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?
loginshell
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?
Best Answer
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 bebash
. 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 tossh 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 asbash
,/etc/zshrc
and~/.zshrc
for zsh,/etc/csh.cshrc
and~/.cshrc
for csh, the file indicated by theENV
variable for POSIX/XSI-compliant shells such as dash, ksh, and bash when invoked assh
,$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.