How to call an external command (as if I’d typed it at the Unix shell or Windows command prompt) from within a Python script?
How to execute a program or call a system command in Python? Answer #1:
subprocess module in the standard library:
import subprocess subprocess.run(["ls", "-l"])
Even the documentation for
os.system recommends using
subprocessmodule provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the
subprocessdocumentation for some helpful recipes.
On Python 3.4 and earlier, use
subprocess.call instead of
Run a system command in Python- Answer #2:
Summary of ways to call external programs, including their advantages and disadvantages:
os.systempasses 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.
os.popenwill do the same thing as
os.systemexcept 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:
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.
subprocess.call. This is basically just like the
Popenclass 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)
subprocess.run. Python 3.5+ only. Similar to the above but even more flexible and returns a
CompletedProcessobject when the command finishes executing.
os.spawnare similar to their C language counterparts, 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.
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 (
stderr=) and it’ll behave like
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
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.
import os os.system("your command")
Note that this is dangerous since the command isn’t cleaned. I leave it up to you to google for the relevant documentation on the ‘os’ and ‘sys’ modules. There are a bunch of functions (exec* and spawn*) that will do similar things.
Answer #6: How to execute a program or call a system command?
There are lots of different libraries which allow you to call external commands with Python. For each library I’ve given a description and shown an example of calling an external command. The command I used as the example is
ls -l (list all files). If you want to find out more about any of the libraries I’ve listed and linked the documentation for each of them.
- subprocess: https://docs.python.org/3.5/library/subprocess.html
- shlex: https://docs.python.org/3/library/shlex.html
- os: https://docs.python.org/3.5/library/os.html
- sh: https://amoffat.github.io/sh/
- plumbum: https://plumbum.readthedocs.io/en/latest/
- pexpect: https://pexpect.readthedocs.io/en/stable/
- fabric: http://www.fabfile.org/
- envoy: https://github.com/kennethreitz/envoy
- commands: https://docs.python.org/2/library/commands.html
These are all the libraries
Hopefully this will help you make a decision on which library to use 🙂
Subprocess allows you to call external commands and connect them to their input/output/error pipes (stdin, stdout, and stderr). Subprocess is the default choice for running commands, but sometimes other modules are better.
subprocess.run(["ls", "-l"]) # Run command subprocess.run(["ls", "-l"], stdout=subprocess.PIPE) # This will run the command and return any output subprocess.run(shlex.split("ls -l")) # You can also use the shlex library to split the command
os is used for “operating system dependent functionality”. It can also be used to call external commands with
os.popen (Note: There is also a subprocess.popen). os will always run the shell and is a simple alternative for people who don’t need to, or don’t know how to use
os.system("ls -l") # Run command os.popen("ls -l").read() # This will run the command and return any output
sh is a subprocess interface which lets you call programs as if they were functions. This is useful if you want to run a command multiple times.
sh.ls("-l") # Run command normally ls_cmd = sh.Command("ls") # Save command as a variable ls_cmd() # Run command as if it were a function
plumbum is a library for “script-like” Python programs. You can call programs like functions as in
sh. Plumbum is useful if you want to run a pipeline without the shell.
ls_cmd = plumbum.local("ls -l") # Get command ls_cmd() # Run command
pexpect lets you spawn child applications, control them and find patterns in their output. This is a better alternative to subprocess for commands that expect a tty on Unix.
pexpect.run("ls -l") # Run command as normal child = pexpect.spawn('scp foo [email protected]:.') # Spawns child application child.expect('Password:') # When this is the output child.sendline('mypassword')
fabric is a Python 2.5 and 2.7 library. It allows you to execute local and remote shell commands. Fabric is simple alternative for running commands in a secure shell (SSH)
fabric.operations.local('ls -l') # Run command as normal fabric.operations.local('ls -l', capture = True) # Run command and receive output
envoy is known as “subprocess for humans”. It is used as a convenience wrapper around the
r = envoy.run("ls -l") # Run command r.std_out # Get output
commands contains wrapper functions for
os.popen, but it has been removed from Python 3 since
subprocess is a better alternative.
Hope you learned something from this post.
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