Writing POSIX-Compatible Shell Scripts

POSIX Shell Compatibility

If your script uses #!/bin/sh, it must conform to the POSIX shell specification. Many systems (Debian, Ubuntu) use dash as /bin/sh, which does not support Bash-specific features.

Common Bashisms to Avoid

Bashism	POSIX Alternative
[[ condition ]]	[ condition ]
== in [ ]	=
${var//old/new}	Use sed: `echo "$var"
Arrays arr=(a b)	Use positional params or multiple vars
function name	name()
source file	. file
(( arithmetic ))	$((arithmetic)) or expr
<<< (here-string)	`echo "string"
select	Manual menu loop
read -p	printf "prompt: "; read var
Process sub <(cmd)	Temp files or pipes

Testing POSIX Compliance

Method 1: Test with dash

dash ./script.sh

Method 2: Use checkbashisms

checkbashisms ./script.sh

Method 3: Use shellcheck with --shell=sh

shellcheck --shell=sh ./script.sh

POSIX-Safe Patterns

String operations:

# Get filename extension (POSIX)
ext="${filename##*.}"

# Get directory name (POSIX)
dir="${filepath%/*}"

# Remove prefix (POSIX)
base="${var#prefix}"

Arithmetic:

# POSIX arithmetic
count=$((count + 1))
result=$((a * b + c))

# POSIX comparison
if [ "$count" -gt 10 ]; then
  echo "More than 10"
fi

Local variables:

# local is not POSIX but widely supported
# For strict POSIX, use subshells for scoping:
my_func() (
  var="local to function"
  echo "$var"
)
# Parentheses instead of braces create a subshell

When to Use #!/bin/bash

Use bash when you need arrays, [[ ]], regex matching, process substitution, or other Bash-specific features. Just be explicit with #!/bin/bash or #!/usr/bin/env bash.