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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 previous event, -2 to the one before that, and so on).
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.
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 specifies the event to replace, and defaults to the current event (not event -1). This command is intended for use in commands that implement 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.
Returns the value of the event given by event. Event defaults to -1. This command causes history revision to occur: see below for details.
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.
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.
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.
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 equality 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 command 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:
| Command | Result |
| 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.
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.
| Command Typed | Command Recorded |
| 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.
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