POSIX Portable grep/sed

Rules for grep and sed that work identically on macOS (BSD) and Linux (GNU).

DO

| Need | Portable | Why | |------|----------|-----| | Match literal $(( | grep -Fv '$((' | -F = fixed string, no regex escaping needed | | Match with alternation | grep -E 'a\|b' or grep -e 'a' -e 'b' | ERE is portable with -E | | Extract substrings | sed 's/.*pattern //' \| sed 's/[")].*//' | POSIX sed pipeline | | Case conversion | echo "$var" \| tr '[:upper:]' '[:lower:]' | tr is POSIX | | In-place edit (macOS) | sed -i '' 's/old/new/' file | macOS sed requires '' after -i | | In-place edit (Linux) | sed -i 's/old/new/' file | GNU sed has no separator | | Newline in replacement | Write sed script to file, use sed -f | macOS sed handles \n in file mode |

DON'T

| Anti-pattern | Problem | Fix | |-------------|---------|-----| | grep -oP '(?<=prefix)\w+' | -P (PCRE) is GNU-only, not on macOS | Use sed pipeline | | grep -v '\$\(\(' | BRE treats \( as group start → Unmatched ( | grep -Fv '$((' | | ${var,,} / ${var^^} | Bash 4+ only; macOS ships bash 3.2 | tr '[:upper:]' '[:lower:]' | | readarray / mapfile | Bash 4+ only | while IFS= read -r line loop | | declare -A | Bash 4+ associative arrays | Use case statement or external tool | | grep -v '\|' (filter pipes) | Too broad — catches any line with \| | grep -v 'echo.*\|' (narrow match) |

Quick Diagnostic

# Check a script for non-portable patterns
grep -nE 'grep.*-[a-zA-Z]*P' script.sh        # grep -P (PCRE)
grep -nE '\$\{[a-zA-Z_]+,,\}' script.sh       # ${var,,}
grep -nE '\$\{[a-zA-Z_]+\^\^\}' script.sh     # ${var^^}
grep -nE 'readarray|mapfile|declare -A' script.sh  # bash 4+

Evidence

• gj-tool spec 006: ${app,,} broke normalize_app() on macOS bash 3.2
• gj-tool spec 008 Codex R2: grep -v '\$\(\(' errored with Unmatched ( or \( in BRE mode — fixed with grep -Fv
• gj-tool spec 008 Codex R2: grep -oP was not portable and missed bare-argument callsites — replaced with sed pipeline