shell - is a program that provides text-only user interface for Unix-like operating systems (it allows you to interact with the system). Its primary function is to read commands that are typed into a console or terminal window and then execute them.
Interactive shell as the name implies allows a user to inteact with it. This type of shell displays a prompt for user to enter commands and can ask you to enter additional input if it's required by the command you run.
You work with interactive shells most of your time. For example, you get an interactive shell when you're logging in to a system via SSH or just open a terminal on your local machine. Rule of thumb: if you see a command prompt - it's an interactive shell.
A non-interactive shell doesn't allow user interaction. You get this type of shell when you run a script. All scripts run in their own subshell (a child process of the parent shell) and this shell closes immediately once the script is executed.
A login shell is started after a successful login to the system. Login is a process of getting access to a system which usually requires you to specify a user name whose permissions you want to assume and some credentials such as a password or a secret key. Thus, whenever you have to login into a system (e.g., when you try to login to a remote system via SSH) you'll get a login shell after successful login.
There are also ways to start a login shell when already logged in. Each of the following commands will launch a login shell:
$ su - [username]
$ su -l [username]
$ su --login [username]
$ sudo -i
You can check if the current shell you're running is a login shell by using the following command:
$ echo $0
-bashIf the output is prepended with a dash (e.g. -bash, -su), it indicates that the current shell is a login shell. A non-login shell won't have a - prefix.
A non-login shell is a shell that you can start without login. For example, opening a new terminal window on you local machine launches a non-login shell, since it doesn't require you to type a user name or password because you're already logged into the system.
You can start a non-login shell, when you're already logged into a system, by calling a shell executable, for example:
$ ssh [email protected] # login to a system
$ echo $0 # check my current shell
-bash # it's a login shell
$ bash # start a non-login shell
$ echo $0 # check my current shell
bash # it's a non-login shell
$ exit # exit a subshell
$ echo $0 # check my current shell
-bash # I returned to my login shellAll scripts run in their own subshell which are most of the times non-login shells (check -l option for bash for login shells).
su command without -, -l or --login also launches a non-login shell.
The real difference between login and non-login shells lies in what startup files they call to configure the shell environment.
When bash is invoked as a login shell 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.
Creating a login shell with su or sudo -i command will also move you to the home directory of a new user.
When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists.
When a non-login shell is started, bash reads and executes commands from ~/.bashrc, if that file exists.
Typically, your ~/.bash_profile contains the line
if [ -f ~/.bashrc ]; then . ~/.bashrc; fi
to source ~/.bashrc for login shells too.
A non-interactive shell is usually present when a shell script is running. It is non-interactive because it is processing a script and not waiting for user input between commands. For these shell invocations, only the environment inherited from the parent shell is used.
/etc/profile- System wide environment variables and startup programs./etc/bashrc- System wide aliases and functions~/.bash_profile- User personal environment variables and startup programs~/.bashrc- User personal aliases and functions