为什么在终端中没有任何输出的情况下我不能在Linux中执行某些命令？

Question

I tried to execute "rmdir * " in terminal. Because some are not directories under current folder, error messages showing some files are not directory will show. My goal is to execute the command silently without any output.

I tried the following:

rmdir * > /dev/null
rmdir * > file.txt
rmdir * |grep "noexist"  (some non-existing characters)
rmdir * 2>&1 > /dev/null
rmdir * 2>&1 > file.txt
rmdir * 2>&1 |grep "noexist"

Only the last command works! I feel a little confused. As the last command works, why don't the 4th and 5th commands work?

Answer 1

> redirects STDOUT. 2> redirects STDERR. &1 redirects to STDOUT. | transfers STDOUT to STDIN for the new process.

Did you try:

rmdir * > /dev/null 2> /dev/null

Which redirects both STDOUT and STDERR to the null device.

Answer 2

1) Doesn't redirect stderr using 2>...
2) See 1.
3) See 2.
4) Redirects stderr to stdout, and stdout is redirected to /dev/null, so you still see the redirected stderr on stdout.
5) See 4. The file to which stdout is redirected is just different.
6) This works because you redirected stderr to stdout. stdout is piped in to grep as grep's stdin.

What REALLY works:

rmdir * >/dev/null 2>&1

Why does that work? Because first stdout was redirected to /dev/null. Then stderr was redirected to stdout, which was already redirected to /dev/null.

Answer 3

除非目录为空，否则您不能使用rmdir。确保执行此操作之前。