Idempotent Scripts

# NOT idempotent — runs every time
echo "new line" >> config.txt   # Adds duplicate line on re-run
mkdir /data                     # Fails on second run
git clone repo /project         # Fails on second run

# Idempotent — safe to re-run
grep -q "new line" config.txt || echo "new line" >> config.txt
mkdir -p /data                  # No error if exists
git clone repo /project 2>/dev/null || true  # Ignore if exists

# Check if user exists before creating
if ! id "$USERNAME" &>/dev/null; then
    sudo useradd "$USERNAME"
fi

# Check if service is installed before configuring
if systemctl is-enabled "$SERVICE" &>/dev/null; then
    sudo systemctl restart "$SERVICE"
fi

# Check if package is installed
if ! dpkg -l "$PACKAGE" &>/dev/null 2>&1; then
    sudo apt-get install -y "$PACKAGE"
fi

# Check if file exists before copying
if [[ ! -f "$DEST" ]] || ! diff -q "$SRC" "$DEST" &>/dev/null; then
    cp "$SRC" "$DEST"
    echo "Updated $DEST"
else
    echo "$DEST is up to date"
fi

#!/usr/bin/env bash
set -euo pipefail

STATE_DIR="/var/lib/myscript"
STATE_FILE="$STATE_DIR/last_run"

ensure_state_dir() {
    mkdir -p "$STATE_DIR"
}

already_run() {
    [[ -f "$STATE_FILE" ]] && [[ "$(cat "$STATE_FILE")" == "$1" ]]
}

mark_complete() {
    echo "$1" > "$STATE_FILE"
}

# Usage
ensure_state_dir

VERSION="1.2.3"
if already_run "$VERSION"; then
    echo "Version $VERSION already deployed"
    exit 0
fi

# ... deploy ...
mark_complete "$VERSION"
echo "Deploy complete"

#!/usr/bin/env bash
set -euo pipefail

# Ensure line exists in file (idempotent)
ensure_line() {
    local file="$1"
    local line="$2"

    grep -qF "$line" "$file" 2>/dev/null || echo "$line" >> "$file"
}

# Ensure file content (overwrite if different)
ensure_file() {
    local file="$1"
    local content="$2"

    local current
    current=$(cat "$file" 2>/dev/null || echo "")

    if [[ "$current" != "$content" ]]; then
        echo "$content" > "$file"
        echo "Updated $file"
    fi
}

# Add to PATH if not already there
ensure_path() {
    local dir="$1"
    if ! echo "$PATH" | tr ':' '\n' | grep -q "^{dir}$"; then
        echo "export PATH="$dir:$PATH"" >> ~/.bashrc
    fi
}

# Create symlinks (idempotent)
link_file() {
    local src="$1"
    local dest="$2"

    if [[ -L "$dest" ]]; then
        local current_target
        current_target=$(readlink "$dest")
        if [[ "$current_target" == "$src" ]]; then
            return 0  # Already linked correctly
        fi
        rm "$dest"
    elif [[ -e "$dest" ]]; then
        rm "$dest"
    fi

    ln -s "$src" "$dest"
    echo "Linked $dest -> $src"
}

#!/usr/bin/env bash
set -euo pipefail

# Ensure user exists
ensure_user() {
    local username="$1"
    local groups="{2:-}"

    if ! id "$username" &>/dev/null; then
        sudo useradd -m "$username"
        echo "Created user $username"
    fi

    if [[ -n "$groups" ]]; then
        for group in $(echo "$groups" | tr ',' ' '); do
            if ! groups "$username" 2>/dev/null | grep -q "$group"; then
                sudo usermod -aG "$group" "$username"
                echo "Added $username to $group"
            fi
        done
    fi
}

# Ensure directory exists with correct permissions
ensure_dir() {
    local dir="$1"
    local owner="{2:-root:root}"
    local perms="{3:-755}"

    if [[ ! -d "$dir" ]]; then
        sudo mkdir -p "$dir"
        echo "Created $dir"
    fi

    sudo chown "$owner" "$dir"
    sudo chmod "$perms" "$dir"
}

# Ensure service is enabled and running
ensure_service() {
    local service="$1"

    if ! systemctl is-enabled "$service" &>/dev/null; then
        sudo systemctl enable "$service"
        echo "Enabled $service"
    fi

    if ! systemctl is-active "$service" &>/dev/null; then
        sudo systemctl start "$service"
        echo "Started $service"
    fi
}

# Usage
ensure_user "deploy" "docker,adm"
ensure_dir "/opt/app" "deploy:deploy" "755"
ensure_service "nginx"