When I ssh into a ubuntu machine, what kind of shell am I using

Question

I keep reading about interactive, non-interactive, login, and non-login shells.

(This is in the context of which of the .bash* files is read).

I don't understand what each type of shell is, so let's start with the basics.

If I ssh from my mac to my ubuntu machine, what type of shell is getting fired up?

Answer 1

If you SSH into your Ubuntu box, you're getting an interactive login shell. Here's the difference:

• Interactive vs. non-interactive: Any shell where you can type at a prompt is interactive. In fact, many scripts test for the variable $PS1 which holds the prompt string to find out whether they're interactive. If a shell is executing a shell script, it's non-interactive.

  So, if you do ssh yourbox.example.com, you'll get an interactive shell, asuming default settings, while if you do ssh yourbox.example.com mighty_shellscript.sh, you'll end up with a non-interactive shell and your SSH session will terminate when the script terminates.

• Login vs. non-login: When you log in from the console or remotely (such as SSH), or when you pass the -l option to bash, you get a login shell. Otherwise--such as when you open up a terminal window--you get a non-login shell.

  To test whether a shell is a login shell, check whether its command name is -bash instead of bash:

  ps -ef | grep [b]ash
  

Answer 2

You get an interactive login shell. But don't take it for granted, check it yourself.

This tells you that you have a login shell (from man bash):

# shopt | grep login
login_shell     on

This tells you that you have an interactive shell, look for the i (from man bash):

# echo $-
himBH

The interactive login shell you get has read /etc/profile and than one of ~/.bash_profile, ~/.bash_login and ~/.profile, as explained in 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 com‐ mands 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

The ssh(1) man page says,

If command is specified, it is executed on the remote host instead of a login shell.

and

When the user's identity has been accepted by the server, the server either executes the given command, or logs into the machine and gives the user a normal shell on the remote machine.

Those suggest to me that the shell you're getting is a login shell.

Answer 4

Try this command.

[max@localhost ~]$ echo $SHELL
/bin/bash

you are getting this output because your shell type is stored in this variable SHELL.

To know your environment variable type this command

[max@localhost ~]$ env
.
.
.
HOSTNAME=localhost.localdomain
SHELL=/bin/bash
HISTSIZE=1000
USER=max
.
.
.

while typing echo $SHELL it will print the value whatever stored here

This value gets updated for every bootup