具有“ 1>＆2”和stderr = STDOUT的Python子进程

Question

I have this code from https://pymotw.com/2/subprocess/

I'm not sure how to interpret the code, in the check_output with 1>&2 output is redirected to stderr, but in the parameter, the stderr is back to stdout stderr=subprocess.STDOUT .

output = subprocess.check_output(
    'echo to stdout; echo to stderr 1>&2; exit 1',
    shell=True,  
    stderr=subprocess.STDOUT, 
    )
print "*****************"
print 'Have %d bytes in output' % len(output)
print output

Running the code, the print commands are not executed meaning nothing is captured.

What does this code trying to accomplish?

EDIT

From the answer and comment, I could run this code to get

try:
    output = subprocess.check_output(
        'echo to stdout; echo to stderr 1>&2; exit 1',
        shell=True,  # No such file or directory error without, maybe 1>&2 requires shell=True
        stderr=subprocess.STDOUT,  
        )

except subprocess.CalledProcessError as e:

    print "*****************"
    print 'Have %d bytes in output' % len(e.output)
    print e.output

this output:

*****************
Have 20 bytes in output
to stdout
to stderr

However, when I commented out the stderr=subprocess.STDOUT line, I got instead

to stderr
*****************
Have 10 bytes in output
to stdout

EDIT2

I tested more with stderr library ( https://github.com/sickill/stderred ) that helps a shell to show characters from stderr in red color.

When I execute this code (comment out the redirection), I can see the to stderr in BLACK color which implies it uses stdout.

output = subprocess.check_output(
        'echo to stdout; echo to stderr 1>&2; exit 1',
        shell=True,  
        #stderr=subprocess.STDOUT, 
        )

From this, I guess (correct me if I'm wrong) that Python's check_output method prints out the data into the stderr redirect to stdout so that it prints out the error message into stderr.

Answer 1

The 1 >&2 shell code applies only to the (echo) command it appears on. It is how to tell the shell to direct the output of that echo to the shell's stderr stream.

The python code stderr=subprocess.STDOUT tells the subprocess module that you want the process's stderr stream to be the same file descriptor as its stdout stream so that you will read whatever the process writes to either stream interleaved together in one stream.

The exit 1 in the shell command means that the shell exits with an error (non-zero) status.

The purpose of the code is to demonstrate that the python function subprocess.check_output will check the exit status and raise an exception when it is non-zero.

If the exit code was non-zero it raises a CalledProcessError. The CalledProcessError object will have the return code in the returncode attribute and output in the output attribute.

Your description of:

Running the code, the print commands are not executed

is a bit misleading since you neglect to mention the output that does occur:

Traceback (most recent call last):
  File "t.py", line 6, in <module>
    stderr=subprocess.STDOUT, 
  File "/usr/lib/python2.7/subprocess.py", line 573, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command 'echo to stdout; echo to stderr 1>&2; exit 1' returned non-zero exit status 1