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。
点击(此处)折叠或打开
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@[admin@fedora bash-5.2.21]$ echo $PS1
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[\u@\h \W]\$
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@[admin@fedora bash-5.2.21]$
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
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export CFLAGS=“-g -O0"
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export LDFLAGS="-g -O0"
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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
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/* Issue a prompt, or prepare to issue a prompt when the next character
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is read. */
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static void
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prompt_again (force)
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int force;
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{
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
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/* Redraw the last line of a multi-line prompt that may possibly contain
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terminal escape sequences. Called with the cursor at column 0 of the
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line to draw the prompt on. */
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static void
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redraw_prompt (char *t)
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{
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char *oldp;
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oldp = rl_display_prompt;
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rl_save_prompt ();
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rl_display_prompt = t;
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local_prompt = expand_prompt (t, PMT_MULTILINE, &prompt_visible_length, &prompt_last_invisible, &prompt_invis_chars_first_line, &prompt_physical_chars);
和
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/* Expand the prompt string S and return the number of visible
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characters in *LP, if LP is not null. This is currently more-or-less
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a placeholder for expansion. LIP, if non-null is a place to store the
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index of the last invisible character in the returned string. NIFLP,
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if non-zero, is a place to store the number of invisible characters in
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the first prompt line. The previous are used as byte counts -- indexes
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into a character buffer. *VLP gets the number of physical characters in
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the expanded prompt (visible length) */
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/* 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
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the returned string; all characters except those between \001 and
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\002 are assumed to be `visible'. */ /* Possible values for FLAGS: PMT_MULTILINE caller indicates that this is part of a multiline prompt */
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/* This approximates the number of lines the prompt will take when displayed */
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#define APPROX_DIV(n, d) (((n) < (d)) ? 1 : ((n) / (d)) + 1)
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static char *
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expand_prompt (char *pmt, int flags, int *lp, int *lip, int *niflp, int *vlp)
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{
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char *r, *ret, *p, *igstart, *nprompt, *ms;
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int l, rl, last, ignoring, ninvis, invfl, invflset, ind, pind, physchars;
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int mlen, newlines, newlines_guess, bound, can_add_invis;
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int mb_cur_max;
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/* We only expand the mode string for the last line of a multiline prompt
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(a prompt with embedded newlines). */
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ms = (((pmt == rl_prompt) ^ (flags & PMT_MULTILINE)) && _rl_show_mode_in_prompt) ? prompt_modestr (&mlen) : 0;
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if (ms)
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{
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：
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{ "show-all-if-unmodified", &_rl_complete_show_unmodified, 0 },
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{ "show-mode-in-prompt", &_rl_show_mode_in_prompt, 0 },
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{ "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。
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#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。
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[admin@fedora bash-5.2.21]$ echo $PS1
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[\u@\h \W]\$
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[admin@fedora bash-5.2.21]$
遇事不决，源码解决，诚不我欺也！
