Should You Use `shell=True` with Python's `subprocess` Module?

The Significance of 'shell=True' in Subprocess Module

The subprocess module facilitates the execution of various processes. However, understanding the role of 'shell=True' parameter is crucial.

Consider the code snippets below:

callProcess = subprocess.Popen(['ls', '-l'], shell=True)

callProcess = subprocess.Popen(['ls', '-l']) # without shell

Both code blocks execute the 'ls -l' command, but the presence of 'shell=True' in the first code has a significant impact. When 'shell=True', the command is executed through the system's shell (specified by the SHELL environment variable on POSIX, cmd.exe on Windows). In contrast, without 'shell', the process is directly initiated.

Benefits of Using Shell:

• Environment variable expansion: The shell interprets and expands environment variables, such as $HOME, in the command.
• File glob expansion: On POSIX systems, file globs (e.g., ".") are expanded to a list of files by the shell.

Benefits of Not Using Shell:

• Avoids mystery program invocation: Without the shell, the exact program executed is known, avoiding potential user-controlled variations.
• Protects against ILS attacks: Exploiting shell interpretation vulnerabilities (e.g., inadvertent command injection) is mitigated.

Recommended Practice:

Generally, it is advisable to use 'shell=False' for the following reasons:

• Security: Avoids potential exploits.
• Transparency: Executes the specified program directly without introducing any other interpretation layer.
• Efficiency: Can be faster, especially with simple commands.

Therefore, unless environment variable expansion or file glob expansion is explicitly required, it is recommended to use 'shell=False' for greater security and efficiency.

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