python code execution bash command - python3 cook book

python code execution bash commands related - python3 cook book

refer: https://python3-cookbook.readthedocs.io/zh_CN/latest/c13/p06_executing_external_command_and_get_its_output.html

Execute external command and get its output

problem

You want to execute an external command and get the results as a Python string.

solution

Use subprocess.check_output()functions. E.g:

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

This code performs a specified command and returns an execution result as a byte string. If you need to return the form of text, plus a decoding steps. E.g:

out_text = out_bytes.decode('utf-8')

If a command is executed to a non-zero return code will throw an exception. The following examples will trap and obtain return 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 the return value input to the standard output. If you need to collect standard output and error output at the same time, use stderrparameters:

out_bytes = subprocess.check_output(['cmd','arg1','arg2'],
                                    stderr=subprocess.STDOUT)

If you need to use a timeout mechanism to execute commands, use timeoutparameters:

try:
    out_bytes = subprocess.check_output(['cmd','arg1','arg2'], timeout=5)
except subprocess.TimeoutExpired as e:
    ...

Generally speaking, it does not require execution of the command to the underlying shell environment (such as sh, bash). A list of strings are passed to a low-level system commands, such as os.execve(). If you want a shell command is executed, passing a string parameter, and set the parameters shell=True. Sometimes you want to perform a complex Python shell command when this is very useful, such as pipeline flow, I / O redirection and other features. E.g:

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

Note that the command execution there will be a security risk in the shell, especially when parameter input from the user. This time you can use the shlex.quote()function to correct parameters in double quote quotes.

discuss

Use check_output()function is to perform an external command and easiest way to get its return value. However, if a child process you need to do more complex interactions, such as sending input to it, you have to use another method. This time can be used directly subprocess.Popencategory. E.g:

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')

subprocessTTY module dependent external commands for inappropriate use. For example, you can not use it to automate tasks a user to enter a password (for example a ssh session). At this time, you need to use a third-party modules, such as based on the famous expectfamily of tools (pexpect or similar)