# GNU History Library

## 1 Using History Interactively

This chapter describes how to use the gnu History Library interactively, from a user's standpoint. It should be considered a user's guide. For information on using the gnu History Library in your own programs, see Programming with GNU History.

Up: Using History Interactively

### 1.1 History Expansion

The History library provides a history expansion feature that is similar to the history expansion provided by `csh`. This section describes the syntax used to manipulate the history information.

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 takes place in two parts. The first is to determine which line from the history list should be used during substitution. The second is to select portions of that line for inclusion into the current one. The line selected from the history is called the event, and the portions of that line that are acted upon are called words. Various modifiers are available to manipulate the selected words. The line is broken into words in the same fashion that Bash does, so that several words surrounded by quotes are considered one word. History expansions are introduced by the appearance of the history expansion character, which is ! by default.

Next: Word Designators, Up: History Interaction

#### 1.1.1 Event Designators

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

`!`
  Start a history substitution, except when followed by a space, tab, the end of the line, = or (.
`!`n
  Refer to command line n.
`!-`n
  Refer to the command n lines back.
`!!`
  Refer to the previous command. This is a synonym for !-1.
`!`string
  Refer to the most recent command starting with string.
`!?`string`[?]`
  Refer to the most recent command containing string. The trailing ? may be omitted if the string is followed immediately by a newline.
`^`string1`^`string2`^`
  Quick Substitution. Repeat the last command, replacing string1 with string2. Equivalent to `!!:s/`string1`/`string2`/`.
`!#`
  The entire command line typed so far.

Next: Modifiers, Previous: Event Designators, Up: History Interaction

#### 1.1.2 Word Designators

Word designators are used to select desired words from the 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.

For example,

`!!`
  designates the preceding command. When you type this, the preceding command is repeated in toto.
`!!:$`
  designates the last argument of the preceding command. This may be shortened to `!$`.
`!fi:2`
  designates the second argument of the most recent command starting with the letters `fi`.

Here are the word designators:

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

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

Previous: Word Designators, Up: History Interaction

#### 1.1.3 Modifiers

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

`h`
  Remove a trailing pathname component, leaving only the head.
`t`
  Remove all leading pathname components, leaving the tail.
`r`
  Remove a trailing suffix of the form .suffix, leaving the basename.
`e`
  Remove all but the trailing suffix.
`p`
  Print the new command but do not execute it.
`s/`old`/`new`/`
  Substitute new for the first occurrence of old in the event line. Any delimiter may be used in place of /. The delimiter may be quoted in old and new with a single backslash. If & appears in new, it is replaced by old. A single backslash will quote the &. The final delimiter is optional if it is the last character on the input line.
`&`
  Repeat the previous substitution.
`g`
  Cause changes to be applied over the entire event line. Used in conjunction with s, as in `gs/`old`/`new`/`, or with &.

## 2 Programming with GNU History

This chapter describes how to interface programs that you write with the gnu History Library. It should be considered a technical guide. For information on the interactive use of gnu History, see Using History Interactively.

Next: History Storage, Up: Programming with GNU History

### 2.1 Introduction to History

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 in composing new ones.

The 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 similar to the history substitution provided by `csh`.

If the programmer desires, he can 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 `<readline/history.h>` 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.

Next: History Functions, Previous: Introduction to History, Up: Programming with GNU History

### 2.2 History Storage

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

         typedef void *histdata_t;

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

The history list itself might therefore be declared as

         HIST_ENTRY **the_history_list;

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 the entries themselves. */
           int offset;           /* The location pointer within this array. */
           int length;           /* Number of elements within this array. */
           int size;             /* Number of slots allocated to this array. */
           int flags;
         } HISTORY_STATE;

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

Next: History Variables, Previous: History Storage, Up: Programming with GNU History

### 2.3 History Functions

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

Next: History List Management, Up: History Functions

#### 2.3.1 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.

— Function: void using_history (void)

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

— Function: HISTORY_STATE * history_get_history_state (void)

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

— Function: void history_set_history_state (HISTORY_STATE *state)

> Set the state of the history list according to state.

Next: Information About the History List, Previous: Initializing History and State Management, Up: History Functions

#### 2.3.2 History List Management

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

— Function: void add_history (const char *string)

> Place string at the end of the history list. The associated data field (if any) is set to `NULL`.

— Function: HIST_ENTRY * remove_history (int which)

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

— Function: HIST_ENTRY * replace_history_entry (int which, const char *line, histdata_t data)

> Make the history entry at offset which have line and data. This returns the old entry so you can dispose of the data. In the case of an invalid which, a `NULL` pointer is returned.

— Function: void clear_history (void)

> Clear the history list by deleting all the entries.

— Function: void stifle_history (int max)

> Stifle the history list, remembering only the last max entries.

— Function: int unstifle_history (void)

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

— Function: int history_is_stifled (void)

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

Next: Moving Around the History List, Previous: History List Management, Up: History Functions

#### 2.3.3 Information About the History List

These functions return information about the entire history list or individual list entries.

— Function: HIST_ENTRY ** history_list (void)

> 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`.

— Function: int where_history (void)

> Returns the offset of the current history element.

— Function: HIST_ENTRY * current_history (void)

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

— Function: HIST_ENTRY * history_get (int offset)

> Return the history entry at position offset, starting from `history_base` (see History Variables). If there is no entry there, or if offset is greater than the history length, return a `NULL` pointer.

— Function: int history_total_bytes (void)

> 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.

Next: Searching the History List, Previous: Information About the History List, Up: History Functions

#### 2.3.4 Moving Around the History List

These functions allow the current index into the history list to be set or changed.

— Function: int history_set_pos (int pos)

> 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.

— Function: HIST_ENTRY * previous_history (void)

> 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.

— Function: HIST_ENTRY * next_history (void)

> Move the current history offset forward to the next history entry, and return the a pointer to that entry. If there is no next entry, return a `NULL` pointer.

Next: Managing the History File, Previous: Moving Around the History List, Up: History Functions

#### 2.3.5 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.

— Function: int history_search (const char *string, int direction)

> 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 a -1 is returned.

— Function: int history_search_prefix (const char *string, int direction)

> 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 a -1 is returned.

— Function: int history_search_pos (const char *string, int direction, int pos)

> 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.

Next: History Expansion, Previous: Searching the History List, Up: History Functions

#### 2.3.6 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.

— Function: int read_history (const char *filename)

> 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.

— Function: int read_history_range (const char *filename, int from, int to)

> 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.

— Function: int write_history (const char *filename)

> 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.

— Function: int append_history (int nelements, const char *filename)

> 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.

— Function: int history_truncate_file (const char *filename, int nlines)

> 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.

Previous: Managing the History File, Up: History Functions

#### 2.3.7 History Expansion

These functions implement history expansion.

— Function: int history_expand (char *string, char **output)

> Expand string, placing the result into output, a pointer to a string (see History Interaction). 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 (see Modifiers).

> If an error ocurred in expansion, then output contains a descriptive error message.

— Function: char * get_history_event (const char *string, int *cindex, int qchar)

> 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 “normal” terminating characters.

— Function: char ** history_tokenize (const char *string)

> 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.

— Function: char * history_arg_extract (int first, int last, const char *string)

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

Next: History Programming Example, Previous: History Functions, Up: Programming with GNU History

### 2.4 History Variables

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

— Variable: int history_base

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

— Variable: int history_length

> The number of entries currently stored in the history list.

— Variable: int history_max_entries

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

— Variable: char history_expansion_char

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

— Variable: char history_subst_char

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

— Variable: char history_comment_char

> 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.

— Variable: char * history_word_delimiters

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

— Variable: char * history_no_expand_chars

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

— Variable: char * history_search_delimiter_chars

> 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.

— Variable: int history_quotes_inhibit_expansion

> If non-zero, single-quoted words are not scanned for the history expansion character. The default value is 0.

— Variable: rl_linebuf_func_t * history_inhibit_expansion_function

> 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`.

Previous: History Variables, Up: Programming with GNU History

### 2.5 History Programming Example

The following program demonstrates simple use of the gnu History Library.

         #include <stdio.h>
         #include <readline/history.h>

         main (argc, argv)
              int argc;
              char **argv;
         {
           char line[1024], *t;
           int len, done = 0;

           line[0] = 0;

           using_history ();
           while (!done)
             {
               printf ("history$ ");
               fflush (stdout);
               t = fgets (line, sizeof (line) - 1, stdin);
               if (t && *t)
                 {
                   len = strlen (t);
                   if (t[len - 1] == '\n')
                     t[len - 1] = '\0';
                 }

               if (!t)
                 strcpy (line, "quit");

               if (line[0])
                 {
                   char *expansion;
                   int result;

                   result = history_expand (line, &expansion);
                   if (result)
                     fprintf (stderr, "%s\n", expansion);

                   if (result < 0 || result == 2)
                     {
                       free (expansion);
                       continue;
                     }

                   add_history (expansion);
                   strncpy (line, expansion, sizeof (line) - 1);
                   free (expansion);
                 }

               if (strcmp (line, "quit") == 0)
                 done = 1;
               else if (strcmp (line, "save") == 0)
                 write_history ("history_file");
               else if (strcmp (line, "read") == 0)
                 read_history ("history_file");
               else if (strcmp (line, "list") == 0)
                 {
                   register HIST_ENTRY **the_list;
                   register int i;

                   the_list = history_list ();
                   if (the_list)
                     for (i = 0; the_list[i]; i++)
                       printf ("%d: %s\n", i + history_base, the_list[i]->line);
                 }
               else if (strncmp (line, "delete", 6) == 0)
                 {
                   int which;
                   if ((sscanf (line + 6, "%d", &which)) == 1)
                     {
                       HIST_ENTRY *entry = remove_history (which);
                       if (!entry)
                         fprintf (stderr, "No such entry %d\n", which);
                       else
                         {
                           free (entry->line);
                           free (entry);
                         }
                     }
                   else
                     {
                       fprintf (stderr, "non-numeric arg given to `delete'\n");
                     }
                 }
             }
         }

## Appendix A Concept Index

- anchored search: Searching the History List
- event designators: Event Designators
- history events: Event Designators
- history expansion: History Interaction
- History Searching: Searching the History List

## Appendix B Function and Variable Index

- `add_history`: History List Management
- `append_history`: Managing the History File
- `clear_history`: History List Management
- `current_history`: Information About the History List
- `get_history_event`: History Expansion
- `history_arg_extract`: History Expansion
- `history_base`: History Variables
- `history_comment_char`: History Variables
- `history_expand`: History Expansion
- `history_expansion_char`: History Variables
- `history_get`: Information About the History List
- `history_get_history_state`: Initializing History and State Management
- `history_inhibit_expansion_function`: History Variables
- `history_is_stifled`: History List Management
- `history_length`: History Variables
- `history_list`: Information About the History List
- `history_max_entries`: History Variables
- `history_no_expand_chars`: History Variables
- `history_quotes_inhibit_expansion`: History Variables
- `history_search`: Searching the History List
- `history_search_delimiter_chars`: History Variables
- `history_search_pos`: Searching the History List
- `history_search_prefix`: Searching the History List
- `history_set_history_state`: Initializing History and State Management
- `history_set_pos`: Moving Around the History List
- `history_subst_char`: History Variables
- `history_tokenize`: History Expansion
- `history_total_bytes`: Information About the History List
- `history_truncate_file`: Managing the History File
- `history_word_delimiters`: History Variables
- `next_history`: Moving Around the History List
- `previous_history`: Moving Around the History List
- `read_history`: Managing the History File
- `read_history_range`: Managing the History File
- `remove_history`: History List Management
- `replace_history_entry`: History List Management
- `stifle_history`: History List Management
- `unstifle_history`: History List Management
- `using_history`: Initializing History and State Management
- `where_history`: Information About the History List
- `write_history`: Managing the History File

## Table of Contents

- 1 Using History Interactively
  - 1.1 History Expansion
    - 1.1.1 Event Designators
    - 1.1.2 Word Designators
    - 1.1.3 Modifiers
- 2 Programming with GNU History
  - 2.1 Introduction to History
  - 2.2 History Storage
  - 2.3 History Functions
    - 2.3.1 Initializing History and State Management
    - 2.3.2 History List Management
    - 2.3.3 Information About the History List
    - 2.3.4 Moving Around the History List
    - 2.3.5 Searching the History List
    - 2.3.6 Managing the History File
    - 2.3.7 History Expansion
  - 2.4 History Variables
  - 2.5 History Programming Example
- Appendix A Concept Index
- Appendix B Function and Variable Index