How to perform command line interactions using os.popen in Python
Python offers different modules for interacting with the operating system (OS). One of these modules is os, and it contains an os.popen() function. Even though it has been deprecated since Python 3.3 in favor of the more modern subprocess module, understanding how os.popen() functions can provide insight into the evolution of Python’s capabilities for handling external processes.
The os.popen() function
The os.popen() function of the os module allows us to open a
Syntax
os.popen(command[, mode[, bufsize]]);
Parameters
command: This represents the command to be executed in the shell.mode (optional): This is the mode in which the command’s standard input or output should be opened. By default, it’s"r"(read) but it can be either"r"(read) or"w"(write).bufsize (optional): This specifies the buffer size. If it is omitted, the default buffering scheme is used.
Return value
The os.popen() method returns a file object connected to the command’s standard output (if the mode is "r") or standard input (if the mode is "w"). The returned file object can read or write data to the command, depending on the specified mode.
Code example
Here’s a simple code example showing the use of os.popen():
import os# Open a pipe to the "ls" command for readingpipe = os.popen("ls", "r")# Read and print the pipe contentsprint("The contents in the current directory:")print(pipe.read())# Close the pipepipe.close()
In the above-given code example:
Line 4: Opens a pipe to the
"ls"command for reading (specified by the"r"mode).Line 8: Reads and prints the output of the
"ls"command.Line 11: Closes the pipe.
Deprecated status
Despite its simplicity and ease of use, os.popen() has been deprecated in recent versions of Python. The deprecation is primarily due to security concerns related to shell injection vulnerabilities. When constructing commands dynamically by concatenating strings, there’s a risk of unintentionally introducing security vulnerabilities. Let’s look at an example for better demonstration.
The os.popen() example
Here’s a code example that uses deprecated and insecure practices, demonstrating the potential risks of command injection in Python applications:
import os# User input obtained, for example, from a web form or external sourceuser_input = "file.txt; rm -rf /"# Constructing a command using os.popen() with string concatenationcommand = "cat " + user_input# Executing the command using os.popen()result = os.popen(command, "r").read()# Output the resultprint(result)
In this example, the os.popen() function relies on the shell to execute the command. The command constructed is cat file.txt; rm -rf /. The semicolon (;) is a command separator in the shell, and it executes the subsequent command regardless of the success or failure of the preceding one. As a result, both cat file.txt and rm -rf / are executed, leading to the printing of the content of the file with some warnings like:
“rm: it is dangerous to operate recursively on '/’”
“rm: use --no-preserve-root to override this failsafe”
Note: These are the security measures implemented by the Educative’s platform that are being populated due to the injected
rm -rf /command. Otherwise, this can lead to unintended and potentially harmful consequences, such as deleting files.
The subprocess example
Now, let’s try to execute the same command using subprocess module:
import subprocess# User input obtained, for example, from a web form or external sourceuser_input = "file.txt; rm -rf /"# Constructing a command using subprocess with proper argument passingcommand = ["cat", user_input]# Executing the command using subprocessresult = subprocess.run(command, capture_output=True, text=True)# Output the resultprint(result.stdout)
In this example, subprocess.run() treats each element in the command list as a separate argument. The command constructed is ["cat", "file.txt; rm -rf /"]. It attempts to execute a single command with the file name containing the semicolon. The cat command will treat the entire string as a file name, and since it doesn’t find a file with that exact name, it produces no output. The subsequent rm -rf / is not executed as a separate command.
Note: To see how subprocess output will appear, you need to fix the user input from user_input = 'file.txt; rm -rf /' to user_input = 'file.txt'.
Conclusion
To summarize, while os.popen() was once a convenient way to run shell commands from within Python, it has been deprecated due to such security concerns. Developers are encouraged to use the subprocess module as a safer alternative. Understanding the transition from os.popen() to subprocess is essential for writing modern and secure Python code, especially when dealing with external processes and command-line interactions.
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