scp的prompt为啥隐藏不掉

scp的prompt为啥隐藏不掉

 

把scp的标准输出和标准错误都重定向到一个文件，为什么shell界面还能打出"Enter windows password:"的字符串呢？ 

scp fromfilename username@x.x.x.x:/topath/tofilename 2>&1 > /tmp/tmp.txt 

Enter windows password: 

 

查了openssh的代码，答案很明显了。 

char * 

readpassphrase(const char *prompt, char *buf, size_t bufsiz, int flags) 

{ 

ssize_t nr; 

int input, output, save_errno; 

char ch, *p, *end; 

struct termios term, oterm; 

struct sigaction sa, savealrm, saveint, savehup, savequit, saveterm; 

struct sigaction savetstp, savettin, savettou, savepipe; 

 

/* I suppose we could alloc on demand in this case (XXX). */ 

if (bufsiz == 0) { 

errno = EINVAL; 

return(NULL); 

} 

 

restart: 

signo = 0; 

/* 

* Read and write to /dev/tty if available.  If not, read from 

* stdin and write to stderr unless a tty is required. 

*/ 

if ((flags & RPP_STDIN) || 

    (input = output = open(_PATH_TTY, O_RDWR)) == -1) { 

if (flags & RPP_REQUIRE_TTY) { 

errno = ENOTTY; 

return(NULL); 

} 

input = STDIN_FILENO; 

output = STDERR_FILENO; 

} 

 

/* 

* Catch signals that would otherwise cause the user to end 

* up with echo turned off in the shell.  Don't worry about 

* things like SIGXCPU and SIGVTALRM for now. 

*/ 

sigemptyset(&sa.sa_mask); 

sa.sa_flags = 0; /* don't restart system calls */ 

sa.sa_handler = handler; 

(void)sigaction(SIGALRM, &sa, &savealrm); 

(void)sigaction(SIGHUP, &sa, &savehup); 

(void)sigaction(SIGINT, &sa, &saveint); 

(void)sigaction(SIGPIPE, &sa, &savepipe); 

(void)sigaction(SIGQUIT, &sa, &savequit); 

(void)sigaction(SIGTERM, &sa, &saveterm); 

(void)sigaction(SIGTSTP, &sa, &savetstp); 

(void)sigaction(SIGTTIN, &sa, &savettin); 

(void)sigaction(SIGTTOU, &sa, &savettou); 

 

/* Turn off echo if possible. */ 

if (input != STDIN_FILENO && tcgetattr(input, &oterm) == 0) { 

memcpy(&term, &oterm, sizeof(term)); 

if (!(flags & RPP_ECHO_ON)) 

term.c_lflag &= ~(ECHO | ECHONL); 

#ifdef VSTATUS 

if (term.c_cc[VSTATUS] != _POSIX_VDISABLE) 

term.c_cc[VSTATUS] = _POSIX_VDISABLE; 

#endif 

(void)tcsetattr(input, _T_FLUSH, &term); 

} else { 

memset(&term, 0, sizeof(term)); 

term.c_lflag |= ECHO; 

memset(&oterm, 0, sizeof(oterm)); 

oterm.c_lflag |= ECHO; 

} 

 

if (!(flags & RPP_STDIN)) 

(void)write(output, prompt, strlen(prompt));

         end = buf + bufsiz - 1; 

for (p = buf; (nr = read(input, &ch, 1)) == 1 && ch != '\n' && ch != '\r';) { 

if (p < end) { 

if ((flags & RPP_SEVENBIT)) 

ch &= 0x7f; 

if (isalpha(ch)) { 

if ((flags & RPP_FORCELOWER)) 

ch = tolower(ch); 

if ((flags & RPP_FORCEUPPER)) 

ch = toupper(ch); 

} 

*p++ = ch; 

} 

} 

 

scp会运行一个ssh子进程来完成连接鉴权. 

13996 root       0:00 /usr/local/bin/ssh -x -oForwardAgent no -oPermitLocalCommand no -oClearAllForwardings yes -lusername x.x.x.x scp -t /topath/tofilename 

root@444444[]:/proc/13996/fd> ls -lrt 

total 0 

lrwx------    1 root     root            64 Jan  4 04:12 9 -> /tmp/lock/lock9997 

lrwx------    1 root     root            64 Jan  4 04:12 6 -> /dev/tty 

lrwx------    1 root     root            64 Jan  4 04:12 5 -> socket:[158339] 

lrwx------    1 root     root            64 Jan  4 04:12 4 -> socket:[158338] 

lrwx------    1 root     root            64 Jan  4 04:12 3 -> socket:[160359] 

lrwx------    1 root     root            64 Jan  4 04:12 2 -> /dev/pts/0 

l-wx------    1 root     root            64 Jan  4 04:12 1 -> pipe:[160353] 

lr-x------    1 root     root            64 Jan  4 04:12 0 -> pipe:[160352] 

 

从此可知，ssh就是通过file descriptor 6直接写/dev/tty来把prompt显示到shell界面的，和标准输出1/标准错误2没有任何关系。 

lrwx------    1 root     root            64 Jan  4 04:12 6 -> /dev/tty