Shell Script Portability

Techniques for writing shell scripts that work across different platforms and environments.

Shebang Selection

Bash Scripts

#!/usr/bin/env bash

Most portable for bash scripts

Works on Linux, macOS, BSD

POSIX Shell Scripts

#!/bin/sh

For maximum portability

Use only POSIX features

Bash vs POSIX Differences

Arrays (Bash only)

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

Test Syntax

Bash - extended test

if [[ "$var" == "value" ]]; then echo "match" fi

POSIX - basic test

if [ "$var" = "value" ]; then echo "match" fi

String Operations

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

Arithmetic

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

Platform Differences

macOS vs Linux

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

sed Differences

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 }

readlink Differences

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 }

Detecting Environment

Operating System

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

Architecture

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

Shell Detection

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

Portable Patterns

Reading Files

Portable line reading

while IFS= read -r line || [ -n "$line" ]; do echo "$line" done < "$file"

The || [ -n "$line" ] handles files without trailing newline

Temporary Files

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 }

Command Existence Check

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

String Contains

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

ShellCheck Compatibility

Disabling Warnings for Portability

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'

Testing Multiple Shells

#!/usr/bin/env bash

shellcheck shell=bash

Or for POSIX:

#!/bin/sh

shellcheck shell=sh

Best Practices

• Choose the right shebang for your needs

• Document shell requirements in README

• Use #!/usr/bin/env bash for bash scripts

• Test on multiple platforms when possible

• Prefer POSIX features when portability matters

• Abstract platform differences into functions

• Use ShellCheck with appropriate shell directive

• Provide fallbacks for platform-specific commands