Commands

This section summarizes the syntax of commands in the shell language. Commands (in the broad sense) are instructions to the shell to perform actions such as running programs, changing the environment, or controlling execution flow. For details, see the linked sections below.

Whole scripts

A shell script consists of a sequence of lists separated by newlines. The shell reads and parses input line by line until it forms a complete list, executes that list, then continues to the next.

$ echo "Hello, World!"
Hello, World!
$ for fruit in apple banana cherry; do
>     echo "I like $fruit"
> done
I like apple
I like banana
I like cherry

Lists

A list is a sequence of and-or lists separated by ; or &. Lists let you write multiple commands on one line or run commands asynchronously.

$ echo "Hello"; echo "World"
Hello
World

And-or lists

An and-or list is a sequence of pipelines separated by && or ||. This lets you control execution flow based on the success or failure of previous commands.

$ test -f /nonexistent/file && echo "File exists" || echo "File does not exist"
File does not exist

Pipelines

A pipeline is a sequence of commands connected by |, where the output of one command is passed as input to the next. Pipelines let you combine commands to process data in a stream.

You can prefix a pipeline with the ! reserved word to negate its exit status:

$ ! tail file.txt | grep TODO
TODO: Fix this issue

Commands

A command (in the narrow sense) is a pipeline component: a simple command, a compound command, or a function definition.

A simple command runs a utility or function, or assigns values to variables.

Compound commands control execution flow and include:

• Grouping commands: Group multiple commands to run as a unit, in the current shell or a subshell.
• If commands: Run commands conditionally based on exit status.
• Case commands: Run commands based on pattern matching a value.
• For loops: Iterate over a list, running commands for each item.
• While loops: Repeat commands while a condition is true.
• Until loops: Repeat commands until a condition becomes true.

A function definition creates a reusable block of code that can be invoked by name.