# PARAMETER EXPANSION

## Indirection

| Parameter | What does it do? |
| --------- | ---------------- |
| `${!VAR}` | Access a variable based on the value of `VAR`.
| `${!VAR*}` | Expand to `IFS` separated list of variable names starting with `VAR`. |
| `${!VAR@}` | Expand to `IFS` separated list of variable names starting with `VAR`. If double-quoted, each variable name expands to a separate word. |


## Replacement

| Parameter | What does it do? |
| --------- | ---------------- |
| `${VAR#PATTERN}` | Remove shortest match of pattern from start of string. |
| `${VAR##PATTERN}` | Remove longest match of pattern from start of string. |
| `${VAR%PATTERN}` | Remove shortest match of pattern from end of string. |
| `${VAR%%PATTERN}` | Remove longest match of pattern from end of string. |
| `${VAR/PATTERN/REPLACE}` | Replace first match with string.
| `${VAR//PATTERN/REPLACE}` | Replace all matches with string.
| `${VAR/PATTERN}` | Remove first match.
| `${VAR//PATTERN}` | Remove all matches.

## Length

| Parameter | What does it do? |
| --------- | ---------------- |
| `${#VAR}` | Length of var in characters.
| `${#ARR[@]}` | Length of array in elements.

## Expansion

| Parameter | What does it do? |
| --------- | ---------------- |
| `${VAR:OFFSET}` | Remove first `N` chars from variable.
| `${VAR:OFFSET:LENGTH}` | Get substring from `N` character to `N` character. <br> (`${VAR:10:10}`: Get sub-string from char `10` to char `20`)
| `${VAR:: OFFSET}` | Get first `N` chars from variable.
| `${VAR:: -OFFSET}` | Remove last `N` chars from variable.
| `${VAR: -OFFSET}` | Get last `N` chars from variable.
| `${VAR:OFFSET:-OFFSET}` | Cut first `N` chars and last `N` chars. | `bash 4.2+` |

## Case Modification

| Parameter | What does it do? | CAVEAT |
| --------- | ---------------- | ------ |
| `${VAR^}` | Uppercase first character. | `bash 4+` |
| `${VAR^^}` | Uppercase all characters. | `bash 4+` |
| `${VAR,}` | Lowercase first character. | `bash 4+` |
| `${VAR,,}` | Lowercase all characters. | `bash 4+` |
| `${VAR~}` | Reverse case of first character. | `bash 4+` |
| `${VAR~~}` | Reverse case of all characters. | `bash 4+` |


## Default Value

| Parameter | What does it do? |
| --------- | ---------------- |
| `${VAR:-STRING}` | If `VAR` is empty or unset, use `STRING` as its value.
| `${VAR-STRING}` | If `VAR` is unset, use `STRING` as its value.
| `${VAR:=STRING}` | If `VAR` is empty or unset, set the value of `VAR` to `STRING`.
| `${VAR=STRING}` | If `VAR` is unset, set the value of `VAR` to `STRING`.
| `${VAR:+STRING}` | If `VAR` is not empty, use `STRING` as its value.
| `${VAR+STRING}` | If `VAR` is set, use `STRING` as its value.
| `${VAR:?STRING}` | Display an error if empty or unset.
| `${VAR?STRING}` | Display an error if unset.


<!-- CHAPTER END -->