subprocess.getoutput fails on win32

[99b26d71-7cc7-468f-91b8-8e0a28da3740]

BPO	10197
Nosy	@terryjreedy, @gpshead, @ncoghlan, @vstinner, @tjguk, @ned-deily, @merwok, @bitdancer, @briancurtin, @florentx
Files	subprocess_getoutput.patch subprocess.patch: Patch to implement ( ) wrapping for Windows issue10197.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = 'https://github.com/tjguk' closed_at = created_at = labels = ['type-bug', 'OS-windows'] title = 'subprocess.getoutput fails on win32' updated_at = user = 'https://bugs.python.org/jldm' ``` bugs.python.org fields: ```python activity = actor = 'gregory.p.smith' assignee = 'tim.golden' closed = True closed_date = closer = 'tim.golden' components = ['Windows'] creation = creator = 'jldm' dependencies = [] files = ['20985', '23433', '32450'] hgrepos = [] issue_num = 10197 keywords = ['patch', 'needs review'] message_count = 33.0 messages = ['119605', '119607', '120558', '123130', '123141', '123150', '123232', '123237', '123246', '123298', '123410', '129967', '129974', '145735', '145764', '145824', '145835', '146365', '146379', '157670', '201907', '202026', '202031', '202044', '202048', '202050', '202208', '202450', '202504', '202630', '212918', '236481', '236483'] nosy_count = 14.0 nosy_names = ['terry.reedy', 'gregory.p.smith', 'ncoghlan', 'vstinner', 'tim.golden', 'ned.deily', 'eric.araujo', 'Arfrever', 'r.david.murray', 'brian.curtin', 'flox', 'jldm', 'python-dev', 'bpoaugust'] pr_nums = [] priority = 'normal' resolution = 'fixed' stage = 'resolved' status = 'closed' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue10197' versions = ['Python 3.3', 'Python 3.4'] ```

[99b26d71-7cc7-468f-91b8-8e0a28da3740]

Hi, first of all sorry for my English.

On windows XP SP3, the following code:

import subprocess
subprocess.getoutput("dir")

returns '"{" is not recognized as an internal or external command,\noperable program or batch file.'

I made a workaround by changing in the file Lib/subprocess.py the line 574 (I thin in 3.2a3 is 584) (in the getstatusoutput(cmd) function definition) from: pipe = os.popen('{ ' + cmd + '; } 2>&1', 'r')

to: pipe = os.popen('( ' + cmd + '; ) 2>&1', 'r')

I have tested it with:

ActivePython 3.1.2.3 (ActiveState Software Inc.) based on Python 3.1.2 (r312:79147, Mar 22 2010, 12:20:29) [MSC v.1500 32 bit (Intel)] on win32

and

Python 3.2a3 (r32a3:85355, Oct 10 2010, 17:11:45) [MSC v.1500 32 bit (Intel)] on win32

Regards from Spain. José Luis Domenech

[bitdancer]

Oddly, the test suite skips getoutput and getstatusoutput on windows with the comment that the source says it is relevant only for posix, but the documentation does not have 'availability: unix' tags. (It is also odd that getoutput isn't documented, but that's a different issue.)

Your workaround can't be used as a fix, since the semantics of {}s in the shell are different from those of ()s.

It's not clear to me what the point of the {}s is, but I have a fear that eliminating them would introduce subtle changes in the behavior of getoutput calls. Perhaps not, though.

It looks like this issue amounts to an RFE for support of getoutput/getstatusoutput on Windows, though the fact that it is not documented as unix-only may make it a bug instead :)

The appropriate fix is probably to conditionalize the code based on platform. A complete patch will require unit test changes and documentation changes (since the docs currently mention the braces).

All of that said, it also appears that the new check_output should be preferred to either getoutput or getstatusoutput. Perhaps those functions could be re-implemented in terms of check_output.

[merwok]

() is used to launch a command in a sub-shell and {} is used to group commands, for example to set up a stream redirection for all commands in brackets.

[merwok]

I think we should implement getstatusoutput and getoutput with Popen objects to gain portability and avoid spawning subshells.

[bitdancer]

Do you have in implementation in mind? I'm not clear how this would work.

[merwok]

My idea is simply using Popen with appropriate args for stdout and stderr instead of using a shell command with redirections:

--- Lib/subprocess.py (révision 86943) +++ Lib/subprocess.py (copie de travail) @@ -560,11 +560,7 @@ return ''.join(result)

-# Various tools for executing commands and looking at their output and status.
-#
-# NB This only works (and is only relevant) for UNIX.
-
-def getstatusoutput(cmd):
+def getstatusoutput(cmd, shell=True):
     """Return (status, output) of executing cmd in a shell.

     Execute the string 'cmd' in a shell with os.popen() and return a 2-tuple
@@ -581,14 +577,21 @@
     >>> subprocess.getstatusoutput('/bin/junk')
     (256, 'sh: /bin/junk: not found')
     """
-    pipe = os.popen('{ ' + cmd + '; } 2>&1', 'r')
-    text = pipe.read()
-    sts = pipe.close()
-    if sts is None: sts = 0
-    if text[-1:] == '\n': text = text[:-1]
+    # Wish I could use with...
+    popen = Popen(cmd, shell=shell, stdout=PIPE, stderr=STDOUT)
+    sts = popen.communicate() #or wait() or whatever is the right thing
+    try:
+        text = process.stdout.read()
+    finally:
+        process.stdout.close()
+    if sts is None:
+        sts = 0
+    if text.endswith('\n'):
+        text = text[:-1]
     return sts, text

(The new “shell” argument is icing on the cake, allowing us to later support a list as cmd argument like Popen does.)

[vstinner]

-def getstatusoutput(cmd): +def getstatusoutput(cmd, shell=True):

shell=True is dangerous, it can lead to shell command injection. I would prefer to set its default value to False. The function already exists in Python 3.1, but it is not used in Python source code. Is it too late to fix its API to avoid security vulnerabilities?

[merwok]

The function already exists in Python 3.1, but it is not used in Python source code

We don’t know what code out there uses. This would be an incompatible change.

[bitdancer]

Ah, I did not realize that getstatusoutput was implemented using os.popen. I thought it already used Popen. Now, in python3, os.popen is in turn implemented using subprocess.Popen, so removing that level of indirection seems sensible.

The question that remains is, does removing the {} change the output obtained from a command sequence in any way?

Note that for backward compatibility you will need to re-munge the status code into C format. Which makes me wonder if getoutput/getstatusoutput should just be documentationally deprecated instead. (I never use them myself, FWIW)

[ned-deily]

See also bpo-9922

[merwok]

The question that remains is, does removing the {} change the output obtained from a command sequence in any way? {} are used to group output from the commands into one stream. I believe the stdout and stderr arguments to Popen allow us to get compatible behavior. Tests need to prove that, of course.

Which makes me wonder if getoutput/getstatusoutput should just be documentationally deprecated instead. (I never use them myself, FWIW) They were relocated from the commands module, resulting in slightly overlapping functions that don’t share a naming patter: call, check_call, getoutput, getstatusoutput.

[vstinner]

subprocess_getoutput.patch: patch subprocess.getstatusoutput() to use directly Popen, instead of os.popen, with stderr=subprocess.STDOUT instead of "2>&1" shell redirection. It strips also all trailing spaces and newlines, not just the last one. And finally, it removes "Availability: UNIX." from the documentation.

I tried to add a shell argument (to be able to disable the shell) and to accept any Popen keyword, but I don't know how to implement shell=False if the input is a list of arguments. list2cmdline() is unsafe on UNIX (see bpo-8972). And if getstatusoutput() doesn't accept argument list, it becomes useless with shell=False (it doesn't support to call a program with arguments).

Note: the status is still shifted on UNIX to be compatible with the wait() format.

[vstinner]

I tried to add a shell argument (to be able to disable the shell) and to accept any Popen keyword, but I don't know how to implement shell=False if the input is a list of arguments. list2cmdline() is unsafe on UNIX (see bpo-8972).

Example of function to escape a list of arguments on UNIX:

def escapeargs(*args):
   return ' '.join(pipes.quote(arg) for arg in args)

R. David Murray disagree with me to allow getoutput(list) (shell=True) because Popen(list, shell=True) behaves differently.

subprocess.Popen(['echo Hello'], shell=True) writes 'Hello', whereas subprocess.Popen(['echo', 'Hello'], shell=True) writes nothing (because echo has no argument.

I would like to do something like that: getoutput(['echo', 'Hello']) calls Popen('echo Hello', shell=True) using escapeargs() function defined above. So getoutput(list) calls shell -c "arg1 arg2", whereas Popen(list, shell=True) calls shell -c "arg1" arg2 arg3 ...

See also issue bpo-7839 for Popen(str, shell=False) and Popen(list, shell=True) cases.

[44b4c87c-fe02-447d-bbdd-58861e3ff3f2]

subprocess.getoutput does not currently work at all on Windows. So it's not necessary to maintain backwards compatibility.

The following fix works for me on WinXP/Python 3.2.2.

Replace

    pipe = os.popen('{ ' + cmd + '; } 2>&1', 'r') # line 613 of subprocess.py

with

    if mswindows:
        pipe = os.popen(cmd + ' 2>&1', 'r') # Windows does not support { }
    else:
        pipe = os.popen('{ ' + cmd + '; } 2>&1', 'r')

[44b4c87c-fe02-447d-bbdd-58861e3ff3f2]

A better fix, which supports multiple windows commands:

    if mswindows:
        pipe = os.popen('( ' + cmd + ' ) 2>&1', 'r') # Windows uses () rather than { }
    else:
        pipe = os.popen('{ ' + cmd + '; } 2>&1', 'r')

This works with the command

subprocess.getoutput("echo before & python -V & echo after")

Note that python -V writes to stderr, so without the enclosing ( ) the version information is not captured.

[merwok]

If Windows shell syntax is similar to POSIX one, then () will run in a sub-shell, which would be a different behavior than using {} (which merely group statements and their streams).

[44b4c87c-fe02-447d-bbdd-58861e3ff3f2]

I got the () syntax from:

http://technet.microsoft.com/en-us/library/cc737438%28WS.10%29.aspx

which refers to grouping, not subshell.

[ncoghlan]

Without knowing this issue existed, I recently started working on adding some convenience APIs for shell invocation to shutil: http://bugs.python.org/issue13238

I think the getstatus and getstatusoutput APIs were copied from the commands module in 3.0 without sufficient thought being given to whether or not they fit with the design principles of the subprocess module. IMO, both should be deprecated:

• they're not cross-platform
• they invoke the shell implicitly, which subprocess promises never to do

[merwok]

IMO, both should be deprecated:

• they're not cross-platform Isn’t the purpose of this report to fix that? :)

• they invoke the shell implicitly, which subprocess promises never to do One could argue that it’s not implicit if it’s documented. Nonetheless, I agree that they don’t fit well with the subprocess promises.

So, +1 on deprecating and +1 on new, safer helpers.

[merwok]

I think that adding safer wrappers and deprecating things are valuable but different bugs. In the short term, we could apply the proposed small patch to just fix the issue at hand. Can one of the Windows experts weigh in?

The patch does this:

    if mswindows:
        pipe = os.popen('( ' + cmd + ' ) 2>&1', 'r')
    else:
        pipe = os.popen('{ ' + cmd + '; } 2>&1', 'r')

It was tested manually; a test should be simple to write.

[tjguk]

Patched according to Nick Coghlan's suggestion in http://bugs.python.org/issue9922#msg150093. Ad hoc tests look ok on Windows. I'll add tests & look at *nix later.

[1762cc99-3127-4a62-9baf-30c3d0f51ef7]

New changeset c34e163c0086 by Tim Golden in branch '3.3': Issue bpo-10197 Rework subprocess.get[status]output to use subprocess functionality and thus to work on Windows. Patch by Nick Coghlan. http://hg.python.org/cpython/rev/c34e163c0086

New changeset 05ce1bd1a4c2 by Tim Golden in branch 'default': Issue bpo-10197 Rework subprocess.get[status]output to use subprocess functionality and thus to work on Windows. Patch by Nick Coghlan. http://hg.python.org/cpython/rev/05ce1bd1a4c2

New changeset b6efaa97ee0e by Tim Golden in branch '3.3': Issue bpo-10197: merge heads http://hg.python.org/cpython/rev/b6efaa97ee0e

New changeset 28a0ae3dcb16 by Tim Golden in branch 'default': Issue bpo-10197: merge heads http://hg.python.org/cpython/rev/28a0ae3dcb16

New changeset fe828884a077 by Tim Golden in branch 'default': Issue bpo-10197: merge 3.3 http://hg.python.org/cpython/rev/fe828884a077

[tjguk]

Code & tests now work on Windows. Applied to 3.3 & 3.4.

[gpshead]

The documentation needs updating to state that these are available on Windows (currently it says UNIX) with a versionchanged annotation.

http://docs.python.org/3.3/library/subprocess.html#legacy-shell-invocation-functions

[1762cc99-3127-4a62-9baf-30c3d0f51ef7]

New changeset 2924a63aab73 by Tim Golden in branch '3.3': Issue bpo-10197: Indicate availability of subprocess.get[status]output on Windows and add a note about the effects of universal newlines http://hg.python.org/cpython/rev/2924a63aab73

New changeset effad2bda4cb by Tim Golden in branch 'default': Issue bpo-10197: Indicate availability of subprocess.get[status]output on Windows and add a note about the effects of universal newlines http://hg.python.org/cpython/rev/effad2bda4cb

[tjguk]

Good point. I've added the versionchanged tag.

The issue with bytes-string encoding goes all the way back to Popen.communicate if universal newlines mode is used so I've simply put in a reference to the existing notes on the subject higher up in the docs.

[1762cc99-3127-4a62-9baf-30c3d0f51ef7]

New changeset 0aa2aedc6a21 by Tim Golden in branch 'default': Issue bpo-10197 Tweak docs for subprocess.getstatusoutput and align the documentation, the module docstring, and the function docstring. http://hg.python.org/cpython/rev/0aa2aedc6a21

[terryjreedy]

Is this ready to be reclosed?

[80036ac5-bb84-4d39-8416-02cd8e51707d]

Lib/subprocess.py still has outdated comment:

# NB This only works (and is only relevant) for POSIX.

[tjguk]

Thanks: final outdated comments removed

[1762cc99-3127-4a62-9baf-30c3d0f51ef7]

New changeset 34df43c9c74a by R David Murray in branch '3.3': bpo-10197: Update get[status]output versionchanged with actual version. http://hg.python.org/cpython/rev/34df43c9c74a

New changeset ee277b383d33 by R David Murray in branch 'default': bpo-10197: Update get[status]output versionchanged with actual version. http://hg.python.org/cpython/rev/ee277b383d33

[gpshead]

A side effect of the changes made within are that getstatusoutput() on POSIX systems now returns a different value for status.

The old implementation present in Python 2 and Python 3.3 before this patch returned the raw waitpid() status result as the status value. ie: getstatusoutput("exit 1")[0] == 256. the lower 8 bits were reserved for the signal number the process died with, if any.

Now it returns the sanitized subprocess style returncode: positive numbers are the process exit code (so the above example returns 1) and negative numbers are the negative signal number the process died with.

I prefer the new behavior, but this API change is not documented anywhere that I can find.

[gpshead]

http://bugs.python.org/issue23508 to track the fall out of that.