Question

How does the vncpasswd program produce output on the console even though both standard out and standard error have been redirected to /dev/null?

$ vncpasswd > /dev/null 2> /dev/null
Password: 

Edit: Here’s a partial strace:

open("/dev/tty", O_RDWR|O_CREAT|O_TRUNC|O_CLOEXEC, 0666) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(3, SNDCTL_TMR_CONTINUE or TCSETSF, {B38400 opost -isig icanon -echo ...}) = 0
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost -isig icanon -echo ...}) = 0
fstat(3, {st_mode=S_IFCHR|0666, st_rdev=makedev(5, 0), ...}) = 0
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost -isig icanon -echo ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fb7c3eda000
write(3, "Password: ", 10Password: )              = 10
read(3, 

Answer 1

It’s opening the underlying terminal directly (something like open("/dev/tty", ...)). No amount of redirecting will get rid of that. If you don’t want to see it, you’ll have to run it not attached to a tty (e.g. through cron or something).