Variable Substitution

ksh93 provides structured variables, such as pos.x and pos.y. To create either one, pos must already exist, and braces must be used to retrieve their values. Names beginning with .sh are reserved for use by ksh.

No spaces should be used in the following expressions. The colon (:) is optional; if it’s included, var must be nonnull as well as set.

var=value ...

Set each variable var to a value.

${ var }

Use value of var; braces are optional if var is separated from the following text. They are required for array variables, and in ksh93 if a variable name contains periods.

${ var :- value }

Use var if set; otherwise, use value.

${ var := value }

Use var if set; otherwise, use value and assign value to var.

${ var :? value }

Use var if set; otherwise, print value and exit (if not interactive). If value isn’t supplied, print the phrase “parameter null or not set.”

${ var :+ value }

Use value if var is set; otherwise, use nothing.

${# var }

Use the length of var.

${#*}

Use the number of positional parameters.

${#@}

Same.

${ var # pattern }

Use value of var after removing pattern from the left. Remove the shortest matching piece.

${ var ## pattern }

Same as # pattern, but remove the longest matching piece.

${ var % pattern }

Use value of var after removing pattern from the right. Remove the shortest matching piece.

${ var %% pattern }

Same as % pattern, but remove the longest matching piece.

In ksh93 and Bash:

${!prefix ...

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