http://kvakaren.blogspot.com/2010/11/gnu-readline-vi-mode-visualization-2.html

vi-readline-indicator.diff

+++ bind.c 2010-10-28 23:09:12.788260925 +0200
@@ -1508,6 +1508,9 @@
 static int sv_histsize PARAMS((const char *));
 static int sv_isrchterm PARAMS((const char *));
 static int sv_keymap PARAMS((const char *));
+static int sv_vi_insert_prompt PARAMS((const char *));
+static int sv_vi_command_prompt PARAMS((const char *));
+static int sv_vi_mode_changed_bin PARAMS((const char *));
 
 static const struct {
   const char * const name;
@@ -1522,6 +1525,9 @@
   { "history-size", V_INT,  sv_histsize },
   { "isearch-terminators", V_STRING, sv_isrchterm },
   { "keymap",  V_STRING, sv_keymap },
+  { "vi-insert-prompt", V_STRING, sv_vi_insert_prompt },
+  { "vi-command-prompt", V_STRING, sv_vi_command_prompt },
+  { "vi-mode-changed-bin", V_STRING, sv_vi_mode_changed_bin },
   { (char *)NULL, 0 }
 };
 
@@ -1694,6 +1700,30 @@
 }
 
 static int
+sv_vi_insert_prompt (value)
+    const char *value;
+{
+  _rl_set_vi_insert_prompt (value);
+  return 0;
+}
+
+static int
+sv_vi_command_prompt (value)
+    const char *value;
+{
+  _rl_set_vi_command_prompt (value);
+  return 0;
+}
+
+static int
+sv_vi_mode_changed_bin (value)
+    const char *value;
+{
+  _rl_set_vi_mode_changed_bin (value);
+  return 0;
+}
+
+static int
 sv_bell_style (value)
      const char *value;
 {
+++ rlprivate.h 2010-10-28 23:09:45.788262324 +0200
@@ -361,6 +361,9 @@
 extern int (_rl_digit_value) PARAMS((int));
 
 /* vi_mode.c */
+extern void _rl_set_vi_insert_prompt PARAMS((const char *));
+extern void _rl_set_vi_command_prompt PARAMS((const char *));
+extern void _rl_set_vi_mode_changed_bin PARAMS((const char *));
 extern void _rl_vi_initialize_line PARAMS((void));
 extern void _rl_vi_reset_last PARAMS((void));
 extern void _rl_vi_set_last PARAMS((int, int, int));
+++ vi_mode.c 2010-10-28 23:21:54.038261767 +0200
@@ -49,6 +49,8 @@
 
 #include <stdio.h>
 
+#include <fcntl.h>
+
 /* Some standard library routines. */
 #include "rldefs.h"
 #include "rlmbutil.h"
@@ -128,16 +130,121 @@
 static int _rl_vi_callback_char_search PARAMS((_rl_callback_generic_arg *));
 #endif
 
+#define _RL_PROMPT_INITIAL_SIZE 256
+#define _VI_MODE_PROMPT_FORMAT_SIZE 256
+#define _VI_MODE_PROMPT_SIZE _RL_PROMPT_INITIAL_SIZE + _VI_MODE_PROMPT_FORMAT_SIZE
+static char _rl_prompt_initial[_RL_PROMPT_INITIAL_SIZE] = "\0";
+static char *_rl_prompt_initial_last_line;
+static char _rl_vi_insert_prompt[_VI_MODE_PROMPT_FORMAT_SIZE] = "\0";
+static char _rl_vi_command_prompt[_VI_MODE_PROMPT_FORMAT_SIZE] = "\0";
+#define _VI_MODE_CHANGED_BIN_SIZE 256
+static char _rl_vi_mode_changed_bin[_VI_MODE_CHANGED_BIN_SIZE] = "\0";
+
+void _rl_set_vi_insert_prompt PARAMS((const char *));
+void _rl_set_vi_command_prompt PARAMS((const char *));
+void _rl_set_vi_mode_changed_bin PARAMS((const char *));
+
+static void vi_mode_changed_prompt PARAMS((void));
+static void vi_mode_changed_bin PARAMS((void));
+static void vi_mode_changed PARAMS((void));
+
+static char *
+safe_strncpy (dest, src, n)
+    char *dest;
+    const char *src;
+    int n;
+{
+  if (src) {
+    strncpy (dest, src, n);
+    dest[n - 1] = '\0';
+  }
+  else {
+    dest[0] = '\0';
+  }
+  return dest;
+}
+
+static char *
+parse_prompt_format_escapes (src)
+    char *src;
+{
+  char *r, *w, c, o;
+  static char buf[4] = "000";
+
+  r = src;
+  w = r;
+  while (*r) {
+    c = *r;
+    if (c == '\\') {
+      r++;
+      if (*r == '\\') {
+        c = '\\';
+      }
+      else if (*r == '0') {
+        c = *r;
+        buf[1] = *(r + 1);
+        buf[2] = *(r + 2);
+        o = strtol (buf, NULL, 8);
+        if (o > 0) {
+          r += 2;
+          c = o;
+        }
+      }
+    }
+    r++;
+    if (c > 0) {
+      *w = c;
+      w++;
+    }
+  }
+  *w = '\0';
+  return src;
+}
+
+void
+_rl_set_vi_insert_prompt (value)
+    const char *value;
+{
+  safe_strncpy (_rl_vi_insert_prompt, value, _VI_MODE_PROMPT_FORMAT_SIZE);
+  parse_prompt_format_escapes (_rl_vi_insert_prompt);
+}
+
+void
+_rl_set_vi_command_prompt (value)
+    const char *value;
+{
+  safe_strncpy (_rl_vi_command_prompt, value, _VI_MODE_PROMPT_FORMAT_SIZE);
+  parse_prompt_format_escapes (_rl_vi_command_prompt);
+}
+
+void
+_rl_set_vi_mode_changed_bin (value)
+    const char *value;
+{
+  safe_strncpy (_rl_vi_mode_changed_bin, value, _VI_MODE_CHANGED_BIN_SIZE);
+}
+
 void
 _rl_vi_initialize_line ()
 {
   register int i, n;
+  char *p;
 
   n = sizeof (vi_mark_chars) / sizeof (vi_mark_chars[0]);
   for (i = 0; i < n; i++)
     vi_mark_chars[i] = -1;
 
   RL_UNSETSTATE(RL_STATE_VICMDONCE);
+
+  safe_strncpy (_rl_prompt_initial, rl_prompt, _RL_PROMPT_INITIAL_SIZE);
+  p = strrchr (_rl_prompt_initial, '\n');
+  if (p) {
+      p++;
+  }
+  else {
+    p = _rl_prompt_initial;
+  }
+  _rl_prompt_initial_last_line = p;
 }
 
 void
@@ -655,6 +762,86 @@
 
 /* Insertion mode stuff. */
 
+/* This is meant to be called after vi mode changes. */
+static void
+vi_mode_changed_prompt ()
+{
+  char *prompt, *p;
+  char pattern[] = "{}";
+  static char buf[_VI_MODE_PROMPT_SIZE];
+  int i, j;
+
+  if (VI_INSERT_MODE()) {
+    prompt = _rl_vi_insert_prompt;
+  }
+  else if (VI_COMMAND_MODE()) {
+    prompt = _rl_vi_command_prompt;
+  }
+  if (strlen (prompt)) {
+    i = _rl_prompt_initial_last_line - _rl_prompt_initial;
+    memcpy (buf, _rl_prompt_initial, i);
+    buf[i] = '\0';
+    p = strstr (prompt, pattern);
+    if (p) {
+      j = p - prompt;
+      memcpy (buf + i, prompt, j);
+      buf[i + j] = '\0';
+      strcat (buf, _rl_prompt_initial_last_line);
+      strcat (buf, p + strlen (pattern));
+    }
+    else {
+      strcat (buf, prompt);
+    }
+    prompt = buf;
+  }
+  else {
+    prompt = _rl_prompt_initial;
+  }
+  rl_set_prompt (prompt);
+  _rl_redisplay_after_sigwinch ();
+}
+
+static void
+vi_mode_changed_bin ()
+{
+  pid_t pid;
+  int status, fd_devnull;
+  char *bin = _rl_vi_mode_changed_bin;
+
+  if (!strlen (bin)) {
+    return;
+  }
+  pid = fork ();
+  if (pid < 0) {
+    perror ("vi_mode_changed_bin: fork failed");
+    return;
+  }
+  else if (pid == 0) {
+    close (STDIN_FILENO);
+    fd_devnull = open ("/dev/null", O_RDONLY);
+    dup2 (fd_devnull, STDIN_FILENO);
+    if (VI_INSERT_MODE()) {
+      execl (bin, bin, "insert", NULL);
+    }
+    else if (VI_COMMAND_MODE()) {
+      execl (bin, bin, "command", NULL);
+    }
+    perror ("vi_mode_changed_bin: execv failed");
+    exit (1);
+  }
+  waitpid (pid, &status, 0);
+}
+
+static void
+vi_mode_changed ()
+{
+  if (!isatty (STDIN_FILENO) || !isatty (STDOUT_FILENO)) {
+    return;
+  }
+  vi_mode_changed_prompt ();
+  vi_mode_changed_bin ();
+}
+
 /* Switching from one mode to the other really just involves
    switching keymaps. */
 int
@@ -663,6 +850,9 @@
 {
   _rl_keymap = vi_insertion_keymap;
   _rl_vi_last_key_before_insert = key;
+
+  vi_mode_changed ();
+
   return (0);
 }
 
@@ -747,6 +937,9 @@
     rl_free_undo_list ();
 
   RL_SETSTATE (RL_STATE_VICMDONCE);
+
+  vi_mode_changed ();
+
   return (0);
 }