python Fabric库学习

fabric官网
http://www.fabfile.org/

wrapper封装
v1.12
fabric是一个python库，同时也是一个命令行工具，其利用ssh进行应用开发或是系统管理任务。
它提供了一个基础的操作套件，可用来在本地亦或是远端执行shell命令，也可用来上传或是下载文件，同时也附带其他功能 such as prompting the running user for input, or aborting execution.

1、Overview and Tutorial
(1)它是能让你通过命令行来执行python函数的工具
(2)它是一个库帮助你通过ssh来执行shell命令
例如：
fabfile.py

def hello():
    print("Hello world!")

$ fab hello
Hello world!

Done.

发生了什么：fab工具导入fabfile.py文件，然后执行函数。

对fabric的封装，把它的local接口封装成try_execute

使用fabric远程执行命令
import fabric.operations

fabric.operations.run(cmd)
_run_command()
_execute()

fabric local函数中使用了subprocess
local和run实现方式不一致。。。

local 的capture=True和False
也就是标准输出，标准错误输出，是直接打印出来还是捕获到变量中。

  if capture:
        out_stream = subprocess.PIPE
        err_stream = subprocess.PIPE
    else:
        dev_null = open(os.devnull, 'w+')
        # Non-captured, hidden streams are discarded.
        out_stream = None if output.stdout else dev_null
        err_stream = None if output.stderr else dev_null
    try:
        cmd_arg = wrapped_command if win32 else [wrapped_command]
        p = subprocess.Popen(cmd_arg, shell=True, stdout=out_stream,
                             stderr=err_stream, executable=shell,
                             close_fds=(not win32))
        (stdout, stderr) = p.communicate()
    finally:
        if dev_null is not None:
            dev_null.close()
    # Handle error condition (deal with stdout being None, too)
    out = _AttributeString(stdout.strip() if stdout else "")
    err = _AttributeString(stderr.strip() if stderr else "")
    out.command = given_command
    out.real_command = wrapped_command
    out.failed = False
    out.return_code = p.returncode
    out.stderr = err
    if p.returncode not in env.ok_ret_codes:
        out.failed = True
        msg = "local() encountered an error (return code %s) while executing '%s'" % (p.returncode, command)
        error(message=msg, stdout=out, stderr=err)
    out.succeeded = not out.failed
    # If we were capturing, this will be a string; otherwise it will be None.
    return out

子进程
subprocess.PIPE
p = subprocess.Popen(cmd , shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

p.stdout.read()
communicate()

python中使用管道
subprocess.PIPE实际上为文本流提供一个缓存区。child1的stdout将文本输出到缓存区，随后child2的stdin从该PIPE中将文本读取走

child1 = subprocess.Popen([“cat”,”/etc/passwd”], stdout=subprocess.PIPE)
child2 = subprocess.Popen([“grep”,”0:0”],stdin=child1.stdout, stdout=subprocess.PIPE)
out = child2.communicate()