Korn Shell Compound Commands . Built-in Commands . Conditional Expressions . A compound command is a list of simple commands, a pipeline, or it can begin with a reserved word. Most of the time you will use compound commands such as if , while , and for when you are writing shell scripts.
Execution ends when there are no more words in the list.
The PS3 prompt is printed and a line is read from the standard input. If this line consists of the number of one of the listed words, then the value of the Identifier parameter is set to the word corresponding to this number. If the line read from standard input is empty, the selection list is printed again. Otherwise, the value of the Identifier parameter is set to null. The form of the patterns is the same as that used for file name substitution.