M commands may be abbreviated to a defined prefix. Most commands have arguments. However, some commands have either optional arguments or no arguments. When a command has no argument and is followed by more commands on the same line, at least two spaces (<SP>) must follow the command without arguments. Commands that accept arguments generally accept multiple arguments on the same command. M treats multiple arguments the same as multiple occurrences of the same command, each with its own argument.
M provides postconditionals as a tool for placing a condition on the execution of a single command and, in some cases, a single command argument. A postconditional consists of a colon (:) delimiter followed by a truth-valued expression. When the expression evaluates to true, M executes the command occurrence. When the expression evaluates to false, M does not execute the command occurrence.
Command postconditionals appear immediately following a command and apply to all arguments for the command when it has multiple arguments. All commands except commands that themselves have a conditional aspect accept a command postconditional. Among the M standard commands, ELSE, FOR, and IF do not accept command postconditionals. All the GT.M command extensions accept command postconditionals.
Commands that affect the flow of control may accept postconditionals on individual command arguments. Because multiple arguments act as multiple commands, this is a straight-forward application of the same principal as command postconditional. The only M standard commands that accept argument postconditionals are DO, GOTO, and XECUTE. The GT.M command extensions that accept argument postconditionals are BREAK, ZGOTO, and ZSYSTEM.
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.
GT.M has been designed to allow large timeout values, and to protect against arithmetic overflow when converting large timeout values to internal representations. 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.