20. Command Grouping in Shell Scripting

Command grouping is a powerful technique in shell scripting that allows you to execute multiple commands as a single unit. This is particularly useful when you need to redirect the combined output of several commands to a single file or apply other shell operations to a sequence of commands.

Without command grouping, you would typically need to redirect the output of each command individually, which can make your scripts more verbose and harder to manage. Command grouping simplifies this process.

20.1. Executing Commands in a Subshell with Parentheses

You can group a list of commands to be executed within a subshell by enclosing them in parentheses (). When commands are run in a subshell, they operate in a separate environment, and any changes made to variables or the current directory within the subshell do not affect the parent shell.

Example: To run the date and uptime commands and redirect all their output to a file named file.txt:

(date; uptime) > file.txt

20.2. Executing Commands in the Current Shell with Curly Braces

Alternatively, you can group commands to be executed in the current shell context using curly braces {}. Unlike parentheses, curly braces do not create a subshell. This means that any modifications to the environment within the grouped commands will persist in the current shell.

Important: When using curly braces, there must be a space after the opening brace and before the closing brace, and the commands within must be separated by newlines or semicolons. The closing brace must also be followed by a semicolon or a newline.

Example: To achieve the same result as the parentheses example, but executing in the current shell:

{date; uptime;} > file.txt

This method is efficient as it avoids the overhead of creating a new subshell.

Benefits of Command Grouping

• Simplified Redirection: Easily redirect the combined output of multiple commands.
• Code Readability: Organizes related commands into logical blocks.
• Environment Control: Choose between subshell execution (parentheses) for isolation or current shell execution (curly braces) for persistent changes.

Understanding and utilizing command grouping is essential for writing more efficient and maintainable shell scripts.