Shell language

The shell interprets input as commands written in the shell language. The language has a syntax (how commands are written and structured) and semantics (how commands are executed). This page gives a brief overview of shell commands.

Simple commands

A simple command is the most basic command type. It consists of a sequence of words that are not reserved words or operators. For example:

ls

or:

echo "Hello, world!"

Most simple commands run a utility—a program that performs a specific task. The first word is the utility name; the rest are arguments.

All words (except redirection operators) in a simple command are expanded before the utility runs. See Words, tokens, and fields for details on parsing and expansion.

You can use parameters to change command behavior dynamically. There are three types: variables, special parameters, and positional parameters.

See Simple commands for more on assignments, redirections, and command search.

Other commands

Other command types construct more complex behavior by combining commands. See Commands for the full list. For example:

• Compound commands group commands, control execution, and handle conditions and loops. Examples: if, for, while, case.
• Pipelines connect the output of one command to the input of another, letting you chain commands.
• And-or lists control execution flow based on command success or failure.
• Lists let you run multiple commands in sequence or in parallel.

Functions

Functions are reusable blocks of code you can define and call in the shell. They help organize scripts and interactive sessions.

Redirections

Redirections control where command input and output go. Use them to save output to files or read input from files. See the File descriptors and redirections section for more.

Aliases

Aliases are shortcuts for longer commands or command sequences. They let you create custom names for commands you use often. See the Aliases section for details.