What Is a Shell?

A shell is a program that lets you control a Unix system by typing commands. Common shells include Bash, Zsh, Fish, and Dash.

When you open a terminal, you usually see a terminal emulator running a shell:

terminal emulator → PTY → shell → programs

The terminal emulator draws the screen and sends your keystrokes. The PTY gives the shell a terminal-shaped connection. The shell reads your command, starts the requested program, and prints the result.

A shell reads commands

When you type:

ls -la

the shell:

1. reads the line;
2. splits it into a command and arguments;
3. finds the ls program;
4. starts it as a child process;
5. waits for it to finish;
6. prints another prompt.

The shell does not usually implement ls itself. It starts the ls program and connects that program to the terminal.

A shell connects programs

The shell also provides syntax for composing programs:

cat access.log | grep 500 > errors.txt

Here the shell:

• starts cat and grep;
• connects cat's output to grep's input with a pipe;
• writes grep's output to errors.txt;
• manages the whole pipeline as one job.

This is why small Unix programs can do useful work together. The shell acts as the glue between them.

Shell features

A shell commonly provides:

• commands, such as cd, export, and alias;
• program launching, such as git status;
• pipes, using |;
• redirection, using <, >, and >>;
• variables, such as $HOME;
• globbing, such as *.log;
• conditionals and loops;
• scripts, which are saved sequences of shell commands;
• job control, including &, fg, bg, and Ctrl-Z.

Some commands are builtins. cd is the classic example. A child process cannot change the shell's current directory, so the shell must implement cd itself.

Other commands are separate executable files. You can often find one with:

command -v git

Shell versus terminal

These are different programs:

• the terminal emulator displays text and sends input;
• the PTY provides the terminal-shaped connection;
• the shell reads commands and starts programs;
• the program does the work requested by the command.

For example, when you run vim in a terminal, vim is not part of the shell. The shell starts it, and the PTY connects it to the terminal emulator.

A shell is therefore best understood as a command interpreter and process launcher. It is the layer that turns typed text into running programs and connected streams.