posix-shell-pro

name	posix-shell-pro
description	Expert in strict POSIX sh scripting for maximum portability across Unix-like systems. Specializes in shell scripts that run on any POSIX-compliant shell (dash, ash, sh, bash --posix).
risk	critical
source	community
date_added	2026-02-27

Use this skill when

• Working on posix shell pro tasks or workflows
• Needing guidance, best practices, or checklists for posix shell pro

Do not use this skill when

• The task is unrelated to posix shell pro
• You need a different domain or tool outside this scope

Instructions

• Clarify goals, constraints, and required inputs.
• Apply relevant best practices and validate outcomes.
• Provide actionable steps and verification.
• If detailed examples are required, open resources/implementation-playbook.md.

Focus Areas

• Strict POSIX compliance for maximum portability
• Shell-agnostic scripting that works on any Unix-like system
• Defensive programming with portable error handling
• Safe argument parsing without bash-specific features
• Portable file operations and resource management
• Cross-platform compatibility (Linux, BSD, Solaris, AIX, macOS)
• Testing with dash, ash, and POSIX mode validation
• Static analysis with ShellCheck in POSIX mode
• Minimalist approach using only POSIX-specified features
• Compatibility with legacy systems and embedded environments

POSIX Constraints

• No arrays (use positional parameters or delimited strings)
• No [[ conditionals (use [ test command only)
• No process substitution <() or >()
• No brace expansion {1..10}
• No local keyword (use function-scoped variables carefully)
• No declare, typeset, or readonly for variable attributes
• No += operator for string concatenation
• No ${var//pattern/replacement} substitution
• No associative arrays or hash tables
• No source command (use . for sourcing files)

Approach

• Always use #!/bin/sh shebang for POSIX shell
• Use set -eu for error handling (no pipefail in POSIX)
• Quote all variable expansions: "$var" never $var
• Use [ ] for all conditional tests, never [[
• Implement argument parsing with while and case (no getopts for long options)
• Create temporary files safely with mktemp and cleanup traps
• Use printf instead of echo for all output (echo behavior varies)
• Use . script.sh instead of source script.sh for sourcing
• Implement error handling with explicit || exit 1 checks
• Design scripts to be idempotent and support dry-run modes
• Use IFS manipulation carefully and restore original value
• Validate inputs with [ -n "$var" ] and [ -z "$var" ] tests
• End option parsing with -- and use rm -rf -- "$dir" for safety
• Use command substitution $() instead of backticks for readability
• Implement structured logging with timestamps using date
• Test scripts with dash/ash to verify POSIX compliance

Compatibility & Portability

• Use #!/bin/sh to invoke the system's POSIX shell
• Test on multiple shells: dash (Debian/Ubuntu default), ash (Alpine/BusyBox), bash --posix
• Avoid GNU-specific options; use POSIX-specified flags only
• Handle platform differences: uname -s for OS detection
• Use command -v instead of which (more portable)
• Check for command availability: command -v cmd >/dev/null 2>&1 || exit 1
• Provide portable implementations for missing utilities
• Use [ -e "$file" ] for existence checks (works on all systems)
• Avoid /dev/stdin, /dev/stdout (not universally available)
• Use explicit redirection instead of &> (bash-specific)

Readability & Maintainability

• Use descriptive variable names in UPPER_CASE for exports, lower_case for locals
• Add section headers with comment blocks for organization
• Keep functions under 50 lines; extract complex logic
• Use consistent indentation (spaces only, typically 2 or 4)
• Document function purpose and parameters in comments
• Use meaningful names: validate_input not check
• Add comments for non-obvious POSIX workarounds
• Group related functions with descriptive headers
• Extract repeated code into functions
• Use blank lines to separate logical sections

Safety & Security Patterns

• Quote all variable expansions to prevent word splitting
• Validate file permissions before operations: [ -r "$file" ] || exit 1
• Sanitize user input before using in commands
• Validate numeric input: case $num in *[!0-9]*) exit 1 ;; esac
• Never use eval on untrusted input
• Use -- to separate options from arguments: rm -- "$file"
• Validate required variables: [ -n "$VAR" ] || { echo "VAR required" >&2; exit 1; }
• Check exit codes explicitly: cmd || { echo "failed" >&2; exit 1; }
• Use trap for cleanup: trap 'rm -f "$tmpfile"' EXIT INT TERM
• Set restrictive umask for sensitive files: umask 077
• Log security-relevant operations to syslog or file
• Validate file paths don't contain unexpected characters
• Use full paths for commands in security-critical scripts: /bin/rm not rm

Performance Optimization

• Use shell built-ins over external commands when possible
• Avoid spawning subshells in loops: use while read not for i in $(cat)
• Cache command results in variables instead of repeated execution
• Use case for multiple string comparisons (faster than repeated if)
• Process files line-by-line for large files
• Use expr or $(( )) for arithmetic (POSIX supports $(( )))
• Minimize external command calls in tight loops
• Use grep -q when you only need true/false (faster than capturing output)
• Batch similar operations together
• Use here-documents for multi-line strings instead of multiple echo calls

Documentation Standards

• Implement -h flag for help (avoid --help without proper parsing)
• Include usage message showing synopsis and options
• Document required vs optional arguments clearly
• List exit codes: 0=success, 1=error, specific codes for specific failures
• Document prerequisites and required commands
• Add header comment with script purpose and author
• Include examples of common usage patterns
• Document environment variables used by script
• Provide troubleshooting guidance for common issues
• Note POSIX compliance in documentation

Working Without Arrays

Since POSIX sh lacks arrays, use these patterns:

• Positional Parameters: set -- item1 item2 item3; for arg; do echo "$arg"; done
• Delimited Strings: items="a:b:c"; IFS=:; set -- $items; IFS=' '
• Newline-Separated: items="a\nb\nc"; while IFS= read -r item; do echo "$item"; done <<EOF
• Counters: i=0; while [ $i -lt 10 ]; do i=$((i+1)); done
• Field Splitting: Use cut, awk, or parameter expansion for string splitting

Portable Conditionals

Use [ ] test command with POSIX operators:

• File Tests: [ -e file ] exists, [ -f file ] regular file, [ -d dir ] directory
• String Tests: [ -z "$str" ] empty, [ -n "$str" ] not empty, [ "$a" = "$b" ] equal
• Numeric Tests: [ "$a" -eq "$b" ] equal, [ "$a" -lt "$b" ] less than
• Logical: [ cond1 ] && [ cond2 ] AND, [ cond1 ] || [ cond2 ] OR
• Negation: [ ! -f file ] not a file
• Pattern Matching: Use case not [[ =~ ]]

CI/CD Integration

• Matrix testing: Test across dash, ash, bash --posix, yash on Linux, macOS, Alpine
• Container testing: Use alpine:latest (ash), debian:stable (dash) for reproducible tests
• Pre-commit hooks: Configure checkbashisms, shellcheck -s sh, shfmt -ln posix
• GitHub Actions: Use shellcheck-problem-matchers with POSIX mode
• Cross-platform validation: Test on Linux, macOS, FreeBSD, NetBSD
• BusyBox testing: Validate on BusyBox environments for embedded systems
• Automated releases: Tag versions and generate portable distribution packages
• Coverage tracking: Ensure test coverage across all POSIX shells
• Example workflow: shellcheck -s sh *.sh && shfmt -ln posix -d *.sh && checkbashisms *.sh

Embedded Systems & Limited Environments

• BusyBox compatibility: Test with BusyBox's limited ash implementation
• Alpine Linux: Default shell is BusyBox ash, not bash
• Resource constraints: Minimize memory usage, avoid spawning excessive processes
• Missing utilities: Provide fallbacks when common tools unavailable (mktemp, seq)
• Read-only filesystems: Handle scenarios where /tmp may be restricted
• No coreutils: Some environments lack GNU coreutils extensions
• Signal handling: Limited signal support in minimal environments
• Startup scripts: Init scripts must be POSIX for maximum compatibility
• Example: Check for mktemp: command -v mktemp >/dev/null 2>&1 || mktemp() { ... }

Migration from Bash to POSIX sh

• Assessment: Run checkbashisms to identify bash-specific constructs
• Array elimination: Convert arrays to delimited strings or positional parameters
• Conditional updates: Replace [[ with [ and adjust regex to case patterns
• Local variables: Remove local keyword, use function prefixes instead
• Process substitution: Replace <() with temporary files or pipes
• Parameter expansion: Use sed/awk for complex string manipulation
• Testing strategy: Incremental conversion with continuous validation
• Documentation: Note any POSIX limitations or workarounds
• Gradual migration: Convert one function at a time, test thoroughly
• Fallback support: Maintain dual implementations during transition if needed

Quality Checklist

• Scripts pass ShellCheck with -s sh flag (POSIX mode)
• Code is formatted consistently with shfmt using -ln posix
• Test on multiple shells: dash, ash, bash --posix, yash
• All variable expansions are properly quoted
• No bash-specific features used (arrays, [[, local, etc.)
• Error handling covers all failure modes
• Temporary resources cleaned up with EXIT trap
• Scripts provide clear usage information
• Input validation prevents injection attacks
• Scripts portable across Unix-like systems (Linux, BSD, Solaris, macOS, Alpine)
• BusyBox compatibility validated for embedded use cases
• No GNU-specific extensions or flags used

Output

• POSIX-compliant shell scripts maximizing portability
• Test suites using shellspec or bats-core validating across dash, ash, yash
• CI/CD configurations for multi-shell matrix testing
• Portable implementations of common patterns with fallbacks
• Documentation on POSIX limitations and workarounds with examples
• Migration guides for converting bash scripts to POSIX sh incrementally
• Cross-platform compatibility matrices (Linux, BSD, macOS, Solaris, Alpine)
• Performance benchmarks comparing different POSIX shells
• Fallback implementations for missing utilities (mktemp, seq, timeout)
• BusyBox-compatible scripts for embedded and container environments
• Package distributions for various platforms without bash dependency

Essential Tools

Static Analysis & Formatting

• ShellCheck: Static analyzer with -s sh for POSIX mode validation
• shfmt: Shell formatter with -ln posix option for POSIX syntax
• checkbashisms: Detects bash-specific constructs in scripts (from devscripts)
• Semgrep: SAST with POSIX-specific security rules
• CodeQL: Security scanning for shell scripts

POSIX Shell Implementations for Testing

• dash: Debian Almquist Shell - lightweight, strict POSIX compliance (primary test target)
• ash: Almquist Shell - BusyBox default, embedded systems
• yash: Yet Another Shell - strict POSIX conformance validation
• posh: Policy-compliant Ordinary Shell - Debian policy compliance
• osh: Oil Shell - modern POSIX-compatible shell with better error messages
• bash --posix: GNU Bash in POSIX mode for compatibility testing

Testing Frameworks

• bats-core: Bash testing framework (works with POSIX sh)
• shellspec: BDD-style testing that supports POSIX sh
• shunit2: xUnit-style framework with POSIX sh support
• sharness: Test framework used by Git (POSIX-compatible)

Common Pitfalls to Avoid

• Using [[ instead of [ (bash-specific)
• Using arrays (not in POSIX sh)
• Using local keyword (bash/ksh extension)
• Using echo without printf (behavior varies across implementations)
• Using source instead of . for sourcing scripts
• Using bash-specific parameter expansion: ${var//pattern/replacement}
• Using process substitution <() or >()
• Using function keyword (ksh/bash syntax)
• Using $RANDOM variable (not in POSIX)
• Using read -a for arrays (bash-specific)
• Using set -o pipefail (bash-specific)
• Using &> for redirection (use >file 2>&1)

Advanced Techniques

• Error Trapping: trap 'echo "Error at line $LINENO" >&2; exit 1' EXIT; trap - EXIT on success
• Safe Temp Files: tmpfile=$(mktemp) || exit 1; trap 'rm -f "$tmpfile"' EXIT INT TERM
• Simulating Arrays: set -- item1 item2 item3; for arg; do process "$arg"; done
• Field Parsing: IFS=:; while read -r user pass uid gid; do ...; done < /etc/passwd
• String Replacement: echo "$str" | sed 's/old/new/g' or use parameter expansion ${str%suffix}
• Default Values: value=${var:-default} assigns default if var unset or null
• Portable Functions: Avoid function keyword, use func_name() { ... }
• Subshell Isolation: (cd dir && cmd) changes directory without affecting parent
• Here-documents: cat <<'EOF' with quotes prevents variable expansion
• Command Existence: command -v cmd >/dev/null 2>&1 && echo "found" || echo "missing"

POSIX-Specific Best Practices

• Always quote variable expansions: "$var" not $var
• Use [ ] with proper spacing: [ "$a" = "$b" ] not ["$a"="$b"]
• Use = for string comparison, not == (bash extension)
• Use . for sourcing, not source
• Use printf for all output, avoid echo -e or echo -n
• Use $(( )) for arithmetic, not let or declare -i
• Use case for pattern matching, not [[ =~ ]]
• Test scripts with sh -n script.sh to check syntax
• Use command -v not type or which for portability
• Explicitly handle all error conditions with || exit 1

References & Further Reading

POSIX Standards & Specifications

• POSIX Shell Command Language - Official POSIX.1-2024 specification
• POSIX Utilities - Complete list of POSIX-mandated utilities
• Autoconf Portable Shell Programming - Comprehensive portability guide from GNU

Portability & Best Practices

• Rich's sh (POSIX shell) tricks - Advanced POSIX shell techniques
• Suckless Shell Style Guide - Minimalist POSIX sh patterns
• FreeBSD Porter's Handbook - Shell - BSD portability considerations

Tools & Testing

• checkbashisms - Detect bash-specific constructs

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.