M provides timeouts as a tool to retain program control over commands of indefinite duration. A timeout consists of a colon (:) delimiter on an argument, followed by a numeric expression specifying the number of seconds for M to attempt to execute the command. When the timeout is zero (0), M makes a single attempt to complete the command.

When a command has a timeout, M maintains the $TEST intrinsic special variable as the command completes. If the command completes successfully, M sets $TEST to TRUE (1). If the command times out before successful completion, M sets $TEST to FALSE (0). When a command argument does not specify a timeout, M does not maintain $TEST.

The following commands accept timeouts:

When a READ times out, M returns any characters that have arrived between the start of the command and the timeout. M does not produce any partial results for any of the other timed commands.