bash-portability

Guidance for writing portable POSIX-compatible scripts and understanding when to leverage bash-specific features.

Shebang Selection

Use #!/usr/bin/env bash for Bash Scripts

#!/usr/bin/env bash

Why: Searches PATH for bash, works across systems where bash may be in different locations.

Use #!/bin/sh for POSIX Scripts

#!/bin/sh

Why: Maximum portability when bash features aren't needed. On many systems, /bin/sh is dash or another POSIX shell.

Direct Path When Required

#!/bin/bash

Use only when: System requirements guarantee bash location, or security policy requires absolute paths.

POSIX vs Bash Feature Matrix

| Feature | POSIX | Bash | Recommendation | | -------------- | ------- | ---- | ------------------------------ | ---- | | [[ ]] | No | Yes | Use [ ] for POSIX | | (( )) | No | Yes | Use [ ] with -eq etc. | | Arrays | No | Yes | Use positional params or files | | local | Partial | Yes | Generally safe | | ${var,,} | No | 4+ | Use tr for POSIX | | <<< | No | Yes | Use echo | cmd | | =~ regex | No | Yes | Use grep or expr | | source | No | Yes | Use . (dot) command | | function f() | No | Yes | Use f() only | | $'...' | No | Yes | Use printf | | {1..10} | No | Yes | Use seq or while loop |

POSIX-Compatible Patterns

Conditionals

# POSIX - use [ ] with proper quoting
if [ -f "$file" ]; then
    echo "File exists"
fi

# String comparison
if [ "$var" = "value" ]; then
    echo "Match"
fi

# Numeric comparison
if [ "$num" -gt 10 ]; then
    echo "Greater"
fi

# Compound conditions
if [ -f "$file" ] && [ -r "$file" ]; then
    echo "Readable file"
fi

Case Conversion (POSIX)

# Lowercase
lower=$(echo "$string" | tr '[:upper:]' '[:lower:]')

# Uppercase
upper=$(echo "$string" | tr '[:lower:]' '[:upper:]')

Substring Operations (POSIX)

# Get substring - use expr or cut
substr=$(expr "$string" : '.\{3\}\(.\{5\}\)')  # chars 4-8
substr=$(echo "$string" | cut -c4-8)

# String length
length=$(expr length "$string")
length=${#string}  # This is actually POSIX

Reading Files (POSIX)

# Line by line
while IFS= read -r line; do
    echo "$line"
done < "$file"

# Read entire file (without cat)
content=$(cat "$file")  # cat is POSIX

Command Substitution

# Modern syntax (preferred even in POSIX)
result=$(command)

# Legacy syntax (avoid)
result=`command`

# Nested (why modern is better)
result=$(echo $(date))      # Clear
result=`echo \`date\``      # Escape nightmare

Bash-Specific Features Worth Using

When portability isn't required, these bash features improve code quality:

Extended Test [[ ]]

# Pattern matching
[[ "$file" == *.txt ]]

# Regex matching
[[ "$input" =~ ^[0-9]+$ ]]

# No word splitting worries
[[ -f $file ]]  # Quotes optional (but still recommended)

# Logical operators inside
[[ -f "$file" && -r "$file" ]]

Arrays

# Indexed arrays
declare -a files=()
files+=("one.txt")
files+=("two.txt")
for f in "${files[@]}"; do
    process "$f"
done

# Associative arrays (Bash 4+)
declare -A config
config[host]="localhost"
config[port]="8080"

Parameter Expansion

# Default value
"${var:-default}"

# Case conversion (Bash 4+)
"${var,,}"  # lowercase
"${var^^}"  # uppercase

# Substring
"${var:0:10}"  # first 10 chars
"${var: -5}"   # last 5 chars

# Search/replace
"${var//old/new}"

Here Strings

# Bash
read -r var <<< "input string"

# POSIX equivalent
var=$(echo "input string")

Process Substitution

# Bash - compare two command outputs
diff <(sort file1) <(sort file2)

# POSIX equivalent (with temp files)
sort file1 > /tmp/sorted1
sort file2 > /tmp/sorted2
diff /tmp/sorted1 /tmp/sorted2

Detecting Shell Type

# Check if running in bash
if [ -n "${BASH_VERSION:-}" ]; then
    echo "Running in Bash"
fi

# Check bash version for features
if [ "${BASH_VERSINFO[0]:-0}" -ge 4 ]; then
    echo "Bash 4+ available"
fi

# Generic shell detection
case "${SHELL##*/}" in
    bash) echo "bash" ;;
    zsh)  echo "zsh" ;;
    *)    echo "other" ;;
esac

Portable Utility Functions

Code examples

Portability Decision Guide

Use POSIX when:

• Script runs on minimal systems (containers, embedded)
• Target includes dash, ash, or busybox sh
• Maximum compatibility is required
• Script is part of system initialization

Use Bash when:

• Target systems guaranteed to have bash
• Need arrays, associative arrays, or regex
• Complex string manipulation required
• Code clarity significantly improved
• Interactive features needed

Common Portability Pitfalls

echo vs printf

# Problematic - behavior varies
echo -n "no newline"
echo -e "with\ttabs"

# Portable
printf '%s' "no newline"
printf 'with\ttabs\n'

Variable Assignment

# Works everywhere
var="value"

# May fail on some shells
var = "value"  # Spaces around = are wrong

Export with Assignment

# POSIX - separate commands
var="value"
export var

# Bash/modern - combined (works most places)
export var="value"

Array-like Operations Without Arrays

# Use positional parameters
set -- "item1" "item2" "item3"
for item in "$@"; do
    echo "$item"
done

# Or IFS-based splitting
items="item1:item2:item3"
IFS=':' read -r item1 item2 item3 <<EOF
$items
EOF