[B/D] acts upon separate lines of input, where a line of input is ended typically by pressing the ENTER key once, and consists of no more than 253 alphanumeric characters. (It is possible both to extend a [B/D] command line over more than one physical line, as we consider in §1.5, and to force more than one [B/D] command line onto one physical line, as we consider in §1.4.3. However, the maximum of 253 characters remains in force whatever the circumstances.) The legal alphanumeric characters are shown in Table 1.1. Otherwise, any illegal characters or invisible control characters supplied to [B/D] are treated as though they are spaces.
Each line of input is treated either as a command line, where [B/D] is intended to interpret a command and act accordingly, or not. The first non-space character of a line determines which is the case. Generally, all lines of input are assumed to be command lines unless the first non-space character is one of the following:
The first non-space character may alse be an opening square bracket, `[', implying that the line begins with the name of a string or some other quantity which will be parsed according to the rules given in the next section. In this situation, the line after parsing must obey the rules given above.
Lines consisting only of spaces are ignored.
A command line should consist of a valid [B/D] command, followed by any pertinent arguments. The arguments must be separated from the command by at least one space, or by one colon. The command itself must consist of contiguous characters. Thereafter, as many spaces as desired can be strewn amongst the characters forming the arguments. For example, the following commands are all legal and equivalent:
