posix-shell-pro

Documentation & Productivité

Expert in strict POSIX sh scripting for maximum portability across

Documentation

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 <

●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](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html) - Official POSIX.1-2024 specification

●[POSIX Utilities](https://pubs.opengroup.org/onlinepubs/9699919799/idx/utilities.html) - Complete list of POSIX-mandated utilities

●[Autoconf Portable Shell Programming](https://www.gnu.org/software/autoconf/manual/autoconf.html#Portable-Shell) - Comprehensive portability guide from GNU

`Portability & Best Practices`

●[Rich's sh (POSIX shell) tricks](http://www.etalabs.net/sh_tricks.html) - Advanced POSIX shell techniques

●[Suckless Shell Style Guide](https://suckless.org/coding_style/) - Minimalist POSIX sh patterns

●[FreeBSD Porter's Handbook - Shell](https://docs.freebsd.org/en/books/porters-handbook/makefiles/#porting-shlibs) - BSD portability considerations

`Tools & Testing`

●[checkbashisms](https://manpages.debian.org/testing/devscripts/checkbashisms.1.en.html) - Detect bash-specific constructs

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