changing shells

i don't know if this a fedora-specific thing or a newer linux thing, but in the past changing your default shell was as simple as editing the /etc/passwd file. today, there is the chsh which can perform this for you. there is a catch, however. the shell must exist in the /etc/shells file to be allowed to be changed.

to make this work, i needed to add my shell to the /etc/shells file. then i issued the chsh to make my shell change.

$ chsh -l
/bin/sh
/bin/bash
/usr/bin/sh
/usr/bin/bash
/usr/bin/tmux
/bin/tmux
/usr/bin/xonsh

$ sudo chsh -s /usr/bin/xonsh mock

Changing shell for mock.

Shell changed.

my previous post about xonsh is what led me here. i had manually changed the shell directly in the /etc/passwd file, but then my gnome session did not recognize my user. this was the strange side effect of changing the default shell not on the /etc/shells list. gnome (gdm) started the gnome-initial-setup process which runs when gnome is freshly installed.

and thing are still messed up. the gnome settings > users section shows no one, even though i am logged in under my account. (figuring out how to fix this will be another lesson for a future time.)

xonsh

there was small thread on the knoxdev's slack account about shells the other day. as i was trying to continue the discussion cleverly using the shell names as regular words in sentences, i did a quick search for available shells from my package manager (i.e. dnf search shell). as i was looking through the results, i found xonsh.

the description listed in my package manager tells me this is what it is:

Description  : xonsh is a Python-ish, BASHwards-compatible shell language and
             : command prompt. The language is a superset of Python 3.4 with
             : additional shell primitives. xonsh (pronounced *conch*) is
             : meant for the daily use of experts and novices alike.

i have toyed around with the idea of using python's interactive shell as my main terminal shell for some time, but i always felt it would be too hard to do some of the simple things, such as file management. sure, i could write equivalent scripts to do those tasks, but i never made the effort.

this accomplishes exactly what i wanted to do. with xonsh, i can do this:

~ $ ls -l gPodder/

total 124

-rw-r--r--. 1 mock mock 90112 Feb 19  2018 Database

drwxrwxr-x. 3 mock mock  4096 Feb 12  2018 Downloads

drwxrwxr-x. 2 mock mock  4096 Feb 19  2018 Logs

-rw-rw-r--. 1 mock mock  2715 Feb 19  2018 Settings.json

-rw-r--r--. 1 mock mock 20480 Feb 19  2018 gpodder.net

~ $ from datetime import datetime

~ $ datetime.now()

datetime.datetime(2018, 10, 23, 8, 35, 50, 863981)

there is much more to xonsh than just this. the full documentation can be found at http://xon.sh.

python string concatenation is a bad practice

i think because python is a scripting language, people tend to thing it's super fine to do something like this:

>>> val + '-string'

the problem is that the + operator does not do any casting of the variable to make it compatible with the string. this sort of thing will throw an error if the type of val is anything other than a string.

check it:

>>> val = None

>>> val + '-string'

Traceback (most recent call last):

  File "<input>", line 1, in <module>

    val + '-string'

TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

>>> '{}-string'.format(val)

'None-string'

>>> val = 4

>>> val + '-string'

Traceback (most recent call last):

  File "<input>", line 1, in <module>

    val + '-string'

TypeError: unsupported operand type(s) for +: 'int' and 'str'

>>> '{}-string'.format(val)

'4-string'

now, you could force cast it using str(val), but that seems silly when you can just use the format() method on the string object to handle things for you.

python is referential

this is often misunderstood about python, especially if you are accustomed to other languages, such as C/C++.

>>> class thing(object):
...     def __init__(self, some):
...         self.some = some
...     def add_on(self, addition):
...         self.some = self.some + addition
... 
>>> a = thing("hello")
>>> print(a.some)
hello

>>> b = a
>>> print(b)
<__main__.thing object at 0x7f9d267cf7f0>
>>> print(a)
<__main__.thing object at 0x7f9d267cf7f0>
>>> print(b.some)
hello

>>> a.add_on(", world")
>>> print(a.some)
hello, world
>>> print(b.some)
hello, world

>>> print(a)
<__main__.thing object at 0x7f9d267cf7f0>
>>> print(b)
<__main__.thing object at 0x7f9d267cf7f0>