Bash

Write reliable Bash scripts with proper quoting, error handling, and parameter expansion.

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