Difference between Login Shell and Non-Login Shell?

Question

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?

Answer 1

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: /etc/profile and ~/.profile for the traditional Bourne shell, ~/.bash_profile additionally for bash^†, /etc/zprofile and ~/.zprofile for zsh^†, /etc/csh.login and ~/.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 /etc/profile and ~/.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 bash, /etc/zshrc and ~/.zshrc for zsh, /etc/csh.cshrc and ~/.cshrc for csh, the file indicated by the ENV variable for POSIX/XSI-compliant shells such as dash, ksh, and bash when invoked as sh, $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 /etc/zshenv and ~/.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.

Answer 2

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.

In Bash, you can also use shopt login_shell:

prompt> shopt login_shell
login_shell     off

(or on in 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
user@localhost's password:
prompt> echo $0
-bash

The importance of using a login shell is that any settings in /home/user/.bash_profile will get executed. Here is a little more information if you are interested (from man bash)

"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 ~/.bash_profile, ~/.bash_login, and ~/.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."

Answer 3

In a login shell, argv[0][0] == '-'. 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.

Answer 4

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.

Answer 5

I'll elaborate on the great answer by Gilles, combined with Timothy's method for checking login shell type.

If you like to see things for yourself, try the snippets and scenarios bellow.

Checking whether shell is (non-)interactive

if tty -s; then echo 'This is interactive shell.'; else echo 'This is non-interactive shell.'; fi

Checking whether shell is (non-)login

If output of echo $0 starts with -, it's login shell (echo $0 output example: -bash). Otherwise it's non-login shell (echo $0 output example: bash).

if echo $0 | grep -e ^\- 2>&1>/dev/null; then echo "This is login shell."; else echo "This is non-login shell."; fi;

Let's combine the two above together to get both pieces of information at once:

THIS_SHELL_INTERACTIVE_TYPE='non-interactive'; 
THIS_SHELL_LOGIN_TYPE='non-login'; 
if tty -s; then THIS_SHELL_INTERACTIVE_TYPE='interactive'; fi; 
if echo $0 | grep -e ^\- 2>&1>/dev/null; then THIS_SHELL_LOGIN_TYPE='login'; fi;
echo "$THIS_SHELL_INTERACTIVE_TYPE/$THIS_SHELL_LOGIN_TYPE"

Scenarios:

Typical SSH session without special options

ssh ubuntu@34.247.105.87
Welcome to Ubuntu 16.04.5 LTS (GNU/Linux 4.4.0-1083-aws x86_64)

ubuntu@ip-172-31-0-70:~$ THIS_SHELL_INTERACTIVE_TYPE='non-interactive';
ubuntu@ip-172-31-0-70:~$ THIS_SHELL_LOGIN_TYPE='non-login';
ubuntu@ip-172-31-0-70:~$ if tty -s; then THIS_SHELL_INTERACTIVE_TYPE='interactive'; fi;
ubuntu@ip-172-31-0-70:~$ if echo $0 | grep -e ^\- 2>&1>/dev/null; then THIS_SHELL_LOGIN_TYPE='login'; fi;
ubuntu@ip-172-31-0-70:~$ echo "$THIS_SHELL_INTERACTIVE_TYPE/$THIS_SHELL_LOGIN_TYPE"

interactive/login

Running script or executing explicitly via new shell

ubuntu@ip-172-31-0-70:~$  bash -c 'THIS_SHELL_INTERACTIVE_TYPE='non-interactive'; THIS_SHELL_LOGIN_TYPE='non-login'; if tty -s; then THIS_SHELL_INTERACTIVE_TYPE='interactive'; fi; if echo $0 | grep -e ^\- 2>&1>/dev/null; then THIS_SHELL_LOGIN_TYPE='login'; fi; 
echo "$THIS_SHELL_INTERACTIVE_TYPE/$THIS_SHELL_LOGIN_TYPE"'

interactive/non-login

Running local script remotely

ssh ubuntu@34.247.105.87 < checkmy.sh
Pseudo-terminal will not be allocated because stdin is not a terminal.
Welcome to Ubuntu 16.04.5 LTS (GNU/Linux 4.4.0-1083-aws x86_64)

non-interactive/login

Running a command over ssh remotely

ssh ubuntu@34.247.105.87 'THIS_SHELL_INTERACTIVE_TYPE='non-interactive'; THIS_SHELL_LOGIN_TYPE='non-login'; if tty -s; then THIS_SHELL_INTERACTIVE_TYPE='interactive'; fi; if echo $0 | grep -e ^\- 2>&1>/dev/null; then THIS_SHELL_LOGIN_TYPE='login'; fi; echo "$THIS_SHELL_INTERACTIVE_TYPE/$THIS_SHELL_LOGIN_TYPE"'

non-interactive/non-login

Running a command over ssh remotely with -t switch

You can explicitly request interactive shell when you want to run command remotely via ssh by using -t switch.

ssh ubuntu@34.247.105.87 -t 'THIS_SHELL_INTERACTIVE_TYPE='non-interactive'; THIS_SHELL_LOGIN_TYPE='non-login'; if tty -s; then THIS_SHELL_INTERACTIVE_TYPE='interactive'; fi; if echo $0 | grep -e ^\- 2>&1>/dev/null; then THIS_SHELL_LOGIN_TYPE='login'; fi; echo "$THIS_SHELL_INTERACTIVE_TYPE/$THIS_SHELL_LOGIN_TYPE"'

interactive/non-login

Note: On topic why running command remotely is not login shell more info here.