[issue16353] add function to os module for getting path to default shell

[STINNER Victor]

STINNER Victor  added the comment:

This issue has been fixed. The subprocess module now uses '/system/bin/sh' by 
default on Android thanks to Xavier de Gaye!

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resolution:  -> fixed
stage: needs patch -> resolved
status: open -> closed

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[issue16353] add function to os module for getting path to default shell

[Chih-Hsuan Yen]

Change by Chih-Hsuan Yen :


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[issue16353] add function to os module for getting path to default shell

[Antoine Pitrou]

Antoine Pitrou  added the comment:

I think the functionality of getting either the system-wide default shell or 
the current user's default shell can be useful regardless of whether the 
specific subprocess issue was fixed.

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stage: patch review -> needs patch
versions: +Python 3.8 -Python 3.5

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[issue16353] add function to os module for getting path to default shell

[Cheryl Sabella]

Cheryl Sabella  added the comment:

3.7 added `sys.getandroidapilevel()` under #28740.  As Xavier mentioned in 
msg271439, that was used for subprocess.Popen in #16255 and also merged as part 
of 3.7.

I'm not sure if there is still interest in adding a `default_shell` to the os 
module using `sys.androidapilevel()` and the last batch that Xavier uploaded in 
2016.

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[issue16353] add function to os module for getting path to default shell

[Xavier de Gaye]

Xavier de Gaye added the comment:

> After several years there is still no consenus, so make it simple - the popen 
> and subprocess shell is one of /bin/sh, cmd.exe, or some other "hard-coded" 
> shell.

I agree, the last patch that is being reviewed in issue 16255 attempts to do 
that.

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[issue16353] add function to os module for getting path to default shell

[Michael Felt]

Michael Felt added the comment:

An interesting read, but I am lost in what the goal is.

e.g., on AIX, which I know well, the system default is /bin/ksh (aka 
/usr/bin/ksh). However, /bin/sh (/usr/bin/sh) is available as well.

My expectation is that on Linux the default shell is /bin/bash, and like AIX 
/bin/sh is also available.

/bin/sh, /bin/ksh, /bin/bash are all (potentially) default shells. However, 
something every posix system is "sh" aka borne shell.

Aside: windows is different, and I expect has a different syntax. 

So, if the goal is to GET the pathname of the so-called "default" shell -again, 
interesting - BUT - what does that buy me?

For Popen or now subprocess I would want a consistent shell syntax, i.e., borne 
shell on posix, cmd.exe on windows, and whatever is correct on platforms that 
do not fit in these two.

Hence, on posix platforms where /bin/sh does not exist (hard)code the correct 
path for that platform.

FYI: CS_PATH does not exist on AIX

IMHO: (as others have stated) having . in PATH is bad for security, being 
dependent on PATH opens other security concerns as well.

IMHO2: KISS principle. After several years there is still no consenus, so make 
it simple - the popen and subprocess shell is one of /bin/sh, cmd.exe, or some 
other "hard-coded" shell.

(Although I thought that subprocess called "program" directly. Python is still 
new, so if not applicable for subprocess - please ignore

rereading, direct call would be when shell=False I expect.

Anyway, my two cents.

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[issue16353] add function to os module for getting path to default shell

[Xavier de Gaye]

Xavier de Gaye added the comment:

Issue 27472 adds test.support.unix_shell as the path to the default shell.

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[issue16353] add function to os module for getting path to default shell

[Christian Heimes]

Changes by Christian Heimes :


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[issue16353] add function to os module for getting path to default shell

[Xavier de Gaye]

Xavier de Gaye added the comment:

Following Serhiy suggestion at msg224477, this tentative patch adds the 
is_android attribute to the posix module and the default_shell attribute to the 
os module. No documentation for the moment. No test case is needed I think, as 
the changes in issue 16255 will test any solution found here.

An alternative to posix.is_android is sys.platform set to 'linux-android' on 
Android. These are the locations in the standard library that do not test for 
"if sys.platform.startswith('linux')":

Lib/distutils/tests/test_dist.py|386 col 16| if sys.platform in ('linux', 
'darwin'):
Lib/test/test_logging.py|527 col 12| if sys.platform in ('linux', 'darwin'):
Lib/test/test_resource.py|135 col 26| @unittest.skipUnless(sys.platform == 
'linux', 'test requires Linux')
Lib/test/test_socket.py|898 col 16| or sys.platform in ('linux', 'darwin')):
Lib/test/test_socket.py|2256 col 23| @skipWithClientIf(sys.platform not in 
{"linux"},
Lib/test/test_socket.py|4508 col 22| @unittest.skipUnless(sys.platform == 
'linux', 'Linux specific test')
Lib/test/test_sysconfig.py|388 col 26| @unittest.skipUnless(sys.platform == 
'linux', 'Linux-specific test')

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Added file: http://bugs.python.org/file42953/default_shell.patch

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[issue16353] add function to os module for getting path to default shell

[Roman Evstifeev]

Changes by Roman Evstifeev :


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[issue16353] add function to os module for getting path to default shell

[Matt Frank]

Matt Frank added the comment:

In msg174930 Christian Heimes (christian.heimes) wrote:
 I've tested confstr(CS_PATH) on Linux, Mac OS X, Solaris, HP-UX
 and BSD. It works and the path to `sh` is always included.

In msg230713 Ned Deily(ned.deily) wrote:
 ignore Lib/macpath.py.
 [...]
 OS X uses Lib/posixpath.py.

These two messages have convinced me that the correct approach is to kick the 
can down the road.  I will file a new issue and patch to fix os.defpath (by 
initializing os.defpath by calling confstr(CS_PATH)).  Then I will file a new 
issue and patch that will define a reasonable os.confstr() for platforms where 
HAVE_CONFSTR is not defined.

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[issue16353] add function to os module for getting path to default shell

[Matt Frank]

Matt Frank added the comment:

In msg230720 Akira Li (akira) wrote:
 os.defpath is supposed to be ':'+CS_PATH, e.g., look at glibc (C library
 used on Linux) sysdeps/posix/spawni.c I don't know whether it is
 possible to change CS_PATH without recompiling every statically linked
 executable on a system, sysdeps/unix/confstr.h:

   #define CS_PATH /bin:/usr/bin

 Though this issue is about the path to the standard shell sh/cmd.exe.
 It is not about os.defpath.

I understand this issue is about the path to the standard shell.
My argument is that os.defpath is the wrong tool for finding that
path, and os.confstr('CS_PATH') is the correct tool, as you originally
suggested in msg224514.

 The patch [1] has tests. Have you tried to run them?
 The tests *pass* at least on one system ;)

 [...]

 os.get_shell_executable() does not use cwd. Neither the documentation
 nor the code in the patch suggest that.

I ran the tests (and the entire Python test suite) before I wrote my
first message.  But you didn't test what happens when there is a bogus
'sh' somewhere along the path.  You also have a bug in your path
searching loop that interprets an empty element on the path as /.
Here's the current failure mode:

Go to root on your Posix system.  With su or sudo create an
executable file named sh.  (for example cd /; sudo touch sh; sudo
chmod +x sh).  Then go back to your python interpreter (with the
patch applied) and run os.get_shell_executable().  The returned
result will be '/sh'.

The loop _should_ be treating empty path elements as current working
directory.  (In the glibc sysdeps/posix/spawni.c file you pointed to
look around line 281, by the comment that says /* Two adjacent
colons, or a colon at the beginning or the end of 'PATH' means to
search the current directory.*/)

But if you fix the loop to iterate over the path correctly (including
'cwd') then you will find that putting an executable named 'sh' in the
current working directory will make os.get_shell_executable() return
the sh in the current working directory (because os.defpath says it
should).

Note also, that glibc's spawni() uses two different default paths.
One is for if the user didn't specify their PATH variable (that's the
one where they use :/bin:/usr/bin) and a completely different (built
in path is used in the spawn*p* version) if it turns out that the file
is a script that needs to run under the system sh.  (On line 290 they
call maybe_script_execute(), which calls script_execute(), which uses
_PATH_BSHELL.)

 os.defpath is the default search path used by exec*p* and spawn*p*
 i.e., if os.defpath is incorrect for these system then os.exec*p* and
 os.spawn*p* functions could also be broken.

I'll look into it.

 I don't remember exactly the motivation to use os.defpath instead of
 os.confstr('CS_PATH').

The motivation was in msg224514.  You wrote:
 In the absense of os.confstr('CS_PATH') on Android (msg175006),
 os.defpath seems appropriate than os.environ['PATH'] to expand 'sh'
 basename to the full path to be used by subprocess later.

But os.defpath doesn't work on Android either (because it is hardcoded
to ':/bin:/usr/bin').

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[issue16353] add function to os module for getting path to default shell

[Akira Li]

Akira Li added the comment:

 Matt Frank added the comment:

 In msg230720 Akira Li (akira) wrote:
 os.defpath is supposed to be ':'+CS_PATH, e.g., look at glibc (C library
 used on Linux) sysdeps/posix/spawni.c I don't know whether it is
 possible to change CS_PATH without recompiling every statically linked
 executable on a system, sysdeps/unix/confstr.h:

   #define CS_PATH /bin:/usr/bin

 Though this issue is about the path to the standard shell sh/cmd.exe.
 It is not about os.defpath.

 I understand this issue is about the path to the standard shell.
 My argument is that os.defpath is the wrong tool for finding that
 path, and os.confstr('CS_PATH') is the correct tool, as you originally
 suggested in msg224514.


I'll repeat os.defpath definition that I've cited in the same message
msg224514:

  The default search path used by exec*p* and spawn*p* if the
  environment doesn’t have a 'PATH' key. Also available via os.path.

1. os.defpath should work (contain the path to the standard shell)
   everywhere where os.confstr(CS_PATH) works.

2. And it might be easier to customize e.g., on systems where there is
   no os.confstr or where changing CS_PATH involves recompiling crucial
   system processes.

If these two statements are not true then os.defpath could be dropped.

 The patch [1] has tests. Have you tried to run them?
 The tests *pass* at least on one system ;)

 [...]

 os.get_shell_executable() does not use cwd. Neither the documentation
 nor the code in the patch suggest that.

 I ran the tests (and the entire Python test suite) before I wrote my
 first message.  But you didn't test what happens when there is a bogus
 'sh' somewhere along the path.  You also have a bug in your path
 searching loop that interprets an empty element on the path as /.
 Here's the current failure mode:

 Go to root on your Posix system.  With su or sudo create an
 executable file named sh.  (for example cd /; sudo touch sh; sudo
 chmod +x sh).  Then go back to your python interpreter (with the
 patch applied) and run os.get_shell_executable().  The returned
 result will be '/sh'.
 But if you fix the loop to iterate over the path correctly (including
 'cwd') then you will find that putting an executable named 'sh' in the
 current working directory will make os.get_shell_executable() return
 the sh in the current working directory (because os.defpath says it
 should).


Comment in the patch explicitly says that '' is interpreted as '/'. The
intent is to exclude the current working directory, thinking that /sh
executable can create only root. And root can do anything to the machine
anyway.

I agree. It is a misfeature. I've uploaded a new patch that excludes ''
completely. The previous code tried to be too cute.

 The loop _should_ be treating empty path elements as current working
 directory.  (In the glibc sysdeps/posix/spawni.c file you pointed to
 look around line 281, by the comment that says /* Two adjacent
 colons, or a colon at the beginning or the end of 'PATH' means to
 search the current directory.*/)

yes. Empty path element stands for the current working directory e.g.,

   $ PATH= python
   $ PATH=: python
   $ PATH=:/ python
   $ PATH=/: python

run ./python on my system.

The current working directory is intentionally excluded (in the original
and in the current patches) otherwise os.get_exec_path(env={}) would be
used.

 Note also, that glibc's spawni() uses two different default paths.
 One is for if the user didn't specify their PATH variable (that's the
 one where they use :/bin:/usr/bin) and a completely different (built
 in path is used in the spawn*p* version) if it turns out that the file
 is a script that needs to run under the system sh.  (On line 290 they
 call maybe_script_execute(), which calls script_execute(), which uses
 _PATH_BSHELL.)

glibc hardcodes the path (like subprocess module does currently):

  #define   _PATH_BSHELL/bin/sh

os.get_shell_executable() is an enhancement that allows
(Unix) systems to configure the path via os.defpath.

 But os.defpath doesn't work on Android either (because it is hardcoded
 to ':/bin:/usr/bin').

As I've mentioned in msg224514 before posting my original patch:

- Andriod uses /system/bin/sh
- os.defpath could be tweaked on Android

i.e., yes. Default os.defpath doesn't work there and it should be
configured on Android to include the path to the standard shell.

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[issue16353] add function to os module for getting path to default shell

[Akira Li]

Changes by Akira Li 4kir4...@gmail.com:


Added file: http://bugs.python.org/file37140/os.get_shell_executable-3.patch

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[issue16353] add function to os module for getting path to default shell

[Matt Frank]

Changes by Matt Frank matthew.i.fr...@intel.com:


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[issue16353] add function to os module for getting path to default shell

[Matt Frank]

Matt Frank added the comment:

Unfortunately os.defpath seems to be hardcoded.  And hardcoded to the wrong 
value on every system I have looked at, including Linux.

Lib/posixpath.py sets defpath=':/bin:/usr/bin' which is _not_ what `getconf 
CS_PATH` returns on my Linux (the extra ':' at the beginning means include the 
cwd[1] which is almost certainly not what anyone wants.)  The hardcoded value 
'/bin:/usr/bin' would also be wrong for Solaris and AIX installations that are 
trying to be POSIX (I think they use /usr/xpg4/bin/sh).

Meanwhile Lib/macpath.py sets defpath=':', which is also almost certainly 
wrong.  (I don't have a Mac and have never programmed one, but StackOverflow[2] 
indicates that one should use `path_helper` (which in turn gets the default 
value by reading it from the files in /etc/paths.d))[3]  So it seems likely 
that this patch breaks Popen() on MacOS (unless, perhaps, a path of ':' means 
something special on Mac?).

And Lib/ntpath.py sets defpath='.;C:\\bin', which doesn't resemble a path that 
even works (as pointed out in now closed http://bugs.python.org/issue5717).  
(The correct value may be 
'%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem;%SYSTEMROOT%\System32\WindowsPowerShell\v1.0\'.)

So I don't know where to go next.  I'm happy to cook up a different patch, but 
I have no idea what it should be.  Here are some possibilities:

1. Kick the can down the road: file a new bug against os.defpath and (somehow) 
fix Lib/*path.py so they do something more reasonable in more cases.  One of 
which might be to have posixpath.py try to call os.confstr() (and then, 
perhaps, special-code something in Modules/posixmodule.c in the case that 
HAVE_CONFSTR is not defined.)

2. Modify @akira's patch to call os.confstr('CS_PATH') instead of os.defpath, 
and then deal with the fact that very few systems actually implement confstr() 
(perhaps by special-coding something in Modules/posixmodule.c as described 
above.)

3. Modify this patch to fall back to `PATH` if `sh` can't be found on 
os.defpath (or os.confstr('CS_PATH') fails).



[1] `man bash` on Linux, search for the description of the PATH variable.  
(e.g., http://man7.org/linux/man-pages/man1/bash.1.html)
[2] 
http://stackoverflow.com/questions/9832770/where-is-the-default-terminal-path-located-on-mac
[3] 
https://developer.apple.com/library/mac/documentation/Darwin/Reference/Manpages/man8/path_helper.8.html
[4] 
http://superuser.com/questions/124239/what-is-the-default-path-environment-variable-setting-on-fresh-install-of-window

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[issue16353] add function to os module for getting path to default shell

[Ned Deily]

Ned Deily added the comment:

Matt, ignore Lib/macpath.py.  It is not used on OS X systems other than in the 
rare case that someone explicitly needs to parse obsolete Classic Mac OS (Mac 
OS 9 or earlier) style path names.  OS X uses Lib/posixpath.py.

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[issue16353] add function to os module for getting path to default shell

[Akira Li]

Akira Li added the comment:

 Matt Frank added the comment:

 Unfortunately os.defpath seems to be hardcoded.  And hardcoded to the
 wrong value on every system I have looked at, including Linux.

os.defpath is supposed to be ':'+CS_PATH, e.g., look at glibc (C library
used on Linux) sysdeps/posix/spawni.c I don't know whether it is
possible to change CS_PATH without recompiling every statically linked
executable on a system, sysdeps/unix/confstr.h:

  #define CS_PATH /bin:/usr/bin

Though this issue is about the path to the standard shell sh/cmd.exe.
It is not about os.defpath.

The patch [1] has tests. Have you tried to run them?
The tests *pass* at least on one system ;)

[1] http://bugs.python.org/review/16353/#ps12569

To run the tests, download the patch into your python checkout:

  $ curl -O \
  http://bugs.python.org/file36196/os.get_shell_executable.patch

and apply it:

  $ patch -p1 os.get_shell_executable.patch

to run only test_os.py:

  $ ./python -mtest test_os

 Lib/posixpath.py sets defpath=':/bin:/usr/bin' which is _not_ what
 getconf CS_PATH` returns on my Linux (the extra ':' at the beginning
 means include the cwd which is almost certainly not what anyone
 wants.) 

os.get_shell_executable() does not use cwd. Neither the documentation
nor the code in the patch suggest that.

 The hardcoded value '/bin:/usr/bin' would also be wrong for
 Solaris and AIX installations that are trying to be POSIX (I think
 they use /usr/xpg4/bin/sh).

os.defpath is the default search path used by exec*p* and spawn*p*
i.e., if os.defpath is incorrect for these system then os.exec*p* and
os.spawn*p* functions could also be broken.

I expect that Windows, OS X, Linux work as is. If the tests fail on
Solaris, AIX then os.defpath should be tweaked on these systems if it
hasn't been already. Note: os.defpath is a very conservative value: it
should be set at python's installation time at the very
latest. Henceforth it should remain the same.

 And Lib/ntpath.py sets defpath='.;C:\\bin', which doesn't resemble a
 path that even works (as pointed out in now closed
 http://bugs.python.org/issue5717).  (The correct value may be
 %SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem;%SYSTEMROOT%\System32\WindowsPowerShell\v1.0\'.)

Please, look at the patch. Neither the documentation nor the code
suggest that os.defpath is used on Windows.

 So I don't know where to go next.  I'm happy to cook up a different
 patch, but I have no idea what it should be.  Here are some
 possibilities:

 1. Kick the can down the road: file a new bug against os.defpath and
 (somehow) fix Lib/*path.py so they do something more reasonable in
 more cases.  One of which might be to have posixpath.py try to call
 os.confstr() (and then, perhaps, special-code something in
 Modules/posixmodule.c in the case that HAVE_CONFSTR is not defined.)

 2. Modify @akira's patch to call os.confstr('CS_PATH') instead of
 os.defpath, and then deal with the fact that very few systems actually
 implement confstr() (perhaps by special-coding something in
 Modules/posixmodule.c as described above.)

Note I'm the author of http://bugs.python.org/issue16353#msg224514
message that references the POSIX recommendation about `getconf PATH`
and *explicitly* mentions os.confstr('CS_PATH').

I don't remember exactly the motivation to use os.defpath instead of
os.confstr('CS_PATH').

A guess: the result is the same (a lone `:` is ignored in the patch) but
os.defpath is easier to modify for uncommon systems where os.confstr
might not be available.

I expect that the tests in the patch will pass on all stable buildbots
[2] successfully without any modifications (with os.defpath as is).

[2] http://buildbot.python.org/all/waterfall?category=3.x.stable

Other systems should probably configure os.defpath appropriately (it
should include the path to the standard shell).

 3. Modify this patch to fall back to `PATH` if `sh` can't be found on
 os.defpath (or os.confstr('CS_PATH') fails).

The standard shell should not depend on PATH envvar. The tests show that
os.get_shell_executable() works even with an empty environment. The
motivation is the same as why `getconf PATH` exists in the first place.

The documentation for os.get_shell_executable() (from the patch):

   Return a path to the standard system shell executable -- ``sh``
   program on POSIX (default: ``'/bin/sh'``) or ``%ComSpec%`` command
   processor on Windows (default: ``%SystemRoot%\system32\cmd.exe``).

   Availability: Unix, Windows

The intent is that os.get_shell_executable() returns the same *shell
path* as used by system(3) on POSIX i.e., (ignoring signals,
etc) os.system(command) could be implemented:

  subprocess.call(command, shell=True, executable=os.get_shell_executable())

(presumably subprocess can use get_shell_executable() internally instead
of /bin/sh but it is another issue)

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[issue16353] add function to os module for getting path to default shell

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Should it be function? Why not use just a variable initialized at import time? 
The path to the default shell shouldn't change during the time of program 
execution.

if sys.platform == 'win32':
default_shell = 'cmd.exe'
else:
default_shell = '/bin/sh'

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[issue16353] add function to os module for getting path to default shell

[eryksun]

eryksun added the comment:

 The path to the default shell shouldn't change during the time 
 of program execution.

On Windows the ComSpec environment variable could change during program 
execution.

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[issue16353] add function to os module for getting path to default shell

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Taking into consideration msg174586 we can ignore the ComSpec environment 
variable and hardcode cmd.exe on Windows, as we ignore the SHELL environment 
variable on Posix systems.

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[issue16353] add function to os module for getting path to default shell

[eryksun]

eryksun added the comment:

Defaulting to just cmd.exe can execute an arbitrary program named cmd.exe 
in the application directory or current directory. 

When CreateProcess needs to find the shell to execute a batch file (.bat or 
.cmd), it doesn't search for cmd.exe -- at least not in NT 6. If ComSpec 
isn't set, it defaults to %SystemRoot%\system32\cmd.exe. If both ComSpec and 
SystemRoot are unset, executing a batch file via CreateProcess fails with 
ERROR_DIRECTORY. 

I think Python should use the same default path if it's ignoring ComSpec:

default_shell = path.join(environ[SystemRoot], system32, cmd.exe)

A KeyError shouldn't be a problem here. CPython won't even run if SystemRoot 
isn't set to the correct path:

C:\set SystemRoot=

C:\py -3
Fatal Python error: Failed to initialize Windows random API (CryptoGen)

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[issue16353] add function to os module for getting path to default shell

[Akira Li]

Akira Li added the comment:

 Should it be function? Why not use just a variable initialized at
 import time? The path to the default shell shouldn't change during
 the time of program execution.

 if sys.platform == 'win32':
 default_shell = 'cmd.exe'
 else:
default_shell = '/bin/sh'

Andriod uses /system/bin/sh (and /sbin/sh if we believe some adb
source on the internet). See issue16255: subrocess.Popen needs
/bin/sh but Android only has /system/bin/sh

POSIX recommends [1] `getconf PATH` (os.confstr('CS_PATH')):

  Applications should note that the standard PATH to the shell cannot
  be assumed to be either /bin/sh or /usr/bin/sh, and should be
  determined by interrogation of the PATH returned by getconf PATH,
  ensuring that the returned pathname is an absolute pathname and not
  a shell built-in.

  For example, to determine the location of the standard sh utility:

$ command -v sh

i.e. shell executable is shutil.which('sh', os.defpath) on POSIX.
os.defpath could be tweaked on Android.

To avoid dependency on shutil something like msg174628 could be
adopted:

  if sys.platform == win32: # use sys.platform to accommodate java
  def get_shell_executable():
  Return path to the command processor specified by %ComSpec%.

  Default: %SystemRoot%\system32\cmd.exe
  
  return (environ.get('ComSpec') or
  path.join(environ.get('SystemRoot', r'C:\Windows'),
r'system32\cmd.exe'))
  else: # POSIX
  def get_shell_executable():
  Return path to the standard system shell executable.

  Default: '/bin/sh'
  
  for dirpath in defpath.split(pathsep):
  sh = dirpath + sep + 'sh' #NOTE: interpret '' as '/'
  if access(sh, F_OK | X_OK) and path.isfile(sh):
  #NOTE: allow symlinks e.g., /bin/sh on Ubuntu may be dash
  return sh
  return '/bin/sh' #XXX either OS or python are broken if we got here

Windows part is based on msg224512. If r'C:\Windows' or '/bin/sh' code
is reached then it means that either OS or python are
broken/misconfigured.

system(), popen() are described on POSIX as [2]:

  # fork()
  execl(shell path, sh, -c, command, (char *)0);

subprocess module that replaces system, popen, spawn*p* calls may use
os.get_shell_executable() to implement shell=True.

os.defpath is [3]:

  The default search path used by exec*p* and spawn*p* if the
  environment doesn’t have a 'PATH' key. Also available via os.path.

In the absense of os.confstr('CS_PATH') on Android (msg175006),
os.defpath seems appropriate than os.environ['PATH'] to expand 'sh'
basename to the full path to be used by subprocess later.

os.get_shell() name might imply that a shell object is returned that
can run commands like for example os.popen() returns a file-like
object that is not true (the intent is to return a path -- a simple
string).

os.get_shell_executable() is similar to sys.executable that returns
path to python executable.

os.get_shell_executable() is not necessary for every python run
therefore it is better to keep it a function to avoid unnecessary stat
calls on startup. Though the result should not change during the
program run.

 On Windows the ComSpec environment variable could change during
 program execution.

Does it? The docs for os.environ say [4]:

  This mapping is captured the first time the os module is imported,
  typically during Python startup as part of processing
  site.py. Changes to the environment made after this time are not
  reflected in os.environ, except for changes made by modifying
  os.environ directly.

I've uploaded a patch with the described implementation (plus docs, tests).

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/sh.html
[2] http://pubs.opengroup.org/onlinepubs/9699919799/functions/popen.html
[3] https://docs.python.org/3.4/library/os.html#os.defpath
[4] https://docs.python.org/3.4/library/os.html#os.environ

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Added file: http://bugs.python.org/file36195/os.get_shell_executable.patch

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[issue16353] add function to os module for getting path to default shell

[Akira Li]

Changes by Akira Li 4kir4...@gmail.com:


--
versions: +Python 3.5 -Python 3.4

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[issue16353] add function to os module for getting path to default shell

[Akira Li]

Changes by Akira Li 4kir4...@gmail.com:


Removed file: http://bugs.python.org/file36195/os.get_shell_executable.patch

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[issue16353] add function to os module for getting path to default shell

[Akira Li]

Changes by Akira Li 4kir4...@gmail.com:


Added file: http://bugs.python.org/file36196/os.get_shell_executable.patch

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[issue16353] add function to os module for getting path to default shell

[Christian Heimes]

Christian Heimes added the comment:

3.4 beta1 will be released next weekend. I'll try to submit a patch in the next 
couple of days.

--
assignee:  - christian.heimes
stage:  - patch review

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[issue16353] add function to os module for getting path to default shell

[Tim Golden]

Changes by Tim Golden m...@timgolden.me.uk:


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[issue16353] add function to os module for getting path to default shell

[Christian Heimes]

Christian Heimes added the comment:

Meh, Python 2.6 from SL4A (scripting languages for Android) doesn't have 
os.confstr(). I just tried it in a Android 2.3.3 emulator. Python 2.6 on Linux 
has the function. I propose we fall back to PATH env and use /bin as last 
resort.

try:
cspath = confstr(CS_PATH)
except (ValueError, NameError):
# installation doesn't have confstr() or doesn't know about CS_PATH
# fall back to PATH environ and use /bin as last resort
cspath = environ.get(PATH, /bin)
for path in cspath.split(pathsep):
...

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[issue16353] add function to os module for getting path to default shell

[Andrew Svetlov]

Andrew Svetlov added the comment:

Confirm it for SL4A Python 3.2

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[issue16353] add function to os module for getting path to default shell

[Taras Lyapun]

Taras Lyapun added the comment:

Andrew, do you mean that Christian Heimes last snippet working on Android?

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[issue16353] add function to os module for getting path to default shell

[Andrew Svetlov]

Andrew Svetlov added the comment:

I mean the last available SL4A doesn't have os.confstr
Fallback should work, os.environ['PATH'] contains '/system/bin' where 'sh' is 
living.

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[issue16353] add function to os module for getting path to default shell

[Andrew Svetlov]

Andrew Svetlov added the comment:

Christian,
Is there ``os.confstr`` supported by MaxOS X?
Is there using of environ['PATH'] makes sense as good callback if former is not 
present?

About COMSPEC. From my point of view it's useful if we need default path.
Or if we have Win9x, where shell is command.com (not supported by 3.4)

I guess to hardcode ``cmd.exe`` for Windows for the same reason as we hardcode 
``sh`` for Unix systems.

I agree to moving code to ``os`` if we no need to use ``shutil.which``.
The function name is still the question: getdefaultshell, getshell, get_shell 
etc.

Adding shutil.getdefaultshell function is interesting suggestion. It function 
intended to return user shell (from SHELL env var or COMSPEC for Windows) and 
not used in subprocess if shell=True (but can be passed as executable param if 
need).

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[issue16353] add function to os module for getting path to default shell

[Chris Jerdonek]

Chris Jerdonek added the comment:

 Any interest in doing like os.get_terminal_size/shutil.get_terminal_size

If functions with two different behaviors are needed, I think the two functions 
should probably have different names (e.g. get_shell() and get_user_shell()).  
Otherwise, it may create confusion as to which to call (or which is being 
called when reading code).

Moreover, it doesn't seem like the following guidance in the docs for which 
get_terminal_size() to use would hold in parallel for the shell variants: 
shutil.get_terminal_size() is the high-level function which should normally be 
used, os.get_terminal_size is the low-level implementation.

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[issue16353] add function to os module for getting path to default shell

[Taras Lyapun]

Taras Lyapun added the comment:

Updated patch.
Moved function to os and used Christian Heimes implementation.
Updated doc, and test.
Also renamed function to get_shell.

Test passes on mac os and windows.

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[issue16353] add function to os module for getting path to default shell

[Christian Heimes]

Christian Heimes added the comment:

I've tested confstr(CS_PATH) on Linux, Mac OS X, Solaris, HP-UX and BSD. It 
works and the path to `sh` is always included.

Taras:
You don't have to perform the platform inside the get_shell() function. I 
suggest that you define the function depending on the value of `name`.

if name == posix:
def get_shell():
...
elif name in {nt, ce}:
def get_shell():
...

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[issue16353] add function to os module for getting path to default shell

[Taras Lyapun]

Taras Lyapun added the comment:

Updated patch in regards to Christian Heimes remark.

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[issue16353] add function to os module for getting path to default shell

[Chris Jerdonek]

Chris Jerdonek added the comment:

Is someone able to test the patch on Android (the impetus for this issue)?

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[issue16353] add function to os module for getting path to default shell

[Chris Jerdonek]

Chris Jerdonek added the comment:

Some minor comments.

+.. function :: get_shell()
+
+  Return the path to default shell.

Three leading spaces needed.  Also, Return the path to the default shell.

+  For unix system returns path to ``sh``, for windows returns path to 
``cmd.exe``.

On Unix systems returns the path to ``sh``, and on Windows returns the path to 
``cmd.exe``.  I would also document and test FileNotFound (e.g. by temporarily 
changing os.confstr).

+Return the path to default shell for Unix.

the default shell

+Return the path to default shell for Windows.

the default shell

+class TestGetShell(unittest.TestCase):
+def test_get_shell(self):
+shell = os.get_shell()
+param = /c if sys.platform == 'win32' else -c
+s = subprocess.Popen([shell, param, echo test],
+stdout=subprocess.PIPE)

Alignment should be:

+s = subprocess.Popen([shell, param, echo test],
+ stdout=subprocess.PIPE)

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[issue16353] add function to os module for getting path to default shell

[Taras Lyapun]

Taras Lyapun added the comment:

Diff with implementation, test and doc.

--
keywords: +patch
nosy: +lyapun
Added file: http://bugs.python.org/file27849/issue16353.diff

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[issue16353] add function to os module for getting path to default shell

[Andrew Svetlov]

Andrew Svetlov added the comment:

I would to prefer *get_default_shell* as function name (it's a bit shorter).

Also please add .. versionadded:: 3.4 tag to docs.

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[issue16353] add function to os module for getting path to default shell

[Taras Lyapun]

Taras Lyapun added the comment:

 I would to prefer *get_default_shell* as function name (it's a bit shorter).

Hm... I'm not sure, because someone can imagine that function returns some 
object or something like this... 'path' indicates that function returns 
something like string.

 Also please add .. versionadded:: 3.4 tag to docs.

Oh, sorry, added.

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[issue16353] add function to os module for getting path to default shell

[Christian Heimes]

Christian Heimes added the comment:

which('sh') isn't correct here. 'which' searches in all PATH environ parts. 
However the shell must be looked up in CS_PATH only.

From man sh(1posix):

 Applications should note that the standard PATH to the shell cannot be 
 assumed to be either /bin/sh or /usr/bin/sh, and should  be determined by 
 interrogation of the PATH returned by getconf PATH , ensuring that the 
 returned pathname is an absolute pathname and not a shell built-in.

'getconf PATH' queries confstr(_CS_PATH).

I suggest that you modify the POSIX part to:

  path = os.confstr(CS_PATH)
  which('sh', path=path)

--
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[issue16353] add function to os module for getting path to default shell

[Chris Jerdonek]

Chris Jerdonek added the comment:

+++ b/Lib/shutil.py Sat Nov 03 13:32:05 2012 +0200
+
+def get_default_shell_path():

Why is the patch putting the function in the shutil module?  The function 
should go in the os module as the title and comments of this issue state.  
shutil seems misplaced to me.

For example, the description of shutil in the docs is, The shutil module 
offers a number of high-level operations on files and collections of files.  In 
particular, functions are provided which support file copying and removal.  In 
contrast, the description of the os module is, This module provides a portable 
way of using operating system dependent functionality.  The default shell is 
operating system dependent functionality.

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[issue16353] add function to os module for getting path to default shell

[Christian Heimes]

Christian Heimes added the comment:

I'm with Chris. The information should be stored in the os module. I suggest 
os.shell var or os.get_shell() function like this:

def get_shell():
for path in confstr(CS_PATH).split(pathsep):
sh = sep.join((path, sh))
try:
mode = stat(sh).st_mode
except OSError:
pass
else:
if st.S_ISREG(mode):
return sh
raise FileNotFound(sh)

According to all examples S_ISREG() is sufficient here.

On Windows the function should use the env var COMSPEC instead of hard coding 
cmd.exe.

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[issue16353] add function to os module for getting path to default shell

[Andrew Svetlov]

Andrew Svetlov added the comment:

Is it ok to import *which* functions from shutil in *os* module?
There is only reason to put function into shutil.

But I like Christian's sketch.

Also, what reason to get shell name from COMSPEC? What should we do if COMSPEC 
points to some another shell, not cmd.exe?

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[issue16353] add function to os module for getting path to default shell

[R. David Murray]

R. David Murray added the comment:

I think it would not be ok to import shutil in os, so I'm glad there is an 
alternative.

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[issue16353] add function to os module for getting path to default shell

[Chris Jerdonek]

Chris Jerdonek added the comment:

 Also, what reason to get shell name from COMSPEC? What should we do if 
 COMSPEC points to some another shell, not cmd.exe?

FWIW, the subprocess module does this (with surrounding code linked after):

comspec = os.environ.get(COMSPEC, cmd.exe)

http://hg.python.org/cpython/file/ed091424f230/Lib/subprocess.py#l1060

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[issue16353] add function to os module for getting path to default shell

[Christian Heimes]

Christian Heimes added the comment:

The os module can't import shutil as it would create a circular import. Also 
shutil.which() does lots of stat calls and we don't want additional stat calls 
in a core module like os. My implementation requires just one stat() call.

COMSPEC can point to an alternative command implementation. That's the point of 
COMSPEC. Perhaps some people remember 4DOS and 4NT.

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[issue16353] add function to os module for getting path to default shell

[Éric Araujo]

Éric Araujo added the comment:

Any interest in doing like os.get_terminal_size/shutil.get_terminal_size with 
the os function being basic (i.e. current patch) and the shutil version 
querying the env var SHELL in addition?

--
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[issue16353] add function to os module for getting path to default shell

[Éric Araujo]

Éric Araujo added the comment:

Please use ``sh`` or maybe :command:`sh` (check the sphinx doc) instead of `sh`.

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[issue16353] add function to os module for getting path to default shell

[Ezio Melotti]

Changes by Ezio Melotti ezio.melo...@gmail.com:


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[issue16353] add function to os module for getting path to default shell

[Antoine Pitrou]

Antoine Pitrou added the comment:

Is it the system default shell or the user's default shell that we want here?

That is, shouldn't we look up `pwd.getpwuid(os.getuid()).pw_shell` ?
(but only when os.getuid() == os.geteuid()?)

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[issue16353] add function to os module for getting path to default shell

[Chris Jerdonek]

Chris Jerdonek added the comment:

The system default shell is what I had in mind when filing this issue (i.e. 
what Popen() uses by default or, in the case of Android, what Popen() should 
use).

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[issue16353] add function to os module for getting path to default shell

[Antoine Pitrou]

Antoine Pitrou added the comment:

 The system default shell is what I had in mind when filing this issue
 (i.e. what Popen() uses by default or, in the case of Android, what
 Popen() should use).

Well, the question then becomes whether Popen() shouldn't use the user's
default shell instead? :)
It probably doesn't make much of a difference in most cases, though.
Also, there's the problem that some special users may have a void shell
entry (or /bin/false or /sbin/nologin), to prevent a human from logging
in as them.

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[issue16353] add function to os module for getting path to default shell

[Andrew Svetlov]

Andrew Svetlov added the comment:

I guess to return sh if supported, cmd.exe for Windows.
The reason is: other shells can have different calling agreements (I mean rules 
to process command line).
subprocess intended to use by library writers, so we need solid well known 
basis.

There are another option — provide two functions: first for shell in terms of 
subprocess requirements and second will return actual user shell.
I like it.

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[issue16353] add function to os module for getting path to default shell

[Tim Golden]

Tim Golden added the comment:

On 02/11/2012 21:00, Andrew Svetlov wrote:
 I guess to return sh if supported, cmd.exe for Windows.

FWIW the canonical approach on Windows is to return whatever %COMSPEC% 
points to.

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[issue16353] add function to os module for getting path to default shell

[Chris Jerdonek]

Chris Jerdonek added the comment:

 Well, the question then becomes whether Popen() shouldn't use the user's 
 default shell instead? :)

That's a good question, too. :)  I was thinking just in terms of supporting the 
status quo.  Maybe two functions would be useful? (as suggested also by Andrew)

I do think the question of changing Popen()'s behavior should be decided 
independently of this issue though.  In other words, even if we change Popen() 
in the future, I think exposing the system default shell would still be 
worthwhile.

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[issue16353] add function to os module for getting path to default shell

[Gregory P. Smith]

Gregory P. Smith added the comment:

 That is, shouldn't we look up `pwd.getpwuid(os.getuid()).pw_shell` ?
 (but only when os.getuid() == os.geteuid()?)

No, you can't use the users shell from the pwd module.  That can be
any crazy program.  Not a functional /bin/sh for use in making
commands when shell=True in subprocess which is where this feature
request came from.

*system* default, not the user's.  subprocess MUST use /bin/sh or
equivalent, ignoring any per-user setting.

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[issue16353] add function to os module for getting path to default shell

[R. David Murray]

R. David Murray added the comment:

The system default shell (which should always be a /bin/sh workalike, I 
think) should always be the default.  Any other shell should be something that 
has to be specified explicitly.  At least, that's the way most posix programs 
work, I think.  As Chris said, different shells can use different syntax, so if 
you want to write portable code you want to be able to expect /bin/sh by 
default.

So at a minimum we want a function for the system default shell.  Anything else 
is a bonus :)

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[issue16353] add function to os module for getting path to default shell

[Andrew Svetlov]

Andrew Svetlov added the comment:

BTW, according to PEP 11 (http://www.python.org/dev/peps/pep-0011/) 
Python 3.4 will remove code for Windows systems where COMSPEC points to 
command.com. There are only winner: cmd.exe as well known shell good as 
default, command.com is gone.

cmd.exe can be assumed as default shell as well as sh for posix platforms.

About subprocess: I like to add something to point str (or pathlib path when 
accepted) as shell, but it's definitely another issue.

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[issue16353] add function to os module for getting path to default shell

[Chris Jerdonek]

Changes by Chris Jerdonek chris.jerdo...@gmail.com:


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[issue16353] add function to os module for getting path to default shell

[Chris Jerdonek]

New submission from Chris Jerdonek:

This issue is to add a function to the os module for getting the path to the 
default shell (e.g. os.getdefaultshell()).

In issue 16255, it was reported that on Android, the path to the default shell 
is /system/bin/sh rather than /bin/sh which it is on other Unix systems.

Such a function in the os module would implement the necessary detection logic 
which could then be used, for example, by the subprocess module when invoking 
Popen() with shell=True (e.g. for issue 16255).

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components: Library (Lib)
messages: 174093
nosy: chris.jerdonek
priority: normal
severity: normal
status: open
title: add function to os module for getting path to default shell
type: enhancement
versions: Python 3.4

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