Section A.3. Modified Backus-Naur Notation

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A.3. Modified Backus-Naur Notation

The syntax of the GNU utilities and Linux system calls in this book are presented in a modified version of a language known as Backus-Naur Form, or BNF for short. In a BNF description, the sequences in Figure A-5 have a special meaning.

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Figure A-5. BNF notations used in this book.

Sequence	Meaning
[ strings ]	Strings may appear zero or one time.
{ strings }*	Strings may appear zero or more times.
{ strings }+	Strings may appear one or more times.
string1|string2	string1 or string2 may appear.
-optionlist	Zero or more options may follow a dash.

The last sequence is the Linux/GNU-oriented modification, which allows me to avoid placing large numbers of brackets around command-line options. To indicate a [, {, |, or - without its special meaning, I precede it with a \ character.

Some variations of commands depend on which option you choose. I indicate this by supplying a separate syntax description for each variation. For example, take a look at the syntax description of the at utility (Figure A-6).

Utility: at -csm time [ date [, year ]][ +increment][ script]

at -r { jobId}+

at -l { jobId}*

The first version of the at utility is selected by any combination of the command-line options -c, -s, and -m. These must then be followed by a time and an optional date specifier. The optional date specifier may be followed by an optional year specifier. Additionally, an increment may be specified and/or a script name.

The second version of at is selected by a -r option, and may be followed by one or more job ID numbers.

The third version of at is selected by the -l option, and may be followed by zero or more job ID numbers.