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Quick Reference

arrays.md

expansion.md

errors.md

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