Basic Operations

Default Shell#

echo $SHELL

/bin/bash

If you start a new Shell in the current terminal (e.g., switching from Bash to Zsh), this command still displays the default login Shell, which is configured in /etc/passwd.

grep root /etc/passwd

root:x:0:0:root:/root:/bin/bash

On macOS, the default Shell information is typically stored in the user account settings, not in the /etc/passwd file. Starting from macOS Catalina (10.15), the default shell has been changed to zsh, previously the default Shell was bash. You can view the default shell using the following method.

dscl . -read /Users/$(whoami) UserShell

UserShell: /bin/zsh

Current Shell#

$0 in Shell returns the Shell name when running, and returns the script name/path when running in a script.

echo $0

-bash

The preceding hyphen - indicates that this Shell is the user’s login Shell.

You can also use the ps command to view the process status.

ps -p $$

    PID TTY          TIME CMD
  17216 pts/0    00:00:00 bash

If you start a new Shell in the current terminal (switching from Bash to Sh), the above two methods will display sh.

Supported Shells#

cat /etc/shells

# /etc/shells: valid login shells
/bin/sh
/bin/bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/usr/bin/sh
/bin/dash
/usr/bin/dash
/usr/bin/tmux
/usr/bin/screen

Change Default Shell#

The content in /etc/passwd will be updated after modification.

chsh#

Modifying the login Shell of the current user will require entering the user password.

chsh -s /bin/bash

Using the root user or sudo can also modify the login Shell of other users.

sudo chsh -s /bin/bash kuga

Note: If we enter a non-existent Shell.

chsh -s /bin/foo

chsh: /bin/foo is an invalid shell

It checks whether the entered Shell is in the /etc/shells file, preventing login failures due to entering an invalid Shell.

usermod#

Using this command requires the root user or a user with sudo privileges.

sudo usermod -s /bin/dash kuga

sudo usermod -s /bin/notexist kuga

The above command will not error, but will cause the kuga user to be unable to log in.

Add sudo Privileges#

View the list of members of the sudo group.

getent group sudo

Give the user sudo group privileges, which requires root execution.

usermod -aG sudo username

Do Not Edit Passwd#

If you accidentally write the configuration incorrectly, it is very likely to cause the entire system to be unable to log in.

Avoid Using Root#

I accidentally changed the login Shell of root to zsh, but Ubuntu did not have zsh installed, so root could not log in. Fortunately, I had another user with sudo privileges, so I could log in normally and successfully changed root’s Shell back. If you’re unlucky and don’t have a user with sudo privileges, it would be troublesome.

sudo chsh -s /bin/bash root