#!/usr/bin/env bash
# Most portable for bash scripts
# Works on Linux, macOS, BSD

#!/bin/sh
# For maximum portability
# Use only POSIX features

# Bash - arrays available
declare -a items=("one" "two" "three")
for item in "${items[@]}"; do
    echo "$item"
done

# POSIX - use positional parameters or space-separated strings
set -- one two three
for item in "$@"; do
    echo "$item"
done

# Bash - extended test
if [[ "$var" == "value" ]]; then
    echo "match"
fi

# POSIX - basic test
if [ "$var" = "value" ]; then
    echo "match"
fi

# Bash - regex matching
if [[ "$input" =~ ^[0-9]+$ ]]; then
    echo "numeric"
fi

# POSIX - use case or external tools
case "$input" in
    *[!0-9]*|'') echo "not numeric" ;;
    *) echo "numeric" ;;
esac

# Bash - arithmetic expansion
(( count++ ))
if (( count > 10 )); then
    echo "greater"
fi

# POSIX - expr or arithmetic expansion
count=$((count + 1))
if [ "$count" -gt 10 ]; then
    echo "greater"
fi

# Date command differences
# GNU (Linux)
date -d "yesterday" +%Y-%m-%d

# BSD (macOS)
date -v-1d +%Y-%m-%d

# Portable approach
if date --version >/dev/null 2>&1; then
    # GNU date
    yesterday=$(date -d "yesterday" +%Y-%m-%d)
else
    # BSD date
    yesterday=$(date -v-1d +%Y-%m-%d)
fi

# GNU sed - in-place edit
sed -i 's/old/new/g' file.txt

# BSD sed - requires backup extension
sed -i '' 's/old/new/g' file.txt

# Portable approach
sed 's/old/new/g' file.txt > file.txt.tmp && mv file.txt.tmp file.txt

# Or use a function
sed_inplace() {
    if sed --version >/dev/null 2>&1; then
        sed -i "$@"
    else
        sed -i '' "$@"
    fi
}

# GNU readlink
readlink -f /path/to/link

# BSD/macOS - no -f option by default
# Use greadlink from coreutils or:
resolve_path() {
    local path="$1"
    if command -v greadlink >/dev/null 2>&1; then
        greadlink -f "$path"
    elif command -v realpath >/dev/null 2>&1; then
        realpath "$path"
    else
        # Fallback
        cd "$(dirname "$path")" && pwd -P
    fi
}

detect_os() {
    case "$(uname -s)" in
        Linux*)  echo "linux" ;;
        Darwin*) echo "macos" ;;
        MINGW*|CYGWIN*|MSYS*) echo "windows" ;;
        FreeBSD*) echo "freebsd" ;;
        *) echo "unknown" ;;
    esac
}

OS=$(detect_os)
case "$OS" in
    linux)  INSTALL_CMD="apt-get install" ;;
    macos)  INSTALL_CMD="brew install" ;;
esac

detect_arch() {
    case "$(uname -m)" in
        x86_64|amd64) echo "amd64" ;;
        aarch64|arm64) echo "arm64" ;;
        armv7l) echo "arm" ;;
        *) echo "unknown" ;;
    esac
}

detect_shell() {
    if [ -n "$BASH_VERSION" ]; then
        echo "bash"
    elif [ -n "$ZSH_VERSION" ]; then
        echo "zsh"
    else
        echo "sh"
    fi
}

# Portable line reading
while IFS= read -r line || [ -n "$line" ]; do
    echo "$line"
done < "$file"
# The || [ -n "$line" ] handles files without trailing newline

# POSIX-compatible temp file
make_temp() {
    if command -v mktemp >/dev/null 2>&1; then
        mktemp
    else
        # Fallback
        local tmp="/tmp/tmp.$$.$RANDOM"
        touch "$tmp" && echo "$tmp"
    fi
}

# POSIX-compatible command check
has_command() {
    command -v "$1" >/dev/null 2>&1
}

# Usage
if has_command curl; then
    curl "$url"
elif has_command wget; then
    wget -O- "$url"
else
    echo "No HTTP client available" >&2
    exit 1
fi

# POSIX-compatible string contains
contains() {
    case "$1" in
        *"$2"*) return 0 ;;
        *) return 1 ;;
    esac
}

# Usage
if contains "$PATH" "/usr/local/bin"; then
    echo "Found in PATH"
fi

# When intentionally using non-portable features
# shellcheck disable=SC2039  # Bash-specific feature
if [[ "$var" =~ regex ]]; then
    :
fi

# Document why
# shellcheck disable=SC2016  # Intentionally not expanding
echo 'Use $HOME for home directory'

#!/usr/bin/env bash
# shellcheck shell=bash

# Or for POSIX:
#!/bin/sh
# shellcheck shell=sh