Scripting¶

Transform your shell knowledge into reusable scripts. This section covers everything from basic script structure to advanced error handling and debugging.

Topics¶

Basics ¶

Script structure, the shebang line, variables, quoting rules, and making scripts executable.

Conditionals ¶

Decision making with if/else, test commands ([[ ]]), and case statements.

Loops ¶

Iteration with for, while, and until loops. Loop control with break and continue.

Arrays ¶

Working with indexed and associative arrays for managing collections of data.

Strings ¶

String manipulation, pattern matching, and parameter expansion techniques.

Arithmetic ¶

Mathematical operations using $(( )), let, and external tools like bc.

Input/Output ¶

Reading user input, formatted output with printf, and working with heredocs.

Error Handling ¶

Making scripts robust with set -e, trap, proper exit codes, and defensive programming.

Debugging ¶

Finding and fixing problems with set -x, PS4, ShellCheck, and systematic debugging techniques.

Script Template¶

Start new scripts with this template:

#!/usr/bin/env bash
#
# script-name - Brief description of what it does
#
# Usage: script-name [options] <arguments>
#

set -euo pipefail

# Constants
readonly SCRIPT_NAME="${0##*/}"
readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"

# Functions
usage() {
    cat << EOF
Usage: $SCRIPT_NAME [options] <argument>

Description of what the script does.

Options:
    -h, --help      Show this help message
    -v, --verbose   Enable verbose output

Examples:
    $SCRIPT_NAME file.txt
    $SCRIPT_NAME -v directory/
EOF
}

main() {
    # Parse arguments
    while [[ $# -gt 0 ]]; do
        case "$1" in
            -h|--help)
                usage
                exit 0
                ;;
            -v|--verbose)
                VERBOSE=true
                shift
                ;;
            *)
                break
                ;;
        esac
    done

    # Main logic here
    echo "Running $SCRIPT_NAME"
}

main "$@"

Learning Path¶

Work through these topics in order:

Basics - Understand script structure
Conditionals - Make decisions
Loops - Repeat operations
Strings - Manipulate text
Arithmetic - Perform calculations
Arrays - Handle collections
I/O - Interact with users and files
Error Handling - Make scripts robust
Debugging - Find and fix issues

Best Practices Preview¶

Throughout this section, you'll learn:

Always quote variables: "$var" not $var
Use [[ ]] instead of [ ] for tests
Start scripts with set -euo pipefail
Use functions to organize code
Handle errors explicitly
Test scripts with ShellCheck

Portability Notes¶

This tutorial focuses on bash. Some features are bash-specific:

Feature	Bash Version	POSIX Alternative
`[[ ]]`	All	`[ ]`
`$(( ))`	All	`expr`
Arrays	All	Not available
`${var,,}`	4.0+	`tr`
Associative arrays	4.0+	Not available
`mapfile`	4.0+	`while read` loop

For maximum portability, use POSIX sh syntax. For practical scripts on modern systems, use bash.