Bash

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name: Bash slug: bash version: 1.0.2 description: Write reliable Bash scripts with proper quoting, error handling, and parameter expansion. metadata: {"clawdbot":{"emoji":"🖥️","requires":{"bins":["bash"]},"os":["linux","darwin"]}}

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Quick Reference

| Topic | File | |-------|------| | Arrays and loops | arrays.md | | Parameter expansion | expansion.md | | Error handling patterns | errors.md | | Testing and conditionals | testing.md |

Quoting Traps

Always quote variables—"$var" not $var, spaces break unquoted

"${arr[@]}" preserves elements—${arr[*]} joins into single string

Single quotes are literal—'$var' doesn't expand

Quote command substitution—"$(command)" not $(command)

Word Splitting and Globbing

Unquoted $var splits on whitespace—file="my file.txt"; cat $file fails

Unquoted * expands to files—quote or escape if literal: "*" or \*

set -f disables globbing—or quote everything properly

Test Brackets

[[ ]] preferred over [ ]—no word splitting, supports &&, ||, regex

[[ $var == pattern* ]]—glob patterns without quotes on right side

[[ $var =~ regex ]]—regex match, don't quote the regex

-z is empty, -n is non-empty—[[ -z "$var" ]] tests if empty

Subshell Traps

Pipes create subshells—cat file | while read; do ((count++)); done—count lost

Use while read < file or process substitution—while read; do ...; done < <(command)

( ) is subshell, { } is same shell—variables in ( ) don't persist

Exit Handling

set -e exits on error—but not in if, ||, && conditions

set -u errors on undefined vars—catches typos

set -o pipefail—pipeline fails if any command fails, not just last

trap cleanup EXIT—runs on any exit, even errors

Arrays

Declare: arr=(one two three)—or arr=() then arr+=(item)

Length: ${#arr[@]}—not ${#arr}

All elements: "${arr[@]}"—always quote

Indices: ${!arr[@]}—useful for sparse arrays

Parameter Expansion

Default value: ${var:-default}—use default if unset/empty

Assign default: ${var:=default}—also assigns to var

Error if unset: ${var:?error message}—exits with message

Substring: ${var:0:5}—first 5 chars

Remove prefix: ${var#pattern}—## for greedy

Arithmetic

$(( )) for math—result=$((a + b))

(( )) for conditions—if (( count > 5 )); then

No $ needed inside $(( ))—$((count + 1)) not $(($count + 1))

Common Mistakes

[ $var = "value" ] fails if var empty—use [ "$var" = "value" ] or [[ ]]

if [ -f $file ] with spaces—always quote: if [[ -f "$file" ]]

local in functions—without it, variables are global

read without -r—backslashes interpreted as escapes

echo portability—use printf for reliable formatting