Define reusable code blocks with the function keyword or parentheses syntax. Functions reduce code duplication and improve readability.

# Both syntaxes are equivalent
function greet() {
    echo "Hello, $1!"
}
 
greet_alt() {
    echo "Hello, $1!"
}
 
# Call the function
greet "Alice"

Return a status code (0-255) with the return statement. The exit code is stored in $? and used by conditionals.

check_file() {
    if [[ -f "$1" ]]; then
        echo "$1 exists"
        return 0
    else
        echo "$1 not found"
        return 1
    fi
}
 
check_file "/etc/passwd" && echo "Success" || echo "Failed"

Access positional parameters inside a function with $1, $2, etc. $0 is the script name, not the first argument.

process_files() {
    echo "Function name: $0"
    echo "First file: $1"
    echo "Second file: $2"
    echo "Total arguments: $#"
}
 
process_files "file1.txt" "file2.txt"

Use $@ to pass all arguments as separate words (expands each one), and $* to pass as a single string. $# gives the argument count.

# Iterate over all arguments
sum_args() {
    local total=0
    for arg in "$@"; do
        total=$((total + arg))
    done
    echo $total
}
 
sum_args 1 2 3 4 5  # Output: 15
 
# Note: "$@" is preferred over "$*" to preserve word boundaries

Shell provides automatic variables for introspection and control flow.

• $$ — Process ID of the current shell
• $! — Process ID of the last background job
• $? — Exit code of the last executed command
• $- — Current shell options (flags like e, u, x)

# Example: Check exit code, get last background PID
ls /nonexistent 2>/dev/null
echo "Exit code: $?"  # Output: Exit code: 2
 
sleep 10 &
echo "Background job PID: $!"
 
# Example: Check if error handling is enabled
bash -e -c 'echo $-'  # Includes 'e' for set -e

Variables are global by default in bash. Use local inside functions to create function-scoped variables.

x=10  # Global variable
 
increment() {
    x=$((x + 1))  # Modifies global x
    local y=5     # Local to this function
    echo "In function: x=$x, y=$y"
}
 
increment
echo "After function: x=$x"  # x is 11
# echo $y  # Error: y is undefined

Environment variables (uppercase by convention) are inherited by child processes. Export them with export.

# Define local variable (not inherited)
DB_USER="admin"
 
# Export to make it available to child processes
export DB_PASS="secret123"
 
# Child process can read DB_PASS
python script.py  # Script sees DB_PASS in os.environ
 
# Child process cannot read DB_USER
# python -c 'import os; print(os.getenv("DB_USER"))'  # Returns None