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# Using History Interactively

This chapter describes how to use the GNU History Library interactively, from a user's standpoint.

## History Interaction

The History library provides a history expansion feature similar to the history expansion in `csh`. The following text describes the syntax you use to manipulate history information.

History expansion takes two parts. In the first part, determine which line from the previous history will be used for substitution. This line is called the event. In the second part, select portions of that line for inclusion into the current line. These portions are called words. GDB breaks the line into words in the same way that the Bash shell does, so that several English (or Unix) words surrounded by quotes are considered one word.

### Event Designators

An event designator is a reference to a command line entry in the history list.

`!`
  Start a history subsititution, except when followed by a space, tab, or the end of the line... `=` or `(`.
`!!`
  Refer to the previous command. This is a synonym for `!-1`.
`!n`
  Refer to command line n.
`!-n`
  Refer to the command line n lines back.
`!string`
  Refer to the most recent command starting with string.
`!?string`[`?`]
  Refer to the most recent command containing string.

### Word Designators

A `:` separates the event designator from the word designator. It can be omitted if the word designator begins with a `^`, `$`, `*` or `%`. Words are numbered from the beginning of the line, with the first word being denoted by a 0 (zero).

`0 (zero)`
  The zero'th word. For many applications, this is the command word.
`n`
  The n'th word.
`^`
  The first argument. that is, word 1.
`$`
  The last argument.
`%`
  The word matched by the most recent `?string?` search.
`x-y`
  A range of words; `-y` Abbreviates `0-y`.
`*`
  All of the words, excepting the zero'th. This is a synonym for `1-$`. It is not an error to use `*` if there is just one word in the event. The empty string is returned in that case.

### Modifiers

After the optional word designator, you can add a sequence of one or more of the following modifiers, each preceded by a `:`.

`#`
  The entire command line typed so far. This means the current command, not the previous command.
`h`
  Remove a trailing pathname component, leaving only the head.
`r`
  Remove a trailing suffix of the form ``.'`suffix, leaving the basename.
`e`
  Remove all but the suffix.
`t`
  Remove all leading pathname components, leaving the tail.
`p`
  Print the new command but do not execute it.

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