gdb: Command Syntax
3.1 Command Syntax
==================
A GDB command is a single line of input. There is no limit on how long
it can be. It starts with a command name, which is followed by
arguments whose meaning depends on the command name. For example, the
command 'step' accepts an argument which is the number of times to step,
as in 'step 5'. You can also use the 'step' command with no arguments.
Some commands do not allow any arguments.
GDB command names may always be truncated if that abbreviation is
unambiguous. Other possible command abbreviations are listed in the
documentation for individual commands. In some cases, even ambiguous
abbreviations are allowed; for example, 's' is specially defined as
equivalent to 'step' even though there are other commands whose names
start with 's'. You can test abbreviations by using them as arguments
to the 'help' command.
A blank line as input to GDB (typing just <RET>) means to repeat the
previous command. Certain commands (for example, 'run') will not repeat
this way; these are commands whose unintentional repetition might cause
trouble and which you are unlikely to want to repeat. User-defined
commands can disable this feature; see 
dont-repeat Define.
The 'list' and 'x' commands, when you repeat them with <RET>,
construct new arguments rather than repeating exactly as typed. This
permits easy scanning of source or memory.
GDB can also use <RET> in another way: to partition lengthy output,
in a way similar to the common utility 'more' (Screen Size Screen
Size.). Since it is easy to press one <RET> too many in this situation,
GDB disables command repetition after any command that generates this
sort of display.
Any text from a '#' to the end of the line is a comment; it does
nothing. This is useful mainly in command files (Command Files
Command Files.).
The 'Ctrl-o' binding is useful for repeating a complex sequence of
commands. This command accepts the current line, like <RET>, and then
fetches the next line relative to the current line from the history for
editing.
Info Catalog
gdb: Commands
gdb: Completion