documentation updates for history builtin to note it does not truncate the history file; documentation updates for HISTFILESIZE noting that it can work on lines or history entries depending on $HISTTIMEFORMAT; documentation update for history saving behavior at shell exit; history -w and history -a should update the number of history entries in the current session if they are using $HISTFILE; history -r now updates the number of history entries in the current session; improvement to history_truncate_file so it leaves fewer partial history entries when operating on lines

lib/readline/doc/hsuser.texi

@@ -85,8 +85,26 @@ by reading history entries from the
 file named by the @env{HISTFILE} variable (default @file{~/.bash_history}).
 This is referred to as the @dfn{history file}.
 The history file is truncated, if necessary,
-to contain no more than the number of history entries
+to contain no more than the number of lines or history entries
 specified by the value of the @env{HISTFILESIZE} variable.
+
+The value of @env{HISTFILESIZE}
+is interpreted as lines or possibly multi-line history
+entries depending on whether the @env{HISTTIMEFORMAT}
+variable has a value,
+since that controls whether or not timestamps are written
+to the history file.
+If @env{HISTTIMEFORMAT}
+has a value,
+@env{HISTFILESIZE}
+is interpreted as a number of
+history entries, including timestamps.
+If it does not, @env{HISTFILESIZE}
+is interpreted as a number of lines,
+which may result in incomplete history entries in the history file,
+or the history file containing more lines than this maximum
+to avoid leaving partial history entries.
+
 If @env{HISTFILESIZE} is unset, or set to null, a non-numeric value,
 or a numeric value less than zero, the history file is not truncated.
 
@@ -98,17 +116,26 @@ These timestamps are optionally displayed depending on the value of the
 When present, history timestamps delimit history entries, making
 multi-line entries possible.
 
-When a shell with history enabled exits, Bash copies the last
-@env{$HISTSIZE} entries from the history list to the file
-named by @env{$HISTFILE}.
+When a shell with history enabled exits, Bash
+copies up to the last
+@env{$HISTSIZE}
+entries from the history list
+to the file named by
+@env{$HISTFILE}.
 If the @code{histappend} shell option is set (@pxref{Bash Builtins}),
-Bash appends the entries to the history file,
-otherwise it overwrites the history file.
+or if the number of history entries entered
+during the current shell session is less than
+@env{$HISTSIZE},
+Bash appends the history entries entered during the current session
+to @env{$HISTFILE}.
+If @code{histappend} is not set, and the number of entries from the current
+shell session exceeds @env{$HISTSIZE},
+it overwrites the history file with the entries from the current session.
 If @env{HISTFILE} is unset or null,
 or if the history file is unwritable, the history is not saved.
 After saving the history, Bash truncates the history file
 to contain no more than @env{$HISTFILESIZE}
-lines as described above.
+entries as described above.
 
 If the @env{HISTTIMEFORMAT}
 variable is set, the shell writes the timestamp information

lib/readline/histfile.c

@@ -613,6 +613,7 @@ history_truncate_file (const char *fname, int lines)
 {
   char *buffer, *filename, *tempname, *bp, *bp1;		/* bp1 == bp+1 */
   int file, chars_read, rv, orig_lines, exists, r;
+  int has_timestamps;
   struct stat finfo, nfinfo;
   size_t file_size;
 
@@ -695,6 +696,11 @@ history_truncate_file (const char *fname, int lines)
     }
   buffer[chars_read] = '\0';	/* for the initial check of bp1[1] */
 
+  /* use a heuristic like in read_history_range() to determine whether the
+     file has timestamps, but don't change the comment character so
+     HIST_TIMESTAMP_START doesn't return true */
+  has_timestamps = history_comment_char == '\0' && buffer[0] == '#' && isdigit ((unsigned char)buffer[1]);
+
   /* Count backwards from the end of buffer until we have passed
      LINES lines.  bp1 is set funny initially.  But since bp[1] can't
      be a comment character (since it's off the end) and *bp can't be
@@ -703,6 +709,8 @@ history_truncate_file (const char *fname, int lines)
      because we decrement it one extra time the first time through the loop
      and we need the final timestamp line. */
   lines += history_write_timestamps;
+  if (history_write_timestamps == 0)
+    lines += has_timestamps;		/* do our best */
   for (bp1 = bp = buffer + chars_read - 1; lines > 0 && bp > buffer; bp--)
     {
       if (*bp == '\n' && HIST_TIMESTAMP_START(bp1) == 0)