A running program is called a process. Each process has its own system state, which includes memory, lists of open files, a program counter that keeps track of the instruction being executed, and a call stack used to hold the local variables of functions.
Normally, a process executes statements one after the other in a single sequence of control flow, which is sometimes called the main thread of the process. At any given time, the program is only doing one thing. A program can create new processes using library functions such as those found in the os or subprocess modules such as os. Popenetc. However, these processes, known as subprocessesrun as completely independent entities-each with their own private system state and main thread of execution.
Because a subprocess is independent, it executes concurrently with the original process. That is, the process that created the subprocess can go on to work on other things while the subprocess carries out its own work behind the scenes.
It offers a higher-level interface than some of the other available modules, and is intended to replace the following functions:. For example, echo name is causing a syntax error because echo is not a built-in statement or function in Python.
So, in Python script, we're using print name instead. To run UNIX commands we need to create a subprocess that runs the command. The recommended approach to invoking subprocesses is to use the convenience functions for all use cases they can handle. Or we can use the underlying Popen interface can be used directly. Also, we got a return value of 0 which is the result of executing this command, which means there was no error in the execution.
If we run the code above os. To see the command output we should redirect it to a file, and the read from it:. Open a pipe to or from command. The return value is an open file object connected to the pipe, which can be read or written depending on whether mode is 'r' default or 'w'. The bufsize argument has the same meaning as the corresponding argument to the built-in open function. The exit status of the command encoded in the format specified for wait is available as the return value of the close method of the file object, except that when the exit status is zero termination without errorsNone is returned.
If we pass everything as a string, then our command is passed to the shell; if we pass them as a list then we don't need to worry about escaping anything. However, it's been deprecated since version 2.Python offers several options to run external processes and interact with the operating system.
However, the methods are different for Python 2 and 3. Python 2 has several methods in the os module, which are now deprecated and replaced by the subprocess module, which is the preferred option in Python 3. Throughout this article we'll talk about the various os and subprocess methods, how to use them, how they're different from each other, on what version of Python they should be used, and even how to convert the older commands to the newer ones.
Hopefully by the end of this article you'll have a better understanding of how to call external commands from Python code and which method you should use to do it. The os module offers four different methods that allows us to interact with the operating system just like you would with the command line and create a pipe to other commands.2: Executing Interactive Commands in Python through Paramiko Part-1
These methods I'm referring to are: popenpopen2popen3and popen4all of which are described in the following sections. The goal of each of these methods is to be able to call other programs from your Python code.
The os. This pipe allows the command to send its output to another command. The output is an open file that can be accessed by other programs. Here the command parameter is what you'll be executing, and its output will be available via an open file. The argument mode defines whether or not this output file is readable 'r' or writable 'w'.
Appending a 'b' to the mode will open the file in binary mode. Thus, for example "rb" will produce a readable binary file object.
In order to retrieve the exit code of the command executed, you must use the close method of the file object. The bufsize parameter tells popen how much data to buffer, and can assume one of the following values:. This method is available for Unix and Windows platforms, and has been deprecated since Python version 2.
If you're currently using this method and want to switch to the Python 3 version, here is the equivalent subprocess version for Python The code above will ask the operating system to list all files in the current directory.
The output of our method, which is stored in pis an open file, which is read and printed in the last line of the code. The of this code in the context of my current directory result is as follows:.
This method is very similar to the previous one. The main difference is what the method outputs. In this case it returns two file objects, one for the stdin and another file for the stdout.
The popen2 method is available for both the Unix and Windows platforms.
Async and await with subprocesses
However, it is found only in Python 2. Again, if you want to use the subprocess version instead shown in more detail belowuse the following instead:. This code will produce the same results as shown in the first code output above.
The difference here is that the output of the popen2 method consists of two files. Thus, the 2nd line of code defines two variables: in and out. In the last line, we read the output file out and print it to the console. This method is very similar to the previous ones.
Functionality may be limited when it's turned off. Read Later on Pocket or Instapaper. Python's subprocess module is one of my favourite modules in the standard library.
If you have ever done some decent amount of coding in python, you might have encountered it. This module is used for dealing with external commands, intended to be a replacement to the old os. The most trivial use might be to get the output of a small shell command like ls or ps. Not that this is the best way to get a list of files in a directory think os.
I am going to put my notes and experiences about this module here. Please note, I wrote this with Python 2. Things are slightly different in other versions even 2. If you find any errors or suggestions, please let me know. For the sake of providing context, lets run the ls command from subprocess and get its output. I'll cover getting output from a command in detail later.
To give more command line arguments.
The first item in the list is the executable and rest are its command line arguments argv equivalent. No quirky shell quoting and complex nested quote rules to digest. Just a plain python list. However, not having shell quoting implies you don't also have the shell niceties.
Like piping for one. The following won't work the way one would expect it to. Here, the ls command gets its first command as and I have no idea what ls would do with it. Perhaps complain that no such file exists. So, instead, we have to use the shell boolean argument. More later down in the article.
If there's just one thing in the subprocess module that you should be concerned with, its the Popen class. Here's the signature from the docs. Subprocess can also run command-line instructions via a shell program. Notice that in this case we pass a string, not a list. This is because we want the shell to interpret the whole of our command. You can even use shell style quoting if you like.
It is up to the shell to decide how to best split the command line into executable and command line arguments. On windows, if you pass a list for args, it will be turned into a string using the same rules as the MS C runtime.
See the doc-string for subprocess.For Python trainingour top recommendation is DataCamp. Datacamp provides online interactive courses that combine interactive coding challenges with videos from top instructors in the field.
Subprocess Overview For a long time I have been using os. The main reason for that, was that I thought that was the simplest way of running Linux commands.
In the official python documentation we can read that subprocess should be used for accessing system commands. Subprocess intends to replace several other, older modules and functions, like: os. We can run the command line with the arguments passed as a list of strings example 1 or by setting the shell argument to a True value example 2 Note, the default value of the shell argument is False.
Let's look at two examples where we show the summary of disk usage using subprocess. Input and Output With subprocess you can suppress the output, which is very handy when you want to run a system call but are not interested about the standard output. Return Codes You can use subprocess. Every process will return an exit code and you can do something with your script based on that code.
If the return code is anything else than zero, it means that an error occurred. If you want to do system administration in Python, I recommend reading Python for Unix and Linux System Administration stdin, stdout and stderr One of the trickiest part I had with subprocess was how to work with pipes and to pipe commands together.
PIPE indicates that a new pipe to the child should be created. The default setting is "None", which means that no redirection will occur. The standard error or stderr can be STDOUT, which indicates that the stderr data from the child process should be captured into the same file handle as for stdout.
Popen The underlying process creation and management in the subprocess module is handled by the Popen class. Let's get started with some real examples. PIPE print p. If you know that you will only work with specific subprocess functions, such as Popen and PIPE, then it is enough to only import those.
Read data from stdout and stderr, until end-of-file is reached. Wait for process to terminate. The optional input argument should be a string to be sent to the child process, or None, if no data should be sent to the child. Basically, when you use communicate it means that you want to execute the command Ping program using subprocess In the "More Reading" section below, you can find links to read more about the subprocess module, but also examples.
Let's write our own ping program where we first ask the user for input, and then perform the ping request to that host. This time we use the host command. If you have any questions or comments, please use the comment field below. Regardless, PythonForBeginners.In this tutorial, you will learn, how to run shell command in python. This module intends to replace several older modules and functions:.
The subprocess module allows users to communicate from their Python script to a terminal like bash or cmd. The os module allows platform independent programming by providing abstract methods. So if you want to capture output then you have to use os. The os. This means that we can access the stream within Python. This is useful since you can now get the output as a variable.
I hope it is helpful for you and if you have any query regarding this post then ask your questions in comment section. Thanks Everyone. Hey friends, this is Gulsanober Saba. A masters student learning Computer Applications belongs from Ranchi. Here I write tutorials related to Python Programming Language. Your email address will not be published.
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Is it possible to execute an arbitrary number of commands in sequence using the same subprocess command? I also need this to work in Python 2. I also need the subprocess command to work in Linux, Windows and macOS which is why I'm just using echo commands as examples here. It makes sense if some commands are not true processes but shell commands that could for example change the shell environment.
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To have this code run under Linux, you would have to replace cmd. For Python 3, you would have to encode the commands and probably decode their output, and to use parentheses with print. Beware: this can only work for little output. If there was enough output to fill the pipe buffer before closing the stdin pipe, this code would deadlock. A more robust way would be to have a second thread to read the output of the commands to avoid that problem.
This is similar to the answer posted by Serge Ballesta, but not quite. Use his for asynchronous execution, where you don't care about the results. Use mine for synchronous processing and result gathering.
Like his answer, I'm showing the Windows solution here - run a bash process in Linux rather than cmd in Windows. If, for example you just read a batch file contents into a string, you could run it this way because it would already have the newlines. If it does not, that final command will fail to execute. Just like if you typed it into your command prompt but didn't hit enter at the end. You will however see a mysterious More? PIPE in your Popen constructor.
When you create a cmd. And I mean that quite literally. If you test this, you will see that the stdout which is returned contains your commands.Python is awesome, and can pretty much do everything you ever wanted, but on rare occasion, you may want to call an external program. Great for running a program, not so great if you need to capture its output. Subprocess is a module dedicated to running other processes. These made life a lot easier, as you could now have easy interaction with the process pipes, running it as a direct call or opening a new shell first, and more.
The downside was the inconvenience of having to switch between them depending on needs, as they all had different outputs considering how you interacted with them. That all changed in Python 3. It is simply the only subprocess command you should ever need!
Now check that response out. Basically, you should use subprocess. But if you had input it was much harder. On Linux you could use signals, but Windows required either a forever running background thread or run in a separate process. Also keep in mind a lot of programs are actual files on Linux, whereas they are shell built-ins on Windows.
There seems to be very odd behavior with the first argument being passed to subprocess functions, including. On Windows you can get away with murder pass either a list or string for either case. The string will be passed directly to the newly spawned shell as is, so it can expand globs and environment variables. However, if a list is passed, it is sent as positional arguments to the shell. So if you have:. And going the other direction can be a pain on Linux. Note that shlex.
Pipes and buffers are both sections of memory used for the storage and exchange of data between processes. Pipes are designed to transfer and hold the data, while buffers act as temporary vessels to transfer data to files. So if you redirect sys.
That way the output is stored into a file, using a buffer to temporarily hold information in memory until a large enough section is worth writing to the file.
If encoding is specified Python 3. However it is not possible to wait for certain output for event before sending input, that is more suited to pexpect or similar. It makes sure the status return code is 0, or will raise subprocess.