Question

I want to use Python to get the group id to a corresponding group name.
The routine must work for Unix-like OS (Linux and Mac OS X).

This is what I found so far

>>> import grp
>>> for g in grp.getgrall():
...     if g[0] == 'wurzel':
...         print g[2]

Answer 1

If you read the grp module documentation you’ll see that grp.getgrnam(groupname) will return one entry from the group database, which is a tuple-like object. You can either access the information by index or by attribute:

>>> import grp
>>> groupinfo = grp.getgrnam('root')
>>> print groupinfo[2]
0
>>> print groupinfo.gr_gid
0

Other entries are the name, the encrypted password (usually empty, if using a shadow file, it’ll be a dummy value) and all group member names. This works fine on any Unix system, including my Mac OS X laptop:

>>> import grp
>>> admin = grp.getgrnam('admin')
>>> admin
('admin', '*', 80, ['root', 'admin', 'mj'])
>>> admin.gr_name
'admin'
>>> admin.gr_gid
80
>>> admin.gr_mem
['root', 'admin', 'mj']

The module also offers a method to get entries by gid, and as you discovered, a method to loop over all entries in the database:

>>> grp.getgrgid(80)
('admin', '*', 80, ['root', 'admin', 'mj'])
>>> len(grp.getgrall())
73

Last but not least, python offers similar functionality to get information on the password and shadow files, in the pwd and spwd modules, which have a similar API.