’, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
for line in p.stdout.readlines():
print line,
retval = p.wait()

You are free to do what you want with the stdout data in the pipe. In fact, you can simply omit those parameters (stdout= and stderr=) and it'll behave like os.system().

Solution 4

Some hints on detaching the child process from the calling one (starting the child process in background).

Suppose you want to start a long task from a CGI script. That is, the child process should live longer than the CGI script execution process.

The classical example from the subprocess module documentation is:

import subprocess
import sys

# Some code here

pid = subprocess.Popen([sys.executable, "longtask.py"]) # Call subprocess

# Some more code here

The idea here is that you do not want to wait in the line 'call subprocess' until the longtask.py is finished. But it is not clear what happens after the line 'some more code here' from the example.

My target platform was FreeBSD, but the development was on Windows, so I faced the problem on Windows first.

On Windows (Windows XP), the parent process will not finish until the longtask.py has finished its work. It is not what you want in a CGI script. The problem is not specific to Python; in the PHP community the problems are the same.

The solution is to pass DETACHED_PROCESS Process Creation Flag to the underlying CreateProcess function in Windows API. If you happen to have installed pywin32, you can import the flag from the win32process module, otherwise you should define it yourself:

DETACHED_PROCESS = 0x00000008

pid = subprocess.Popen([sys.executable, "longtask.py"],
                       creationflags=DETACHED_PROCESS).pid

/* UPD 2015.10.27 @eryksun in a comment below notes, that the semantically correct flag is CREATE_NEW_CONSOLE (0x00000010) */

On FreeBSD we have another problem: when the parent process is finished, it finishes the child processes as well. And that is not what you want in a CGI script either. Some experiments showed that the problem seemed to be in sharing sys.stdout. And the working solution was the following:

pid = subprocess.Popen([sys.executable, "longtask.py"], stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)

I have not checked the code on other platforms and do not know the reasons of the behaviour on FreeBSD. If anyone knows, please share your ideas. Googling on starting background processes in Python does not shed any light yet.

Solution 5

import os
os.system("your command")

Note that this is dangerous, since the command isn't cleaned. See the documentation of the os and sys modules. There are a bunch of functions (exec* and spawn*) that will do similar things.