Bash

Quick Reference

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