13.6 执行外部命令并获取它的输出

问题¶

You want to execute an external command and collect its output as a Python string.

解决方案¶

Use the subprocess.check_output() function. For example:

import subprocess out_bytes = subprocess.check_output([‘netstat’,’-a’])

This runs the specified command and returns its output as a byte string. If you need to interpret the resulting bytes as text, add a further decoding step. For example:

out_text = out_bytes.decode(‘utf-8’)

If the executed command returns a nonzero exit code, an exception is raised. Here is an example of catching errors and getting the output created along with the exit code:

try:

out_bytes = subprocess.check_output([‘cmd’,’arg1’,’arg2’])

except subprocess.CalledProcessError as e:

out_bytes = e.output # Output generated before error code = e.returncode # Return code

By default, check_output() only returns output written to standard output. If you want both standard output and error collected, use the stderr argument:

out_bytes = subprocess.check_output([‘cmd’,’arg1’,’arg2’],

stderr=subprocess.STDOUT)

If you need to execute a command with a timeout, use the timeout argument:

try:

out_bytes = subprocess.check_output([‘cmd’,’arg1’,’arg2’], timeout=5)

except subprocess.TimeoutExpired as e:

...

Normally, commands are executed without the assistance of an underlying shell (e.g., sh, bash, etc.). Instead, the list of strings supplied are given to a low-level system com‐ mand, such as os.execve(). If you want the command to be interpreted by a shell, supply it using a simple string and give the shell=True argument. This is sometimes useful if you’re trying to get Python to execute a complicated shell command involving pipes, I/O redirection, and other features. For example:

out_bytes = subprocess.check_output(‘grep python | wc > out’, shell=True)

Be aware that executing commands under the shell is a potential security risk if argu‐ ments are derived from user input. The shlex.quote() function can be used to properly quote arguments for inclusion in shell commands in this case.

讨论¶

The check_output() function is the easiest way to execute an external command and get its output. However, if you need to perform more advanced communication with a

subprocess, such as sending it input, you’ll need to take a difference approach. For that, use the subprocess.Popen class directly. For example:

import subprocess

# Some text to send text = b’‘’ hello world this is a test goodbye ‘’‘

# Launch a command with pipes p = subprocess.Popen([‘wc’],

stdout = subprocess.PIPE, stdin = subprocess.PIPE)

# Send the data and get the output stdout, stderr = p.communicate(text)

# To interpret as text, decode out = stdout.decode(‘utf-8’) err = stderr.decode(‘utf-8’)

The subprocess module is not suitable for communicating with external commands that expect to interact with a proper TTY. For example, you can’t use it to automate tasks that ask the user to enter a password (e.g., a ssh session). For that, you would need to turn to a third-party module, such as those based on the popular “expect” family of tools (e.g., pexpect or similar).