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?
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: /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.
bash
as a non-interactive login shell?
– Piotr Dobrogost
Jun 16 '13 at 8:47
FOO
is an environment variable (i.e. .profile
contains export FOO=something
) then it's available to all subprocesses, including foo.sh
. If you change .profile
to export FOO=something_else
then ./foo.sh
will still print something
until the next time you log in.
– Gilles
Oct 6 '16 at 15:13
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 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."
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.
man bash
, with emphasis added, "A login shell is one whose first character of argument zero is a -, or one started with the --login option."
– Wildcard
Jan 23 '17 at 11:49
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.