Parameter substitution in the Korn shell or POSIX shell

The Korn shell, or POSIX shell, lets you perform parameter substitutions.

The following are substitutable parameters:

Item	Description
`${Parameter}`	The shell reads all the characters from the `${` to the matching `}` as part of the same word, even if that word contains braces or metacharacters. The value, if any, of the specified parameter is substituted. The braces are required when the `Parameter` parameter is followed by a letter, digit, or underscore that is not to be interpreted as part of its name, or when a named parameter is subscripted. If the specified parameter contains one or more digits, it is a positional parameter. A positional parameter of more than one digit must be enclosed in braces. If the value of the variable is `` or `@`), each positional parameter, starting with `$1`, is substituted (separated by a field separator character). If an array identifier with a subscript `` or `@` is used, then the value for each of the elements (separated by a field separator character) is substituted.
`${#Parameter}`	If the value of the `Parameter` parameter is `*` or `@`, the number of positional parameters is substituted. Otherwise, the length specified by the `Parameter` parameter is substituted.
`${#Identifier`[`*`]`}`	The number of elements in the array specified by the `Identifier` parameter is substituted.
`${Parameter:-Word}`	If the `Parameter` parameter is set and is not null, then its value is substituted; otherwise, the value of the `Word` parameter is substituted.
`${Parameter:=Word}`	If the `Parameter` parameter is not set or is null, then it is set to the value of the `Word` parameter. Positional parameters cannot be assigned in this way.
`${Parameter:?Word}`	If the `Parameter` parameter is set and is not null, then substitute its value. Otherwise, print the value of the `Word` variable and exit from the shell. If the `Word` variable is omitted, then a standard message is printed.
`${Parameter:+Word}`	If the `Parameter` parameter is set and is not null, then substitute the value of the `Word` variable.
`${Parameter#Pattern}` \| `${Parameter##Pattern}`	If the specified shell `Pattern` parameter matches the beginning of the value of the `Parameter` parameter, then the value of this substitution is the value of the `Parameter` parameter with the matched portion deleted. Otherwise, the value of the `Parameter` parameter is substituted. In the first form, the smallest matching pattern is deleted. In the second form, the largest matching pattern is deleted.
`${Parameter%Pattern}` \| `${Parameter%%Pattern}`	If the specified shell `Pattern` matches the end of the value of the `Parameter` variable, then the value of this substitution is the value of the `Parameter` variable with the matched part deleted. Otherwise, substitute the value of the `Parameter` variable. In the first form, the smallest matching pattern is deleted; in the second form, the largest matching pattern is deleted. In the previous expressions, the `Word` variable is not evaluated unless it is to be used as the substituted string. Thus, in the following example, the pwd command is executed only if the -d flag is not set or is null: `echo ${d:-$(pwd)}`

Note: If the : is omitted from the previous expressions, the shell checks only whether the Parameter parameter is set.