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       history - GNU History Library


       The GNU History Library is Copyright (C) 1989-2020 by the Free Software Foundation, Inc.


       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  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 line from the history list to
       use during substitution.  The second is to select portions of that line for inclusion into
       the  current  one.   The  line selected from the history is the event, and the portions of
       that line that are acted upon are words.  Various modifiers are  available  to  manipulate
       the  selected  words.  The line is broken 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).  History
       expansions are introduced by the appearance of the history expansion character, which is !
       by  default.   Only  backslash  (\)  and  single  quotes  can  quote the history expansion

   Event Designators
       An event designator is a reference to a command line entry in the  history  list.   Unless
       the  reference  is  absolute,  events  are relative to the current position in the history

       !      Start a history substitution, except when followed by a blank, newline, = or (.
       !n     Refer to command line n.
       !-n    Refer to the current command minus n.
       !!     Refer to the previous command.  This is a synonym for `!-1'.
              Refer to the most recent command preceding the current position in the history list
              starting with 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, the string from the most recent search is
              used; it is an error if there is no previous search string.
              Quick substitution.  Repeat the  last  command,  replacing  string1  with  string2.
              Equivalent to ``!!:s^string1^string2^'' (see Modifiers below).
       !#     The entire command line typed so far.

   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.

       0 (zero)
              The zeroth word.  For the shell, this is the command word.
       n      The nth word.
       ^      The first argument.  That is, word 1.
       $      The  last  word.   This is usually the last argument, but will expand to the zeroth
              word if there is only one word in the line.
       %      The first word matched by the most recent `?string?' search, if the  search  string
              begins with a character that is part of a word.
       x-y    A range of words; `-y' abbreviates `0-y'.
       *      All  of the words but the zeroth.  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 x is missing, it defaults to

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

       After  the  optional  word  designator,  there may appear 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 file name component, leaving only the head.
       t      Remove all leading file name 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; the last one supplied is used.
              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.  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 &.  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  `:s'  (e.g.,  `:gs/old/new/')  or  `:&'.  If used with `:s', 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 `s' or `&' modifier once to each word in the event line.


       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 <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.

   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;
         char *timestamp;
         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;

       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.

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

       HISTORY_STATE * history_get_history_state (void)
       Return a structure describing the current state of the input history.

       void history_set_history_state (HISTORY_STATE *state)
       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.

       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.   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

       void add_history_time (const char *string)
       Change the time stamp associated with the most recent history entry to string.

       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.

       histdata_t free_history_entry (HIST_ENTRY *histent)
       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.

       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
       the caller can dispose of any application-specific data.  In the case of an invalid which,
       a NULL pointer is returned.

       void clear_history (void)
       Clear the history list by deleting all the entries.

       void stifle_history (int max)
       Stifle  the  history  list,  remembering only the last max entries.  The history list will
       contain only max entries at a time.

       int unstifle_history (void)
       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.

       int history_is_stifled (void)
       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

       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.

       int where_history (void)
       Returns the offset of the current history element.

       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.

       HIST_ENTRY * history_get (int offset)
       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.

       time_t history_get_time (HIST_ENTRY *)
       Return the time stamp associated with the history entry passed as the argument.

       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.

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

       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.

       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.

       HIST_ENTRY * next_history (void)
       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.

       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.

       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.

       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.

   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.

       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.

       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.

       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.

       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.

       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.

   History Expansion
       These functions implement history expansion.

       int history_expand (char *string, char **output)
       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
       If an error occurred in expansion, then output contains a descriptive error message.

       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.

       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.

       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().

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

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

       int history_length
       The number of entries currently stored in the history list.

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

       int history_write_timestamps
       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),  timestamps  will
       not be written.

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

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

       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.

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

       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, \r, and =.

       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.

       int history_quotes_inhibit_expansion
       If non-zero, double-quoted words are not scanned for the history  expansion  character  or
       the history comment character.  The default value is 0.

       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.


              Default filename for reading and writing saved history


       The Gnu Readline Library, Brian Fox and Chet Ramey
       The Gnu History Library, Brian Fox and Chet Ramey


       Brian Fox, Free Software Foundation

       Chet Ramey, Case Western Reserve University


       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  If you  have  a  fix,  you  are  welcome  to  mail  that  as  well!
       Suggestions  and  `philosophical'  bug  reports  may  be mailed to or
       posted to the Usenet newsgroup gnu.bash.bug.

       Comments  and  bug  reports  concerning  this  manual   page   should   be   directed   to