history(n)		    Tcl Built-In Commands		   history(n)


  history - Manipulate the history list

  history ?option? ?arg arg ...?


  The  history	command	 performs  one	of  several  operations	 related   to
  recently-executed  commands  recorded	 in  a	history	 list.	Each of these
  recorded commands is referred to as an ``event''.  When specifying an event
  to the history command, the following forms may be used:

  [1]  A number:  if positive, it refers to the event with that	 number	 (all
       events  are  numbered  starting	at 1).	If the number is negative, it
       selects an event relative to the current event (-1 refers to the	 pre-
       vious event, -2 to the one before that, and so on).

  [2]  A string:  selects the most recent event that matches the string.   An
       event  is  considered  to match the string either if the string is the
       same as the first characters of the event, or if	 the  string  matches
       the event in the sense of the string match command.

  The history command can take any of the following forms:

       Same as history info, described below.

  history add command ?exec?
       Adds the command argument to the history list as a new event.  If exec
       is  specified  (or  abbreviated) then the command is also executed and
       its result is returned.	If exec isn't specified then an empty  string
       is returned as result.

  history change newValue ?event?
       Replaces the value recorded for an event with newValue.	Event  speci-
       fies  the  event	 to  replace,  and defaults to the current event (not
       event -1).  This command is intended for use in commands	 that  imple-
       ment new forms of history substitution and wish to replace the current
       event (which  invokes  the  substitution)  with	the  command  created
       through substitution.  The return value is an empty string.

  history event ?event?
       Returns the value of the event given by event.  Event defaults to  -1.
       This command causes history revision to occur: see below for details.

  history info ?count?
       Returns a formatted string (intended for humans to  read)  giving  the
       event  number  and contents for each of the events in the history list
       except the current event.  If count is specified then  only  the	 most
       recent count events are returned.

  history keep count
       This command may be used to change the size of  the  history  list  to
       count  events.  Initially, 20 events are retained in the history list.
       This command returns an empty string.

  history nextid
       Returns the number of the next event to be  recorded  in	 the  history
       list.   It  is  useful  for  things  like printing the event number in
       command-line prompts.

  history redo ?event?
       Re-executes the command indicated by  event  and	 return	 its  result.
       Event  defaults to -1.  This command results in history revision:  see
       below for details.

  history substitute old new ?event?
       Retrieves the command given by event  (-1  by  default),	 replace  any
       occurrences of old by new in the command (only simple character equal-
       ity is supported; no wild cards), execute the resulting	command,  and
       return  the result of that execution.  This command results in history
       revision:  see below for details.

  history words selector ?event?
       Retrieves from the command given by event (-1 by	 default)  the	words
       given  by  selector,  and  return those words in a string separated by
       spaces.	The selector argument has three forms.	If  it	is  a  single
       number  then  it selects the word given by that number (0 for the com-
       mand name, 1 for its first argument, and so on).	 If  it	 consists  of
       two  numbers  separated	by  a dash, then it selects all the arguments
       between those two.  Otherwise selector is treated as  a	pattern;  all
       words  matching	that  pattern  (in  the	 sense	of  string match) are
       returned.  In the numeric forms $ may be used to select the last	 word
       of  a  command.	 For  example, suppose the most recent command in the
       history list is

	 format	 {%s is %d years old} Alice [expr $ageInMonths/12]
       Below are some history commands and the results they would produce:

	 history words $ [expr $ageInMonths/12]
	 history words 1-2{%s is %d years  old} Alice
	 history words *a*o*{%s is %d years old} [expr $ageInMonths/12]
       History words results in history revision:  see below for details.


  The history options event, redo, substitute, and words result in  ``history
  revision''.  When one of these options is invoked then the current event is
  modified to eliminate the history command and replace it with the result of
  the  history command.	 For example, suppose that the most recent command in
  the history list is

       set a [expr $b+2]
  and suppose that the next command invoked is one of the ones	on  the	 left
  side	of  the	 table	below.	 The command actually recorded in the history
  event will be the corresponding one on the right side of the table.

       history redo    set a [expr $b+2]
       history s a b   set b [expr $b+2]
       set c [history w 2]set c [expr $b+2]
  History revision is needed because event specifiers like -1 are only	valid
  at a particular time:	 once more events have been added to the history list
  a different event specifier would be needed.	History revision occurs	 even
  when	history	 is  invoked  indirectly  from the current event (e.g. a user
  types a command that invokes a Tcl procedure that  invokes  history):	  the
  top-level  command whose execution eventually resulted in a history command
  is replaced.	If you wish to invoke commands	like  history  words  without
  history  revision,  you  can	use history event to save the current history
  event and then use history change to restore it later.

  event, history, record, revision