bash

Quoting

• 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