RISC OS 3 User Guide(PDF). [S.l.]: Acorn Computers Limited. 1 de março de 1992. p. 125
books.google.com
Parker, Steve (2011). «11: Choosing and using shells». Shell Scripting: Expert Recipes for Linux, Bash and more. Col: Programmer to programmer. Indianapolis: John Wiley & Sons. p. 262. ISBN9781118166321. Consultado em 23 de março de 2017. The shell has four different command prompts, called PS1, P52, P53, and PS4. PS stands for Prompt String.