HISTORY(3)

NAME
history - GNU History Library

COPYRIGHT
The GNU History Library is Copyright (C) 1989-2025 by the Free Software Foundation, Inc.

DESCRIPTION
Many programs read input from the user a line at a time. The GNU History library is able to keep track of those lines, associate arbitrary data with each line, and utilize information from previous lines when composing new ones.

The History library provides functions that allow applications to their history, the set of previously-typed lines, which it keeps in a list. Applications can choose which lines to save into a history list, how many commands to save, save a history list to a file, read a history list from a file, and display lines from the history in various formats.

HISTORY EXPANSION
The history library supports a history expansion feature that is identical to the history expansion in bash. This section describes what syntax features are available.

History expansions introduce words from the history list into the input stream, making it easy to repeat commands, insert the arguments to a previous command into the current input line, or fix errors in previous commands quickly.

History expansion is usually performed immediately after a complete line is read. It takes place in two parts. The first is to determine which history list entry to use during substitution. The second is to select portions of that entry to include into the current one.

The entry selected from the history is the event, and the portions of that entry that are acted upon are words. Various modifiers are available to manipulate the selected words. The entry is split into words in the same fashion as bash does when reading input, so that several words that would otherwise be separated are considered one word when surrounded by quotes (see the description of history_tokenize() below). The event designator selects the event, the optional word designator selects words from the event, and various optional modifiers are available to manipulate the selected words.

History expansions are introduced by the appearance of the history expansion character, which is ! by default. History expansions may appear anywhere in the input, but do not nest.

Only backslash (\) and single quotes can quote the history expansion character.

There is a special abbreviation for substitution, active when the quick substitution character (default ^) is the first character on the line. It selects the previous history list entry, using an event designator equivalent to !!, and substitutes one string for another in that entry. It is described below under Event Designators. This is the only history expansion that does not begin with the history expansion character.

  Event Designators
An event designator is a reference to an entry in the history list. The event designator consists of the portion of the word beginning with the history expansion character and ending with the word designator if present, or the end of the word. Unless the reference is absolute, events are relative to the current position in the history list.

!   Start a history substitution, except when followed by a blank, newline, carriage return, =, or (.
!n  Refer to history list entry n.
!-n Refer to the current entry minus n.
!!  Refer to the previous entry. This is a synonym for
!string
    Refer to the most recent command preceding the current position in the history list starting with string.
!?string[?]
    Refer to the most recent command preceding the current position in the history list containing string. The trailing ? may be omitted if string is followed immediately by a newline. If string is missing, this uses the string from the most recent search; it is an error if there is no previous search string.
^string1^string2^
    Quick substitution. Repeat the previous command, replacing string1 with string2. Equivalent to (see Modifiers below).
!#  The entire command line typed so far.

  Word Designators
Word designators are used to select desired words from the event. They are optional; if the word designator isn't supplied, the history expansion uses the entire event. A : separates the event specification from the word designator. It may 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 0 (zero). Words are inserted into the current line separated by single spaces.

0 (zero)
    The zeroth word. For the shell, and many other applications, this is the command word.
n   The nth word.
^   The first argument: word 1.
$   The last word. This is usually the last argument, but expands to the zeroth word if there is only one word in the line.
%   The first word matched by the most recent search, if the search string begins with a character that is part of a word. By default, searches begin at the end of each line and proceed to the beginning, so the first word matched is the one closest to the end of the line.
x-y A range of words; abbreviates
*   All of the words but the zeroth. This is a synonym for It is not an error to use * if there is just one word in the event; it expands to the empty string in that case.
x*  Abbreviates x-$.
x-  Abbreviates x-$ like x*, but omits the last word. If x is missing, it defaults to 0.

If a word designator is supplied without an event specification, the previous command is used as the event, equivalent to !!.

  Modifiers
After the optional word designator, the expansion may include a sequence of one or more of the following modifiers, each preceded by a These modify, or edit, the word or words selected from the history event.

h   Remove a trailing filename component, leaving only the head.
t   Remove all leading filename components, leaving the tail.
r   Remove a trailing suffix of the form .xxx, leaving the basename.
e   Remove all but the trailing suffix.
p   Print the new command but do not execute it.
q   Quote the substituted words, escaping further substitutions.
x   Quote the substituted words as with q, but break into words at blanks and newlines. The q and x modifiers are mutually exclusive; expansion uses the last one supplied.
s/old/new/
    Substitute new for the first occurrence of old in the event line. Any character may be used as the delimiter in place of /. The final delimiter is optional if it is the last character of the event line. A single backslash quotes the delimiter in old and new. If & appears in new, it is replaced with old. A single backslash quotes the &. If old is null, it is set to the last old substituted, or, if no previous history substitutions took place, the last string in a !?string[?] search. If new is null, each matching old is deleted.
&   Repeat the previous substitution.
g   Cause changes to be applied over the entire event line. This is used in conjunction with (e.g., or If used with any delimiter can be used in place of /, and the final delimiter is optional if it is the last character of the event line. An a may be used as a synonym for g.
G   Apply the following or modifier once to each word in the event line.

PROGRAMMING WITH HISTORY FUNCTIONS
This section describes how to use the History library in other programs.

  Introduction to History
A programmer using the History library has available functions for remembering lines on a history list, associating arbitrary data with a line, removing lines from the list, searching through the list for a line containing an arbitrary text string, and referencing any line in the list directly. In addition, a history expansion function is available which provides for a consistent user interface across different programs.

The user using programs written with the History library has the benefit of a consistent user interface with a set of well-known commands for manipulating the text of previous lines and using that text in new commands. The basic history manipulation commands are identical to the history substitution provided by bash.

The programmer can also use the readline library, which includes some history manipulation by default, and has the added advantage of command line editing.

Before declaring any functions using any functionality the History library provides in other code, an application writer should include the file in any file that uses the History library's features. It supplies extern declarations for all of the library's public functions and variables, and declares all of the public data structures.

  History Storage
The history list is an array of history entries. A history entry is declared as follows:

    typedef struct _hist_entry {
      char *line;
      char *timestamp;
      histdata_t data;
    } HIST_ENTRY;

The history list itself might therefore be declared as

The state of the History library is encapsulated into a single structure:

    /*
     * A structure used to pass around the current state of the history.
     */
    typedef struct _hist_state {
      HIST_ENTRY **entries; /* Pointer to entry records. */
      int offset;           /* The current record. */
      int length;           /* Number of records in list. */
      int size;             /* Number of records allocated. */
      int flags;
    } HISTORY_STATE;

If the flags member includes HS_STIFLED, the history has been stifled.

History Functions
This section describes the calling sequence for the various functions exported by the GNU History library.

  Initializing History and State Management
This section describes functions used to initialize and manage the state of the History library when you want to use the history functions in your program.

Begin a session in which the history functions might be used. This initializes the interactive variables.

Return a structure describing the current state of the input history.

Set the state of the history list according to state.

  History List Management
These functions manage individual entries on the history list, or set parameters managing the list itself.

Place string at the end of the history list. The associated data field (if any) is set to NULL. If the maximum number of history entries has been set using stifle_history(), and the new number of history entries would exceed that maximum, the oldest history entry is removed.

Change the time stamp associated with the most recent history entry to string.

Remove history entry at offset which from the history. The removed element is returned so you can free the line, data, and containing structure.

Free the history entry histent and any history library private data associated with it. Returns the application-specific data so the caller can dispose of it.

Make the history entry at offset which have line and data. This returns the old entry so the caller can dispose of any application-specific data. In the case of an invalid which, a NULL pointer is returned.

Clear the history list by deleting all the entries.

Stifle the history list, remembering only the last max entries. The history list will contain only max entries at a time.

Stop stifling the history. This returns the previously-set maximum number of history entries (as set by stifle_history()). history was stifled. The value is positive if the history was stifled, negative if it wasn't.

Returns non-zero if the history is stifled, zero if it is not.

  Information About the History List
These functions return information about the entire history list or individual list entries.

Return a NULL terminated array of HIST_ENTRY * which is the current input history. Element 0 of this list is the beginning of time. If there is no history, return NULL.

Returns the offset of the current history element.

Return the history entry at the current position, as determined by where_history(). If there is no entry there, return a NULL pointer.

Return the history entry at position offset. The range of valid values of offset starts at history_base and ends at history_length - 1. If there is no entry there, or if offset is outside the valid range, return a NULL pointer.

Return the time stamp associated with the history entry passed as the argument.

Return the number of bytes that the primary history entries are using. This function returns the sum of the lengths of all the lines in the history.

  Moving Around the History List
These functions allow the current index into the history list to be set or changed.

Set the current history offset to pos, an absolute index into the list. Returns 1 on success, 0 if pos is less than zero or greater than the number of history entries.

Back up the current history offset to the previous history entry, and return a pointer to that entry. If there is no previous entry, return a NULL pointer.

If the current history offset refers to a valid history entry, increment the current history offset. If the possibly-incremented history offset refers to a valid history entry, return a pointer to that entry; otherwise, return a NULL pointer.

  Searching the History List
These functions allow searching of the history list for entries containing a specific string. Searching may be performed both forward and backward from the current history position. The search may be anchored, meaning that the string must match at the beginning of the history entry.

Search the history for string, starting at the current history offset. If direction is less than 0, then the search is through previous entries, otherwise through subsequent entries. If string is found, then the current history index is set to that history entry, and the value returned is the offset in the line of the entry where string was found. Otherwise, nothing is changed, and the function returns -1.

Search the history for string, starting at the current history offset. The search is anchored: matching lines must begin with string. If direction is less than 0, then the search is through previous entries, otherwise through subsequent entries. If string is found, then the current history index is set to that entry, and the return value is 0. Otherwise, nothing is changed, and the function returns -1.

Search for string in the history list, starting at pos, an absolute index into the list. If direction is negative, the search proceeds backward from pos, otherwise forward. Returns the absolute index of the history element where string was found, or -1 otherwise.

  Managing the History File
The History library can read the history from and write it to a file. This section documents the functions for managing a history file.

Add the contents of filename to the history list, a line at a time. If filename is NULL, then read from ~/.history. Returns 0 if successful, or errno if not.

Read a range of lines from filename, adding them to the history list. Start reading at line from and end at to. If from is zero, start at the beginning. If to is less than from, then read until the end of the file. If filename is NULL, then read from ~/.history. Returns 0 if successful, or errno if not.

Write the current history to filename, overwriting filename if necessary. If filename is NULL, then write the history list to ~/.history. Returns 0 on success, or errno on a read or write error.

Append the last nelements of the history list to filename. If filename is NULL, then append to ~/.history. Returns 0 on success, or errno on a read or write error.

Truncate the history file filename, leaving only the last nlines lines. If filename is NULL, then ~/.history is truncated. Returns 0 on success, or errno on failure.

  History Expansion
These functions implement history expansion.

Expand string, placing the result into output, a pointer to a string. Returns:
    0   If no expansions took place (or, if the only change in the text was the removal of escape characters preceding the history expansion character);
    1   if expansions did take place;
    -1  if there was an error in expansion;
    2   if the returned line should be displayed, but not executed, as with the :p modifier.
If an error occurred in expansion, then output contains a descriptive error message.

Returns the text of the history event beginning at string + *cindex. *cindex is modified to point to after the event specifier. At function entry, cindex points to the index into string where the history event specification begins. qchar is a character that is allowed to end the event specification in addition to the terminating characters.

Return an array of tokens parsed out of string, much as the shell might. The tokens are split on the characters in the history_word_delimiters variable, and shell quoting conventions are obeyed.

Extract a string segment consisting of the first through last arguments present in string. Arguments are split using history_tokenize().

  History Variables
This section describes the externally-visible variables exported by the GNU History Library.

The logical offset of the first entry in the history list.

The number of entries currently stored in the history list.

The maximum number of history entries.  This must be changed using
stifle_history().

If non-zero, timestamps are written to the history file, so they can be
preserved between sessions.  The default value is 0, meaning that
timestamps are not saved.
The current timestamp format uses the value of history_comment_char
to delimit timestamp entries in the history file.
If that variable does
not have a value (the default),
the history library will not write timestamps.

The character that introduces a history event.
The default is !.
Setting this to 0 inhibits history expansion.

The character that invokes word substitution if found at the start of
a line.
The default is ^.

During tokenization, if this character is seen as the first character
of a word, then it and all subsequent characters up to a newline are
ignored, suppressing history expansion for the remainder of the line.
This is disabled by default.

The characters that separate tokens for history_tokenize().
The default value is " \t\n()<>;&|".

The list of characters which inhibit history expansion if found immediately
following history_expansion_char.  The default is space, tab, newline,
\r, and =.

The list of additional characters which can delimit a history search
string, in addition to space, tab, : and ? in the case of
a substring search.  The default is empty.

If non-zero, the history expansion code implements shell-like quoting:
single-quoted words are not scanned for the history expansion
character or the history comment character, and double-quoted words may
have history expansion performed, since single quotes are not special
within double quotes.
The default value is 0.

An application may set this variable to indicate that the current line
being expanded is subject to existing quoting.
If set to ',
history expansion assumes that the line is single-quoted and
inhibit expansion until it reads an unquoted closing single quote;
if set to ", history expansion assumes the line is double quoted
until it reads an unquoted closing double quote.
If set to zero, the default,
history expansion assumes the line is not quoted and
treats quote characters within the line as described above.
This is only effective if history_quotes_inhibit_expansion is set.

This should be set to the address of a function that takes two arguments:
a char * (string)
and an int index into that string (i).
It should return a non-zero value if the history expansion starting at
string[i] should not be performed; zero if the expansion should
be done.
It is intended for use by applications like bash that use the history
expansion character for additional purposes.
By default, this variable is set to NULL.

FILES
Default filename for reading and writing saved history

SEE ALSO
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
bash(1)
readline(3)

AUTHORS
Brian Fox, Free Software Foundation
bfox@gnu.org

Chet Ramey, Case Western Reserve University
chet.ramey@case.edu

BUG REPORTS
If you find a bug in the history library, you should report it. But first, you should make sure that it really is a bug, and that it appears in the latest version of the history library that you have.

Once you have determined that a bug actually exists, mail a bug report to bug-readline@gnu.org. If you have a fix, you are welcome to mail that as well! Suggestions and bug reports may be mailed to bug-readline@gnu.org or posted to the Usenet newsgroup gnu.bash.bug.

Comments and bug reports concerning this manual page should be directed to chet.ramey@case.edu.