In Unix-based operating systems (including Linux distributions and macOS), the shell serves as an intermediary between users and the kernel. Commands typed at the keyboard are parsed by the shell and forwarded to the kernel for execution.

1 Unix Shell Variants

Modern Unix systems typically include multiple shell implementations:

$ cat /etc/shells
/bin/bash
/bin/csh
/bin/dash
/bin/ksh
/bin/sh
/bin/tcsh
/bin/zsh

sh: The original Unix shell, serving as the POSIX standard. Various implementations exist, including the Bourne shell.

bash: Bourne-Again Shell, an enhanced version of sh with features like command completion, history management, aliases, and job control. This was the default shell for most Linux distributions and macOS versions prior to Catalina.

dash: Debian Almquist Shell, a lightweight implementation derived from NetBSD. It offers faster startup times and smaller codebase compared to bash, though with fewer features. On Debian-based systems, dash typically serves as /bin/sh due to its stricter POSIX compliance and quicker initialization.

The relationship between /bin/sh and other shells varies by distribution. On many Linux systems, /bin/sh points to bash:

$ ls -l /bin/sh
lrwxrwx 1 root root 4 Oct 15  2017 /bin/sh -> bash

On Debian and derivatives, /bin/sh typically links to dash instead. To change the default sh interpreter, simply redirect the symlink:

$ ln -sf /bin/bash /bin/sh

For Docker images where you want sh to use bash, add this to you're Dockerfile:

RUN ln -sf /bin/bash /bin/sh

1.2 Switching Shells

To identify the current default shell:

$ echo $SHELL

To switch to a different installed shell:

$ chsh -s /bin/zsh

This change takes effect after restarting the terminal session.

1.3 Shell Execution Modes

Shells operate in two primary modes:

login shell: Initiated during user authentication. This mode reads system-wide configuration files (such as /etc/profile) and user-specific files (like ~/.bash_profile) to establish the environment.

non-login shell: Spawned after authentication, typically when opening a new terminal window. These inherit settings from the parent login shell and handle interactive tasks within the session.

Direct terminal login (tty1-6) triggers login shell initialization, while launching a new terminal within an existing session creates a non-login shell.

2 Configuration Files

Shell environments are configured through various initialization files.

2.1 bash

bash configuration hierarchy includes:

• /etc/profile、/etc/profile.d/*.sh
• /etc/bashrc
• /.bash_profile、/.bash_login、~/.profile
• ~/.bashrc
• ~/.bash_logout

Scope: Files under /etc/ apply globally, while those in the user's home directory affect only that user.

Function: Profile-type files initialize login shells, while bashrc files configure non-login shells.

login shell initialization sequence:

1. System-wide setup: /etc/profile loads /etc/profile.d/*.sh
2. User-specific setup: One file from [~/.bash_profile, ~/.bash_login, ~/.profile] is selected in order of priority, with execution stopping upon finding the first match
3. Cleanup: ~/.bash_logout runs upon shell exit

non-login shell initialization: Reads ~/.bashrc exclusively, falling back to /etc/bashrc if needed.

Q&A

Q1: Why does ~/.bash_profile typically source ~/.bashrc?

A1: The typical pattern in ~/.bash_profile includes conditional sourcing logic to load the bashrc file when present.

Q2: Why does ~/.bashrc reference /etc/bashrc?

A2: The standard ~/.bashrc implementation contains conditional code to load the system-wide bashrc configuration.

Q3: What is ~/.bash_history?

A3: This file persists command history across sessions. Commands executed during the current session remain in memory until logout, at which point they're written to the history file.

2.2 zsh

Starting with macOS Catalina, zsh became the default shell on Apple systems.

zsh configuration loads in this sequence:

1. /etc/zshenv、~/.zshenv
2. /etc/zprofile、~/.zprofile
3. /etc/zshrc、~/.zshrc
4. /etc/zlogin、~/.zlogin
5. ~/.zlogout、/etc/zlogout

Similar to bash, ~/.zshrc initializes for each new terminal, while ~/.zshenv runs whenever zsh starts. If the same variable appears in both files, ~/.zshrc takes precedence.

The zprofile and zlogin files serve similar purposes for login shells, though they originate from different shell traditions. zprofile evolved from bash's profile approach, while zlogin came from csh. Since older macOS versions used bash as default, ~/.zprofile is the recommended choice.

Unlike bash, zsh ignores /etc/profile and ~/.bash_profile entirely. To import existing bash configurations, add the following to ~/.zshrc:

source ~/.bash_profile