Write idiomatic, efficient Bash and Shell scripts

Bash / Shell: Idiomatic Efficiency Reference is a skill of before/after bash coding guidelines across seven areas: quoting and word splitting, conditionals and tests, loops and iteration, pipes and process substitution, functions and return values, error handling, and a summary anti-pattern table.

Use this when writing or reviewing bash or shell scripts and want idiomatic, safer patterns applied consistently rather than ad hoc style. It's language-specific guidance, not an architectural framework, and the skill itself notes that over-compressing a script can hurt readability, so judgement still applies.

Quoting rules: double-quote every $variable and $(command) unless splitting is intentional, use "${files[@]}" in loops, prefer $() over backticks for nesting, and quote both sides of string comparisons. Conditionals: prefer [[ ]] over [ ] for safer && and || with no internal word splitting, test a command's exit status directly (if grep -q pattern file) instead of checking $? afterward, and use (( )) for arithmetic comparisons instead of [ "$count" -gt 10 ]. Loops: glob directly (for f in *.txt) instead of parsing ls output, read files line-by-line with while IFS= read -r line rather than for line in $(cat file), use brace expansion {1..10} or C-style (( )) instead of seq, and redirect input into a while loop rather than piping into it, since piping runs the loop in a subshell that silently discards variable updates made inside it. Pipes: avoid chained grep | grep in favor of a single regex or awk, avoid cat file | grep as a useless use of cat, use process substitution <(cmd1) instead of temp files for diffing two commands' output, and set -o pipefail so a failing early command in a pipeline isn't hidden by a later command's success. Functions: use echo plus command substitution to return string values, since return is only for 0-255 exit codes, keep function-local state with local instead of mutating globals, and define functions as name() {} rather than the non-POSIX function keyword.

Error handling: start every script with set -euo pipefail, removed selectively only where genuinely needed, check command results explicitly (cd /some/dir || exit 1) rather than letting a failed cd fall through into a destructive rm -rf, use trap 'rm -f "$tmpfile"' EXIT for guaranteed cleanup, and never silence errors with 2>/dev/null without an explicit || true showing the failure was anticipated. A closing anti-pattern table maps each bad pattern - ls parsing, cat | grep, unquoted vars, eval with user input, #!/bin/sh with bash features, expr for math - to its preferred replacement.

Developers writing or reviewing bash and shell scripts who want a concrete before/after reference for idiomatic, safer patterns instead of relying on habit or memory.