bash提示符的最开头出现了莫名其妙的@符号时怎么办

近来发现不知为何在shell的提示符的开头莫名其妙的多出了一个@。

一开始懒得管他，以为什么时候把PS1给改错了。今天看了一下PS1并没有错。这就很奇怪。
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@[admin@fedora bash-5.2.21]$ echo $PS1
[\u@\h \W]\$
@[admin@fedora bash-5.2.21]$

在本地配置文件、国内互联网加外网一通翻找，愣是没有找到一丁点线索。这就很诡异。

运行bash -x也没有找到什么有用信息，这就很无奈。

还好可以代码加gdb，上网下载了对应版本的源代码，然后

export CFLAGS=“-g -O0"
export LDFLAGS="-g -O0"
configure && make

一遍过，不错不错。

不用安装，直接就对当前目录的bash进行gdb。

过程繁琐且无聊，无他，就是先从代码里面搜索prompt关键字，我找到的是y.tab.c文件

/* Issue a prompt, or prepare to issue a prompt when the next character
   is read. */
static void
prompt_again (force)
     int force;
{

然后一个函数一个函数的深入，直到找到lib/readline/display.c的expand_prompt函数

/* Redraw the last line of a multi-line prompt that may possibly contain
   terminal escape sequences. Called with the cursor at column 0 of the
   line to draw the prompt on. */
static void
redraw_prompt (char *t)
{
  char *oldp;


  oldp = rl_display_prompt;
  rl_save_prompt ();


  rl_display_prompt = t;
  local_prompt = expand_prompt (t, PMT_MULTILINE, &prompt_visible_length, &prompt_last_invisible, &prompt_invis_chars_first_line, &prompt_physical_chars);

和

/* Expand the prompt string S and return the number of visible

   characters in *LP, if LP is not null. This is currently more-or-less

   a placeholder for expansion. LIP, if non-null is a place to store the

   index of the last invisible character in the returned string. NIFLP,

   if non-zero, is a place to store the number of invisible characters in

   the first prompt line. The previous are used as byte counts -- indexes

   into a character buffer. *VLP gets the number of physical characters in

   the expanded prompt (visible length) */



/* Current implementation: \001 (^A) start non-visible characters \002 (^B) end non-visible characters    all characters except \001 and \002 (following a \001) are copied to
   the returned string; all characters except those between \001 and
   \002 are assumed to be `visible'. */ /* Possible values for FLAGS: PMT_MULTILINE    caller indicates that this is part of a multiline prompt */


/* This approximates the number of lines the prompt will take when displayed */
#define APPROX_DIV(n, d) (((n) < (d)) ? 1 : ((n) / (d)) + 1)


static char *
expand_prompt (char *pmt, int flags, int *lp, int *lip, int *niflp, int *vlp)
{
  char *r, *ret, *p, *igstart, *nprompt, *ms;
  int l, rl, last, ignoring, ninvis, invfl, invflset, ind, pind, physchars;
  int mlen, newlines, newlines_guess, bound, can_add_invis;
  int mb_cur_max;


  /* We only expand the mode string for the last line of a multiline prompt
     (a prompt with embedded newlines). */
  ms = (((pmt == rl_prompt) ^ (flags & PMT_MULTILINE)) && _rl_show_mode_in_prompt) ? prompt_modestr (&mlen) : 0;
  if (ms)
    {

根据gdb和上面的函数调用可以确定：前两个条件(pmt == rl_prompt) 和 (flags & PMT_MULTILINE)都为真，其决定作用的是_rl_show_mode_in_prompt开关项。

去代码中搜索这一项可以看到在lib/readline/bind.c文件中的信息：

{ "show-all-if-unmodified",    &_rl_complete_show_unmodified,    0 },
{ "show-mode-in-prompt",    &_rl_show_mode_in_prompt,    0 },
{ "skip-completed-text",    &_rl_skip_completed_text,    0 },

再去网上搜show-mode-in-prompt，发现是inputrc(vi ~/.inputrc)里面的一个选项。应该是前段时间玩vi mode的bash shell时打开的。

#set show-mode-in-prompt on

也就是说：开头的@应该叫做“模式提示符”

关掉后一切OK。

[admin@fedora bash-5.2.21]$ echo $PS1
[\u@\h \W]\$
[admin@fedora bash-5.2.21]$

遇事不决，源码解决，诚不我欺也！