I think the idea is that the `:` builtin allows expansion of arguments without actually doing anything else. However, the temporary variable $_ is filled with t

I think the idea is that the `:` builtin allows expansion of arguments without actually doing anything else. However, the temporary variable $_ is filled with the content of the expression. That is, after the first : "${1#"${1%%[![:space:]]*}"}" The $_ temporary variable contains the result of removing the leading spaces. In the next line, the spaces at the end are removed from the temporary variable with the "${_%..." syntax. You can test this in your own shell by e.g. doing: : $PATH echo $_