Ubuntu – When I ssh into a ubuntu machine, what kind of shell am I using

The most common problem caused by misuse of shell startup files is that if you define environment variables in .bashrc as opposed to .profile, they are not defined in applications that are not launched from a shell in a terminal but rather from a GUI menu.

Defining environment variables from .bashrc is generally not a good idea, but that's primarily because it's generally useless as the variables should already have been set at login time. It can cause a problem in practice if you (or an application) have deliberately changed the value of a variable (e.g. PATH) in a program and you launch a shell from that program and expect the setting to have remained the same.

You can avoid the problem of environment variables being reset by defining a “sentinel” value, and not defining anything if the sentinel value is set:

if [ -z "$JMLANE_PROFILE_READ" ]; then
  export ...
  export JMLANE_PROFILE_READ=1
fi

Another problem caused by terminals starting login shells is that there are things that should only be done once when you log in, for example starting a password agent (e.g. ssh-agent), or initiating a session (e.g. running startx in certain circumstances). The sentinel variable avoids these issues.

A shell inside a terminal is always interactive. As long as you source .bashrc from .bash_profile when the shell is interactive, you don't need to worry about the terminal starting a login shell.

An additional wart of bash's startup file handling is that .bashrc is read by non-interactive shells invoked by rshd or sshd. For example, when you do rsync somefile host.example.com:, if your login shell on host.example.com is bash, you can use .bashrc there to set the path to include rsync. You can tell you're in this situation because .bashrc is read from a non-interactive shell.

## .bashrc
if [[ $- != *i* ]]; then
  # either .bashrc was executed explicitly or this is a noninteractive rsh/ssh session
fi

These are my thoughts on the different "types"of shells -- unfortunately I didn't witnessed the rise of the Un*x right from the beginning (I assume this concept is grown historically to a good extend), so please be critical.

• When I log into a system (nowadays via the graphical X login), there might be some tasks which should run once, e.g. establish a connection to some kind of server, present me a the today's todo list, autostart some commands, etc. This shouldn't happed every time I open up a new terminal. So there's a set of config files (/etc/profile, ~/.bash_login and so on, refer to the manual for an accurate list) sourced only by login shells.
• Accordingly, to close the connections, kill some programs, run a backup script there is ~/.bash_logout when the login shell exists.
• so, the "normal" shell I use in a terminal, shouldn't ba a login shell, but nevertheless should read my personal preferences from ~/.bashrc, because I want my keybindings to interact with the shell -- hence this is an interactive, non-login shell.
• and last, but not least when bash is used for scripting, neither of this is important. bash should start as fast as it can, i.e. shouldn't read any config files. This is a non-interactive, non-login shell.

So, my answer to your question What would we lose if we only had one type of shell? is in one word: "Flexibility."

The answer to your second question is easy:

$- lists the current set of options. These can either be set by command line parameters to bash or via the set builtin. So you have to look at two places in the manual:

• OPTIONS section:

  -i        If the -i option is present, the shell is interactive.
  

• SHELL BUILTIN COMMANDS section, subsection set:

  -h      Remember the location of commands as they are looked up for execution.  This is enabled by default.
  -m      Monitor  mode.  Job control is enabled.  This option is on by default for interactive shells on systems that sup‐
          port it (see JOB CONTROL above).  Background processes run in a separate process  group  and  a  line  containing
          their exit status is printed upon their completion.
  -B      The shell performs brace expansion (see Brace Expansion above).  This is on by default.
  -H      Enable !  style history substitution.  This option is on by default when the shell is interactive.