The READ command transfers the input from the current device to a global or local variable specified as a READ argument. For convenience, READ also accepts arguments that perform limited output to the current device.
The format of the READ command is:
R[EAD][:tvexpr] (glvn|*glvn|glvn#intexpr)[:numexpr]|strlit|fcc[,...]
The optional truth-valued expression immediately following the command is a command postconditional that controls whether or not GT.M executes the command.
A subscripted or unsubscripted global or local variable name specifies a variable into which to store the input; the variable does not have to exist prior to the READ; if the variable does exist prior to the READ, the READ replaces its old value.
When an asterisk (*) immediately precedes the variable name, READ accepts one character of input and places the ASCII code for that character into the variable.
When a number-sign (#) and a positive non-zero integer expression immediately follow the variable name, the integer expression determines the maximum number of characters accepted as input to the read; such reads terminate when GT.M reads the number of characters specified by the integer expression or a terminator character in the input stream or the optional timeout expires, whichever occurs first.
The optional numeric expression specifies a time in seconds at most, for which the command waits for input to be terminated. When a timeout is specified, if the input has been terminated before the timeout expires, $TEST is set to 1 (true), otherwise, $TEST is set to 0 (false). When a READ times out, the target variable takes the value of the string received before the timeout.
To provide a concise means of issuing prompts, GT.M sends string literal and format control character (!,?intexpr,#) arguments of a READ to the current device as if they were arguments of a WRITE.
An indirection operator and an expression atom evaluating to a list of one or more READ arguments form a legal argument for a READ.
In UTF-8 mode, the READ command uses the character set value specified on the device OPEN as the character encoding of the input device. If character set "M" or "UTF-8" is specified, the data is read with no transformation. If character set is "UTF-16", "UTF-16LE", or "UTF-16BE", the data is read with the specified encoding and transformed to UTF-8. If the READ command encounters an illegal character or a character outside the selected representation, it generates a run-time error. The READ command recognizes all Unicode® line terminators for non-FIXED devices.
For more information on READ, devices, input, output and format control characters, refer to Chapter 9: “Input/Output Processing”.