Shell

Core Idea

A shell is a command interpreter. It reads a line of text, expands shell syntax, starts programs, connects streams, and reports success or failure. Bash is the shell used for this course.

Command Anatomy

grep -R makers ~/src/pages > ~/playground/matches.txt 2>/dev/null

• grep is the command name.
• -R is an option.
• makers and ~/src/pages are arguments.
• > redirects stdout.
• 2> redirects stderr.
• The shell expands ~ before grep starts.

What The Shell Does Before A Program Runs

1. Reads your line.
2. Splits it into words using shell rules.
3. Expands variables such as $USER.
4. Expands ~ and some patterns.
5. Sets up pipes and redirection.
6. Starts the command.
7. Stores the exit status in $?.

Why Quoting Matters

name='Ada Lovelace'
printf '%s\n' $name
printf '%s\n' "$name"

Unquoted variables can split into multiple words. Quoted variables preserve one value. This is why scripts in the course use "$1", "$HOME", and "$name".

Interactive Shell Versus Script

An interactive shell prints a prompt and waits for you. A script is a file of shell commands run by a shell. The same syntax appears in both, but scripts need more discipline: shebang, set -euo pipefail, argument checks, and predictable output.

Common Confusions

• The shell expands $HOME; ls receives the expanded path.
• cd is a shell builtin because it changes the shell’s own current directory.
• man cd may not help because some commands are shell builtins; use help cd in Bash.
• A command’s output is not the same as its exit status.
• sudo is not a shell feature and is not part of ordinary learner work on the shared server.

Proof Check

Run type cd, type grep, and type printf. Explain which are shell builtins and which are external commands on this system.

Docs Pointers

• Run man bash, then read SHELL GRAMMAR, QUOTING, REDIRECTION, and Pipelines.
• Run help, help cd, help type, and help set.
• Read the Bash Reference Manual.
• Read Terminal to separate the shell from the terminal app.
• Read Readline for Bash prompt movement, editing, completion, and history search keystrokes.
• Read I/O, File Descriptor, and Signal to understand streams, redirection, and interrupts.