Bash shortening

bash-shortening

Make Bash scripts shorter without making them harder to read. Source: 51 techniques from Karandeep Singh's "Bash Code Shortening" article, codified here as a cheatsheet plus categorized references.

Philosophy

Shortening is about expressing intent, not saving keystrokes. The win is fewer moving parts (subprocesses, temp files, intermediate variables) — not denser code per line. A 7-line script that creates a temp file and deletes it is worse than a 1-line pipeline; a cryptic ${1:-${X:-${Y/-,/ }}} chain is worse than three clear lines.

Two heuristics decide every call:

1. Does shortening eliminate a class of bugs? (e.g. forgotten rm on temp files, missed branches in nested if, race on directory checks before mkdir -p). If yes, shorten.
2. Does shortening obscure intent for the next reader? If yes, expand.

Both heuristics can fire at once — when they conflict, prefer clarity. See references/anti-patterns.md for the long-form version.

How to use this skill

Architecture: workers gather, main thread writes. Read-only analyzer workers run in parallel to collect violations. Then the main thread is the single writer — it runs the script, verifies the result, and hand-edits the rest. This avoids parallel writes to the same file (the lost-update race the user is right to worry about).

1. Identify the target. File path(s), pasted snippet, or bash-fenced block. Confirm it's actually bash (see "When NOT to use this skill").
2. Dispatch the analyzer sweep. Fire one read-only analyzer worker per reference category in a single batched call so they run concurrently. Analyzers only report violations — they do not write, do not run the script, do not touch the target. See "Parallel sweep — worker contract" below.
3. Merge analyzer reports. Collect each analyzer's path:line — verbose → idiomatic hits. Dedupe overlap (the same line can be flagged by two categories — keep one entry, note both attributions).
4. Run the script from the main thread. Run python3 scripts/bash-shorten.py --apply <target> (one file per invocation — loop over the list if there are several). This applies the high-confidence core rules in one writer. Never delegate --apply to a worker.
5. Verify the script's output. bash -n <target> for syntax, shellcheck <target> if available, and read the diff. If a rule produced something surprising, revert and narrow with --rules or --skip before retrying.
6. Present the remaining punch list. Show the analyzer hits the script didn't cover and let the user pick which categories to apply, especially when anti-pattern hits conflict with shortening hits.
7. Hand-edit the rest. One technique per change with rationale visible. Use the before / after examples in the matching reference file. Preserve quoting ("$var"), set -euo pipefail, and any error handling. Stop before any rewrite needs a comment to explain what it does.

Don't bulk-rewrite silently. Workers gather, main thread writes, every change attributable to one technique the user can learn.

Automated rewrites

The skill ships with scripts/bash-shorten.py — a Python rewriter for the high-confidence patterns. It has no third-party Python dependencies but does require ast-grep (sg) on PATH; see the "ast-grep is required" section below. It defaults to dry-run (prints a unified diff to stdout and per-rule counts to stderr) and only writes when you pass --apply. Always run shellcheck on the output afterwards.

# preview rewrites
python3 scripts/bash-shorten.py path/to/script.sh

# apply (atomic write, preserves the file in place)
python3 scripts/bash-shorten.py --apply path/to/script.sh

# only run a subset of rules
python3 scripts/bash-shorten.py --rules backticks,test-numeric script.sh

# disable a rule
python3 scripts/bash-shorten.py --skip find-exec-rm-delete script.sh

# enable the modernize rule group (off by default — see "Rule groups")
python3 scripts/bash-shorten.py --include modernize --apply script.sh

# explore the ruleset
python3 scripts/bash-shorten.py --list
python3 scripts/bash-shorten.py --explain test-numeric

# verify the rules still work after editing
python3 scripts/bash-shorten.py --self-test

Two rule groups (core, modernize) with embedded positive and negative fixtures for every rule (run --list to see the current counts). The core group is on by default; the modernize group is opt-in. Most core rules mirror source-article examples (sed-replace-first/all, echo-wc-c, cut-c-substring, expr-arith-vars/literal/increment, combined-tests, test-numeric, empty-default, param-default, mkdir-guard, for-range-expansion); the rest are bonus core patterns the article doesn't cover but are obvious wins (backticks, legacy-null-check, empty-string-eq, find-exec-rm-delete, cat-file-pipe-grep — three of which shellcheck flags but doesn't auto-fix).

What the rewriter deliberately can't do: anything that needs data flow analysis (single-use variable inlining), multi-statement detection (temp-file → pipeline), function extraction, parallelization, or behavioral judgment ("is this &&/|| chain safe?"). Those are hand-edits guided by the references.

When a user asks "can you shorten this whole script?", the main thread runs the script after the read-only analyzer sweep returns (see next section). The script handles the boring 60% of the rewrites; the analyzers cover what regex can't see; the main thread is the only writer.

Parallel sweep — worker contract

A single in-context reviewer reliably tunnel-visions on the first one or two categories it looks at, missing whole classes (command substitution, parameter expansion, arithmetic, brace expansion, process substitution, functions, advanced, real-world). The fix is to fan out read-only analyzers — one per reference category — in parallel. They gather violations; they don't write. The main thread runs the script and the hand-edits afterward.

Workers are strictly read-only

Every parallel worker has the same contract: read the target, report violations, return. No worker writes to disk, applies rewrites, or runs scripts/bash-shorten.py --apply. If a job involves writing, it isn't a worker — it's the main thread's job.

Why: parallel writes to the same file race. Even if you scoped each worker to its own category, they'd produce overlapping diffs against shared lines that the main thread would still have to merge by hand. Gather first, write once.

Worker type

Worker	Count	Role	Output
Category analyzer	One per reference category (9 by default)	Read references/<category>.md and scan the target for the patterns it covers	Markdown list of path:line — verbose → idiomatic, no rewriting

Brief for each category analyzer (substitute <category> and <target> — paths in this skill are relative to the skill directory unless noted; pass the repo-relative path of the script being shortened as <target>):

Read references/<category>.md (relative to the bash-shortening skill directory). Scan <target> for every instance of the patterns it covers. Return a markdown list: path:line — verbose form → idiomatic form. Do not rewrite the file, do not run scripts/bash-shorten.py, do not write anything. If a hit conflicts with the anti-patterns reference, flag it but still include it.

Dispatch rule: fire all analyzers in a single batched call so they run concurrently. Sequential dispatch defeats the latency win that's the whole reason to fan out.

Harness-agnostic — pick your primitive

The contract is "fire N read-only tasks in parallel and collect results." Use whichever primitive your harness exposes — the skill does not depend on any one of them:

• A parallel sub-agent / sub-task call (one batched message that spawns all workers at once).
• Multiple parallel tool calls in a single assistant turn.
• A task / node fan-out in an agent-graph framework.
• Plain shell: xargs -P 9 or parallel over the worker briefs piped to your agent CLI, with each worker's stdout captured to a separate file.
• No parallel primitive available: walk the category table sequentially in one pass. Tick each off explicitly — tunnel-vision on the first hits is the failure mode this section exists to prevent.

Categories

These are the complete checklist — do not stop after finding hits in two or three of them. Tunnel-vision on basename/dirname/for-range while skipping command substitution, parameter expansion, arithmetic, brace expansion, process substitution, functions, advanced, and real-world is the failure mode this section exists to prevent.

Category	Reference	Article refs	Patterns to find
Command-substitution sweep	references/command-substitution.md	3-7	$(cmd) temp-file elimination, pipelines vs reused output, xargs patterns, find … | xargs vs process substitution
Parameter-expansion sweep	references/parameter-expansion.md	8-15	if [ -z "$X" ]; then X=… defaults, cut -c/echo | sed substitutions, ${#S} length, basename/dirname shells
Function-pattern sweep	references/functions.md	16-20	repeated logging blocks, default-param boilerplate, echo-returns, named-param patterns
Brace-expansion sweep	references/brace-expansion.md	21-26	sequential mkdir/touch a b c, for i in 1 2 3 4 5 ranges, zero-padded sequences
Process-substitution sweep	references/process-substitution.md	27-31	temp files feeding diff/loops, echo "x" | cmd → <<< here-strings
Arithmetic sweep	references/arithmetic.md	32-38	expr, $(…)+1 increments, [ -gt/-lt/-eq ] numeric tests, [ "$A" ] && [ "$B" ]
Real-world sweep	references/real-world.md	39-45	config parsing, log analysis, health checks, batch processing, backups, API+jq
Advanced sweep	references/advanced.md	48-51	repeated echo lines (→ heredoc), 3+ branch if/elif (→ assoc array or case), sequential independent commands (→ & … & wait), cut -d, in loops (→ custom IFS)
Anti-pattern sweep	references/anti-patterns.md	1-2, 46-47	nested expansions, cryptic one-liners, places where shortening would hurt — flag for the user, do not auto-rewrite

Merge, apply, verify — single writer

After all analyzers return, the main thread owns every write:

1. Collect and dedupe the per-category hit lists. The same line can be flagged by two analyzers — keep one entry, note both attributions.
2. Run the rewriter script from the main thread. Invoke python3 scripts/bash-shorten.py --apply <target> once per file (loop if there are several — the script takes one positional file). Defaults to core; add --include modernize to opt into the sd/rg/fd rewrites. This is the only place --apply runs.
3. Verify the script's output. bash -n <target> for syntax, shellcheck <target> if available, and a manual diff read. If a rule produced something surprising, revert and narrow with --rules or --skip before retrying.
4. Present the remaining analyzer punch list — the hits the script didn't cover. Let the user pick which categories to apply, especially when anti-pattern hits conflict with shortening hits.
5. Apply hand-edits one technique per change with rationale (per the "How to use this skill" workflow above). Each edit is a single write from the main thread; no fan-out, no parallelism.

Why one writer: the analyzers are orthogonal in what they look for, but their proposed rewrites overlap on the same lines (a single for i in 1 2 3 4 5 hit shows up in brace-expansion and arithmetic and maybe real-world). If workers wrote in parallel, the last writer would silently overwrite the others. Gathering all violations first and applying from one place keeps the diff auditable and prevents lost-update bugs.

Why fan out for analysis: the categories are orthogonal, each needs a different reference file in working memory, and parallel workers force coverage that a single pass over many files reliably skips. Read-only tasks don't race, so parallelism is free here — exactly where the single-writer discipline doesn't apply.

Rule groups

Group	Default	Contents
core	on	18 idiomatic-bash rewrites that don't change tooling
modernize	off	sed-replace-to-sd, grep-fixed-to-rg, find-name-to-fd — rewrite to non-coreutils binaries (sd, rg, fd) the user must have installed

The modernize rules are conservative on purpose: only literal sed patterns map to sd, only grep -F (fixed-string) maps to rg -F (plain grep PAT uses BRE which differs enough from rg's regex flavor to risk silent behavior changes), and only find . -type f -name "GLOB" without additional flags maps to fd (and even then the find → fd rewrite changes behavior: fd respects .gitignore by default).

Opt in via --include modernize. The installer (scripts/install.sh) brings down sd, ripgrep, fd, and ast-grep so the rewritten code runs and the rewriter has its required dependencies.

ast-grep is required

bash-shorten.py requires ast-grep (sg) on PATH. Structural patterns (backticks, test-numeric) route through ast-grep first using the rule pack at scripts/sg-rules/, then the remaining regex rules run on the output. Tree-sitter parses the bash once, so context-sensitive rules (skip # comments, skip heredoc bodies) work correctly without ad-hoc lookbehinds in the regex layer.

If sg is missing, the script exits with a friendly diagnostic. Two paths forward:

brew install ast-grep        # macOS / Linuxbrew
cargo install ast-grep --bin sg

Or skip the script entirely and run only the category analyzers from the parallel sweep — the reference-driven methodology in this file is the fallback for environments without ast-grep.

Why some rules are still regex-only.

• mkdir-guard, empty-default, param-default, expr-increment need cross-metavariable equality (same name in two positions). YAML ast-grep rules can't express that constraint cleanly; the Python regex uses a backreference instead.
• combined-tests — [ ... ] flattens into a list of word tokens, so structural matching on the operator side is brittle.
• for-range-expansion — needs runtime arithmetic to verify the captured integers form a step-1 ascending sequence; pure-YAML rules can't compute that.
• sed-replace-* — the literal-pattern guard needs character-class restrictions in the matcher.

test-numeric was previously in this list but moved to sg-rules/ as six per-operator rules — tree-sitter-bash distinguishes test_command ([ ]) from conditional_expression ([[ ]]) so the sg form correctly skips [[ ]] whereas the regex bled into it (issue #18).

These regex-only rules still run after the ast-grep pass. See AGENTS.md for guidance on writing new rules — default to ast-grep, fall back to regex only when one of the constraints above blocks it.

Quick wins

The most common bloat patterns and where to read the full treatment. Numbers in parens are the example numbers from the source article.

Verbose form	Idiomatic form	Reference
if [ -z "$X" ]; then X=default; fi	X=${X:-default}	parameter-expansion (8)
$(echo "$S" | cut -c1-5)	${S:0:5}	parameter-expansion (10)
$(basename "$P") / $(dirname "$P")	${P##*/} / ${P%/*} (hand-edit only — silently breaks on trailing-slash paths; dirname also diverges from ${P%/*} when the path has no / (real dirname returns ., the expansion returns the original string); rewriter does not auto-apply)	parameter-expansion (11-12)
$(echo "$S" | sed 's/a/b/g')	${S//a/b}	parameter-expansion (13-14)
$(echo -n "$S" | wc -c)	${#S}	parameter-expansion (15)
$(expr $A + $B) / C=$(expr $C + 1)	$((A + B)) / ((C+=1)) (or C=$((C + 1)))	arithmetic (32-34)
[ $X -gt 100 ] / [ $A ] && [ $B ]	((X > 100)) / [[ $A && $B ]]	arithmetic (35, 37)
mkdir a; mkdir b; mkdir c	mkdir -p {a,b,c}	brace-expansion (21-22)
for i in 1 2 3 4 5	for i in {1..5} (or {01..10}, {2..10..2})	brace-expansion (23-26)
cmd > /tmp/x; cmd2 < /tmp/x; rm /tmp/x	cmd | cmd2 or cmd2 < <(cmd)	command-substitution (5), process-substitution (29)
sort a > /tmp/a; sort b > /tmp/b; diff ...	diff <(sort a) <(sort b)	process-substitution (27)
if [ "$E" = dev ]; elif ... ; fi (3+ branches)	case or assoc array ${URLS[$E]:-default}	functions (intro), advanced (49)
Repeating echo "[$(date)] [LEVEL] msg"	log() function with ${1^^}	functions (16)
find ... > /tmp/x; while read; ...; done < /tmp/x; rm	find ... | xargs cmd or done < <(find ...)	command-substitution (6), process-substitution (29)
Multi-line echo "..." x N	cat <<EOF ... EOF heredoc	advanced (48)
cmd1; cmd2; cmd3 (sequential, independent)	cmd1 & cmd2 & cmd3 & wait	advanced (50)
cut -d, -f1,2,3 inside loop	while IFS=, read -r a b c	advanced (51)

If the user's pattern doesn't appear here, search the reference index below — every example from the article is preserved.

Reference index

Read the file matching the technique class. Each holds the full before/after from the source article plus gotchas worth knowing.

File	Covers	Examples
references/command-substitution.md	Pipelines, xargs, eliminating temp files, single-shot vs reused command output	3-7
references/parameter-expansion.md	Defaults, alternatives, substring, path extraction, replacement, length	8-15
references/functions.md	Logging, default params, inline conditionals, echo-returns, named params	16-20
references/brace-expansion.md	Directory/file expansion, numeric and char sequences, steps, zero-padding	21-26
references/process-substitution.md	<(cmd), >(cmd), here-strings (<<<), feeding loops from commands	27-31
references/arithmetic.md	$(( )), (( )), [[ ]], comparison operators, ternary gotcha	32-38
references/real-world.md	Config parsing, log analysis, health checks, batch processing, backups, user mgmt, API+jq	39-45
references/anti-patterns.md	When not to shorten — cryptic one-liners, nested expansions, the philosophy	1-2, 46-47
references/advanced.md	Heredocs, associative arrays, parallel execution + wait, custom IFS for CSV	48-51

When NOT to use this skill

• Non-bash shells. Most parameter expansions (${var//x/y}, ${var:offset:length}), [[ ]], arrays, and process substitution are bashisms. If the script's shebang is #!/bin/sh, #!/usr/bin/env dash, or it targets posh/Alpine ash/busybox, switch register or refuse and explain. fish and zsh have their own grammars — none of this applies.
• One-liner golf. If the user explicitly wants the shortest possible line for a code-golf challenge, shortening past readability is the goal, not a bug — but call out the readability cost so they own the choice.
• Critical infrastructure scripts. Boot scripts, init scripts, and scripts that run before logging is set up benefit from being boring. Don't trade clarity for elegance in code that runs at 3 AM during an incident.
• POSIX-portability requirement. When a script is shipped as #!/bin/sh for cross-distro install scripts, stay POSIX. The anti-patterns reference has a portability checklist.

What you don't do

• Don't rewrite the whole file in one pass — one technique per change, with the rationale visible.
• Don't introduce new dependencies (yq, jq, parallel) just to enable a shortening. Suggest them, but only apply if the user agrees.
• Don't strip comments or set -euo pipefail while shortening — those are load-bearing.
• Don't claim a rewrite is faster without measuring. Subprocess elimination usually is, but say "should be faster" not "is 5x faster" unless you ran time against both.

Common mistakes to catch on review

These come up often when LLMs (or humans rushing) try to shorten bash:

• Unquoted $var inside the rewrite. Shortening should never drop quoting; word-splitting bugs are worse than verbosity.
• Arithmetic ternary returning a string. $((C > 10 ? "high" : "low")) does not work — bash arithmetic is integer-only. Use [[ $C -gt 10 ]] && S=high || S=low or a case. (Source article example 36 has this bug; the arithmetic reference flags it.)
• && ... || ... as if-then-else. Only safe when the first branch cannot fail. If the first command has any chance of returning non-zero on success, the || branch fires anyway. Use if/else for non-trivial branches.
• mkdir without -p. Shortening removes the existence check, so the -p flag is what makes the rewrite safe. Don't drop both.
• xargs without -r or -0. Empty input or filenames with spaces blow up xargs. Use -r (don't run on empty) and -0 with find -print0 for path safety. The article doesn't mention this; flag it on review.

Source

51 techniques from https://karandeepsingh.ca/posts/bash-code-shortening-techniques/ by Karandeep Singh (2023). Every numbered example in the source article is preserved in the references — counts and numbering match the original.