Variables are defined and initialized with the set command (17.5). set output=10 defines the variable output and sets it to the (real) number 10. Predefined variables, which are used inside ngspice for specific purposes, are listed in Chapt. 17.7. Variables are accessible globally. The values of variables may be used in commands by writing $varname where the value of the variable is to appear, e.g. $output. The special variable $$ refers to the process ID of the program. With $< a line of input is read from the terminal. If a variable is assigned with to with $&word, then word must be a vector (see below), and word's numeric value is taken to be the new value of the variable. If foo is a valid variable, and is of type list, then the expression $foo[low-high] expands to a range of elements. Either the upper or lower index may be left out, and in addition to slicing also reversing of a list is possible through $foo[len-0] (len is the length of the list, the first valid index is always 1). Furthermore, the notation $?foo evaluates to 1 if the variable foo is defined, 0 otherwise, and $#foo evaluates to the number of elements in foo if it is a list, 1 if it is a number or string, and 0 if it is a Boolean variable.