How can I call an external command (as if I'd typed it at the Unix shell or Windows command prompt) from within a
Look at the subprocess module in the standard library:
import subprocess subprocess.run(["ls", "-l"])
The advantage of
system is that it is more flexible (you can get the
stderr, the "real" status code, better error handling, etc...).
The official documentation recommends the
subprocess module over the alternative
subprocessmodule provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function [
The Replacing Older Functions with the subprocess Module section in the
subprocess documentation may have some helpful recipes.
For versions of Python before 3.5, use
import subprocess subprocess.call(["ls", "-l"])
Here's a summary of the ways to call external programs and the advantages and disadvantages of each:
os.system("some_command with args") 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, etc. On the other hand, this also lets you run commands which are simply shell commands and not actually external programs. See the documentation.
stream = os.popen("some_command with args") 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. See the documentation.
Popen class of the
subprocess module. 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()
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.
call function from the
subprocess module. 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)
See the documentation.
If you're on Python 3.5 or later, you can use the new
subprocess.run function, which is a lot like the above but even more flexible and returns a
CompletedProcess object when the command finishes executing.
The os module also has all of the fork/exec/spawn functions that you'd have in a C program, but I don't recommend using them directly.
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.