Why can't scp's prompt be hidden?

Why can't the scp prompt hide and redirect the scp standard output and standard errors to a file? why can the & quot; Enterwindowspassword: & quot; string be played on the shell interface? Scpfromfilenameusername@x.x.x.x:/topath/tofilename2 & gt; & amp; 1 scp prompt for why hidden not to redirect scp standard output and standard errors to a file, why can I still use the "Enter windows password:" string on the shell interface? Scp fromfilename username@x.x.x.x:/topath/tofilename 2> & 1>/tmp/tmp.txt Enter windows password: check the openssh code, the answer is very obvious. 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 cocould 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 wowould otherwise cause the user to en D * 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 CILS */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, & s A, & 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 runs an ssh sub-process to complete connection authentication. 13996 root/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 64 Jan 4 04:12 9->/tmp/lock/lock9997 lrwx ------ 1 root 64 Jan 4 04:12 6->/dev/tty lrwx ------ 1 root 64 Jan 4 5-> socket: [158339] lrwx ------ 1 root 64 Jan 4 4-> socket: [158338] lrwx ------ 1 root 64 Jan 4 3-> socket: [160359] lrwx ------ 1 root 64 Jan 4 2->/dev/pts/0 l-wx ------ 1 root 64 Jan 4 1-> pipe: [160353] lr-x ------ 1 root 64 Jan 4 0-> pipe: [160352] from now on, ssh directly writes/dev/tty through file descriptor 6 to display the prompt to the shell interface, which has nothing to do with standard output 1/Standard error 2. Lrwx ------ 1 root 64 Jan 4 6->/dev/tty