MPE/iX Shell and Utilities Reference Manual, Vol 1

let(1) MPE/iX Shell and Utilities let(1)
PORTABILITY
let is built into the KornShell on UNIX systems, and is not a Bourne Shell command.
NOTE
This command is built into the shell.
SEE ALSO
sh(1), expr(1) test(1)
1-286 Commands and Utilities