🧱 Extended Functions and Libraries

Functions encapsulate reusable logic. Libraries organize shared code across multiple scripts. This section covers both in depth.

🧭 Defining Functions

Two syntaxes (both equivalent):

# Style 1: function keyword
function greet {
    echo "Hello, $1!"
}

# Style 2: Parentheses (preferred in Bash)
greet() {
    echo "Hello, $1!"
}

Call the function:

greet "Alice"   # Outputs: Hello, Alice!

---

🧪 Passing Arguments

Inside functions, positional parameters are local:

show_args() {
    echo "Script name: $0"
    echo "Function name: $FUNCNAME"   # Bash/Zsh
    echo "First arg: $1"
    echo "All args: $@"
    echo "Arg count: $#"
}

show_args hello world "foo bar"

Output:

Script name: ./script.sh
Function name: show_args
First arg: hello
All args: hello world foo bar
Arg count: 3

---

🧠 Returning Values

Shell functions don't return values like other languages. Instead:

Method 1: Echo Output (Most Common)

get_current_date() {
    date +%Y-%m-%d
}

today=$(get_current_date)
echo "Today is $today"

Method 2: Global Variable

calculate_total() {
    total=$(( $1 + $2 ))
}

calculate_total 10 20
echo "Total: $total"   # 30

⚠️ Modifies global scope — use carefully.

Method 3: Exit Code

validate_input() {
    [ -z "$1" ] && return 1
    return 0
}

if validate_input "$user_input"; then
    echo "Valid"
else
    echo "Invalid"
fi

---

🧪 Local Variables

Prevent pollution of global scope:

process_file() {
    local filename="$1"
    local temp_dir
    temp_dir=$(mktemp -d)

    # Do work with $temp_dir
    cp "$filename" "$temp_dir/"

    # Cleanup
    rm -rf "$temp_dir"
}

local ensures variables don't leak outside the function.

---

🧠 Organizing Libraries

Simple Library File

lib/utils.sh

#!/bin/sh

log_msg() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" >&2
}

die() {
    log_msg "ERROR: $*"
    exit 1
}

require_file() {
    [ -f "$1" ] || die "Required file not found: $1"
}

Including Libraries

#!/bin/sh

# Determine script directory
SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"

# Source library
. "$SCRIPT_DIR/lib/utils.sh"

# Use functions
log_msg "Starting process..."
require_file config.yml
log_msg "Config found"

The . command (or source) loads functions into current shell context.

---

🧪 Advanced Library Patterns

Namespaced Functions

Avoid name collisions:

db_connect() {
    echo "Connecting to database..."
}

db_disconnect() {
    echo "Disconnecting from database..."
}

http_connect() {
    echo "Connecting to HTTP..."
}

Prefix with module name for clarity.

Conditional Loading

Load only if not already defined:

if ! command -v log_msg >/dev/null 2>&1; then
    . ./lib/logging.sh
fi

Or check function existence:

if ! declare -f log_msg >/dev/null; then
    . ./lib/logging.sh
fi

---

🧠 Recursive Functions

Bash supports recursion (enable with shopt -s extdebug in some cases):

factorial() {
    local n=$1
    if [ $n -le 1 ]; then
        echo 1
    else
        local prev=$(factorial $((n - 1)))
        echo $((n * prev))
    fi
}

echo "5! = $(factorial 5)"   # 120

⚠️ Deep recursion may hit stack limits — use iteration for large inputs.

---

🧪 Anonymous Functions (Bash 4.4+)

Define inline functions:

map() {
    local fn=$1
    shift
    for item; do
        "$fn" "$item"
    done
}

map 'echo "Processing: $1"' apple banana cherry

---

🧾 Portability Notes

Feature	POSIX	Bash	Zsh
Function definition	✅	✅	✅
local variables	✅	✅	✅
$FUNCNAME	❌	✅	✅
Recursion	⚠️	✅	✅
Anonymous functions	❌	✅	❌
Arrays as args	❌	✅	✅

---

🧾 Summary

• Use functions to avoid code duplication.
• Always use local for variables inside functions.
• Return values via stdout or exit codes.
• Organize shared code in library files.
• Source libraries with . or source.
• Prefix function names to avoid collisions.

👉 Continue to: Error Handling