📦 deps(thirdparty): update snapshots
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---
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name: bash
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description: "Language-specific super-code guidelines for bash."
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risk: safe
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source: community
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date_added: "2026-06-16"
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---
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# Bash / Shell: Idiomatic Efficiency Reference
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## Table of Contents
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1. [Quoting & Word Splitting](#quoting)
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2. [Conditionals & Tests](#conditionals)
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3. [Loops & Iteration](#loops)
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4. [Pipes & Process Substitution](#pipes)
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5. [Functions & Return Values](#functions)
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6. [Error Handling](#errors)
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7. [Anti-patterns specific to Bash](#antipatterns)
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---
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## 1. Quoting & Word Splitting {#quoting}
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```bash
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# ❌ Unquoted variable (word splitting + globbing)
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for f in $files; do rm $f; done
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# ✅
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for f in "${files[@]}"; do rm -- "$f"; done
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```
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```bash
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# ❌ Unquoted command substitution
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path=$(find . -name config)
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cat $path # breaks on spaces
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# ✅
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path="$(find . -name config)"
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cat "$path"
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```
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```bash
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# ❌ Using backticks for command substitution
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result=`echo hello`
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# ✅ — $() nests cleanly
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result=$(echo hello)
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```
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```bash
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# ❌ String comparison without quotes
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if [ $var = "hello" ]; then # breaks if var is empty or has spaces
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# ✅
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if [[ "$var" = "hello" ]]; then
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```
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**Rule: double-quote every `$variable` and `$(command)` unless you specifically need splitting.**
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---
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## 2. Conditionals & Tests {#conditionals}
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```bash
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# ❌ Single bracket test (POSIX but fragile)
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if [ -f "$file" -a -r "$file" ]; then
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# ✅ — [[ is safer, supports &&/||, no word splitting inside
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if [[ -f "$file" && -r "$file" ]]; then
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```
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```bash
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# ❌ Testing command exit status with if [ $? -eq 0 ]
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grep -q pattern file
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if [ $? -eq 0 ]; then echo "found"; fi
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# ✅ — test command directly
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if grep -q pattern file; then echo "found"; fi
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```
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```bash
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# ❌ String equality with == outside [[
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if [ "$a" == "$b" ]; then # == not POSIX in [ ]
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# ✅
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if [[ "$a" == "$b" ]]; then # Bash
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# or POSIX:
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if [ "$a" = "$b" ]; then
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```
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```bash
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# ❌ Arithmetic with [ ] and string comparison
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if [ "$count" -gt 10 ]; then
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# ✅ — (( )) for arithmetic
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if (( count > 10 )); then
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```
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---
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## 3. Loops & Iteration {#loops}
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```bash
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# ❌ Parsing ls output
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for f in $(ls *.txt); do process "$f"; done
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# ✅ — glob directly
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for f in *.txt; do
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[[ -e "$f" ]] || continue # handle no-match
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process "$f"
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done
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```
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```bash
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# ❌ Reading file line-by-line with for
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for line in $(cat file.txt); do # splits on words, not lines
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# ✅
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while IFS= read -r line; do
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process "$line"
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done < file.txt
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```
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```bash
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# ❌ Seq for counting
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for i in $(seq 1 10); do
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# ✅ — brace expansion (Bash)
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for i in {1..10}; do
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# or C-style:
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for (( i = 1; i <= 10; i++ )); do
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```
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```bash
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# ❌ Processing command output line-by-line with pipe (subshell trap)
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count=0
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cat file.txt | while read -r line; do
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(( count++ )) # count resets after loop — subshell
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done
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echo "$count" # always 0
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# ✅ — redirect, not pipe
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count=0
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while IFS= read -r line; do
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(( count++ ))
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done < file.txt
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echo "$count"
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```
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---
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## 4. Pipes & Process Substitution {#pipes}
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```bash
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# ❌ Chained grep | grep for AND
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grep "error" log.txt | grep "timeout"
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# ✅ — single grep with pattern
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grep -E "error.*timeout|timeout.*error" log.txt
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# or awk for complex logic:
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awk '/error/ && /timeout/' log.txt
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```
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```bash
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# ❌ cat + pipe (UUOC — Useless Use of Cat)
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cat file.txt | grep pattern
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# ✅
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grep pattern file.txt
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```
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```bash
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# ❌ Temp file for diff between commands
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cmd1 > /tmp/a.txt
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cmd2 > /tmp/b.txt
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diff /tmp/a.txt /tmp/b.txt
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rm /tmp/a.txt /tmp/b.txt
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# ✅ — process substitution
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diff <(cmd1) <(cmd2)
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```
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```bash
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# ❌ Ignoring pipe failures (only last command's exit code)
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false | true
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echo $? # 0 — false's failure hidden
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# ✅
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set -o pipefail
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false | true
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echo $? # 1
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```
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---
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## 5. Functions & Return Values {#functions}
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```bash
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# ❌ Using return for string values
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get_name() {
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return "Alice" # return is for exit codes (0-255)
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}
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# ✅ — echo + capture
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get_name() {
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echo "Alice"
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}
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name=$(get_name)
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```
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```bash
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# ❌ Global variables modified inside functions
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result=""
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compute() { result="done"; }
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# ✅ — use local, return via stdout
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compute() {
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local tmp
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tmp=$(do_work)
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echo "$tmp"
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}
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result=$(compute)
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```
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```bash
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# ❌ Function keyword (not POSIX)
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function my_func {
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# ✅
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my_func() {
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```
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---
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## 6. Error Handling {#errors}
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```bash
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# ❌ No error handling — script continues after failure
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cd /some/dir
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rm -rf * # if cd fails, deletes from wrong directory
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# ✅
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set -euo pipefail
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cd /some/dir || { echo "cd failed" >&2; exit 1; }
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rm -rf ./*
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```
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```bash
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# ❌ No cleanup on exit
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tmpfile=$(mktemp)
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# ... script might exit early, leaving tmpfile
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# ✅ — trap for cleanup
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tmpfile=$(mktemp)
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trap 'rm -f "$tmpfile"' EXIT
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```
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```bash
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# ❌ Silencing errors blindly
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command 2>/dev/null
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# ✅ — redirect only when you know what you're suppressing
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command 2>/dev/null || true # explicit: we expect and accept failure
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```
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**Start every script with `set -euo pipefail`. Remove selectively where needed.**
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---
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## 7. Anti-patterns specific to Bash {#antipatterns}
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| Anti-pattern | Preferred |
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| Parsing `ls` output | glob: `for f in *.txt` |
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| `cat file \| grep` | `grep pattern file` |
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| Unquoted `$var` | `"$var"` always |
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| `[ ]` for complex tests | `[[ ]]` |
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| Backtick substitution | `$(command)` |
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| `$?` check after command | `if command; then` |
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| `echo` for debug | `printf '%s\n'` (portable) |
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| No `set -euo pipefail` | always set at script top |
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| Temp files without cleanup | `trap 'rm -f "$tmp"' EXIT` |
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| `eval` with user input | avoid; use arrays for dynamic commands |
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| `#!/bin/sh` with Bash features | `#!/usr/bin/env bash` |
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| String math `expr 1 + 1` | `$(( 1 + 1 ))` |
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| `test -z` for number comparison | `(( ))` for arithmetic |
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## Limitations
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- These are language-specific guidelines and do not cover overall architectural decisions.
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- Over-compression might reduce readability; apply judgement.
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