I understand the basic difference between an interactive shell and a non-interactive shell. But what exactly differentiates a login shell from a non-login shell?
Can you give examples for uses of a non-login interactive shell?
A login shell is the first process that executes under your user ID when you log in for an interactive session. The login process tells the shell to behave as a login shell with a convention: passing argument 0, which is normally the name of the shell executable, with a
- character prepended (e.g.
-bash whereas it would normally be
bash. Login shells typically read a file that does things like setting environment variables:
~/.profile for the traditional Bourne shell,
~/.bash_profile additionally for bash†,
~/.zprofile for zsh†,
~/.login for csh, etc.
When you log in on a text console, or through SSH, or with
su -, you get an interactive login shell. When you log in in graphical mode (on an X display manager), you don't get a login shell, instead you get a session manager or a window manager.
It's rare to run a non-interactive login shell, but some X settings do that when you log in with a display manager, so as to arrange to read the profile files. Other settings (this depends on the distribution and on the display manager) read
~/.profile explicitly, or don't read them. Another way to get a non-interactive login shell is to log in remotely with a command passed through standard input which is not a terminal, e.g.
ssh example.com <my-script-which-is-stored-locally (as opposed to
ssh example.com my-script-which-is-on-the-remote-machine, which runs a non-interactive, non-login shell).
When you start a shell in a terminal in an existing session (screen, X terminal, Emacs terminal buffer, a shell inside another, etc.), you get an interactive, non-login shell. That shell might read a shell configuration file (
~/.bashrc for bash invoked as
~/.zshrc for zsh,
~/.cshrc for csh, the file indicated by the
ENV variable for POSIX/XSI-compliant shells such as dash, ksh, and bash when invoked as
$ENV if set and
~/.mkshrc for mksh, etc.).
When a shell runs a script or a command passed on its command line, it's a non-interactive, non-login shell. Such shells run all the time: it's very common that when a program calls another program, it really runs a tiny script in a shell to invoke that other program. Some shells read a startup file in this case (bash runs the file indicated by the
BASH_ENV variable, zsh runs
~/.zshenv), but this is risky: the shell can be invoked in all sorts of contexts, and there's hardly anything you can do that might not break something.
† I'm simplifying a little, see the manual for the gory details.
To tell if you are in a login shell:
prompt> echo $0 -bash # "-" is the first character. Therefore, this is a login shell. prompt> echo $0 bash # "-" is NOT the first character. This is NOT a login shell.
Information can be found in
man bash (search for Invocation). Here is an excerpt:
A login shell is one whose first character of argument zero is a -, or one started with the --login option.
You can test this yourself. Anytime you SSH, you are using a login shell. For Example:
prompt> ssh user@localhost fervor@localhost's password: prompt> echo $0 -bash
The importance of using a login shell is the any settings in
/home/user/.bash_profile will get executed. Here is a little more information if you are interested (from
"When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and
executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for
~/.profile, in that order, and reads and executes commands from the first one that exists and is readable.
The --noprofile option may be used when the shell is started to inhibit this behavior."
In a login shell,
argv == '-'.
This is how it knows it's a login shell.
And then in some situations it behaves differently depending on its "login shell" status. E.g. a shell, that is not a login shell, would not execute a "logout" command.
A shell started in a new terminal in a GUI would be an interactive non-login shell. It would source your .bashrc, but not your .profile, for example.