shell-portability

# Shell Script Portability

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

## Shebang Selection

### Bash Scripts

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

### POSIX Shell Scripts

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

## Bash vs POSIX Differences

### Arrays (Bash only)

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

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

### String Operations

```bash
# 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
# 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

```bash
# 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

```bash
# 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