history

早期freebsd实现

Command history

	There is a command line editor and history mechanism built
	into calc, which is active when stdin is a terminal.  When
	stdin is not a terminal, then the command line editor is
	disabled.

	Lines of input to calc are always terminated by the return
	(or enter) key.  When the return key is typed, then the current
	line is executed and is also saved into a command history list
	for future recall.

	Before the return key is typed, the current line can be edited
	using emacs-like editing commands.  As examples, ^A moves to
	the beginning of the line, ^F moves forwards through the line,
	backspace removes characters from the line, and ^K kills the
	rest of the line.

	Previously entered commands can be recalled by using the history
	list.  The history list functions in a LRU manner, with no
	duplicated lines.  This means that the most recently entered
	lines are always at the end of the history list where they are
	easiest to recall.

	Typing <esc>h lists all of the commands in the command history
	and numbers the lines.  The most recently executed line is always
	number 1, the next most recent number 2, and so on.  The numbering
	for a particular command therefore changes as lines are entered.

	Typing a number at the beginning of a line followed by <esc>g
	will recall that numbered line.  So that for example, 2<esc>g
	will recall the second most recent line that was entered.

	The ^P and ^N keys move up and down the lines in the history list.
	If they attempt to go off the top or bottom of the list, then a
	blank line is shown to indicate this, and then they wrap around
	to the other end of the list.

	Typing a string followed by a ^R will search backwards through
	the history and recall the most recent command which begins
	with that string.

	Typing ^O inserts the current line at the end of the history list
	without executing it, and starts a new line.  This is useful to
	rearrange old history lines to become recent, or to save a partially
	completed command so that another command can be typed ahead of it.

	If your terminal has arrow keys which generate escape sequences
	of a particular kind (<esc>[A and so on), then you can use
	those arrow keys in place of the ^B, ^F, ^P, and ^N keys.

	The actual keys used for editing are defined in a bindings file,
	usually called /usr/local/lib/calc/bindings.  Changing the entries 
	in this file will change the key bindings used for editing.  If the 
	file is not readable, then a message will be output and command
	line editing is disabled.  In this case you can only edit each
	line as provided by the terminal driver in the operating system.

	A shell command can be executed by typing '!cmd', where cmd
	is the command to execute.  If cmd is not given, then a shell
	command level is started.