中国DOS联盟

The first time I saw NUL used in this way

Hehe, after looking at it for a long time, I just don't understand!~~

Originally posted by chenall at 2006-3-26 14:38:
You can't use handle 3 simultaneously if it's specified twice. Try changing the second one to echo. 1>nul 4>nul and see.
Indeed, it works.
To restore, use echo 2>con 5>con respectively, echo 1>con 6>con. If you only want to...

Among the objects we are discussing, there are two >. Using i and j to represent numbers from 0 to 9, if we regard the handle as a pointer to the device, then by default:
Handle Points to
stdin con
stdout con
stderr con

When the first > in the command cnd i>nul j>nul is executed, the system temporarily stores the target of in an unused handle space, such as , for restoration when the command ends. However, the second > modifies the temporarily stored content, thus leaving an impact on subsequent commands.
In fact, cnd i>nul j>nul will also temporarily store the original content of in another unused space. So
The change in output from cnd 2>nul 3>nul can be restored using cnd 2>&3 4>&3.
Since the temporary storage location is selected by the system and cannot be controlled by the user, the values cannot be determined.
(Each time I test, to ensure the certainty of the handle space, I always re-enter cmd.)

Regarding:

Because the temporary location is selected by the system and users cannot control it, so the value cannot be determined

I guessed in floor 13 that cmd stores the handle before modification in "unvisited undefined handles (3-9)". Here, I will clarify again. The "unvisited" here means that the user has never explicitly specified it in the current cmd command line, not that cmd itself has not used it. And there may be multiple "unvisited" handles, and cmd will choose the first one. If you have counterexamples, feel free to put them forward.

Re regarding:

I have my own counter-evidence. After executing the following two sentences, the command line is still blocked.

echo test 3>nul
echo test 1>nul 3>nul

That is to say, although we have accessed handle 3 in the first sentence, we can still use 3>nul in the second sentence to change the backup handle stdout.

Therefore, the handle backup strategy of cmd may be like this: first use the undefined handle 3, if it is detected that this handle is in the backup state and is changed, then switch to the next handle.

In addition, there are some other discoveries:

1. cd 1>nul 2>nul 3>nul 4>nul can directly change the output of stdout and stderr;
2. cd 0<nul 3<nul or cd 0<nul 3>nul will cause cmd to continuously echo the command line prompt;
3. cd 0<nul 3<file will directly cause the cmd window to close;
4. After cd 1>nul 3>nul, the behavior of cls will also fail, so it is confirmed that cls is also an output behavior;

Finally, regarding the copying of handles, it can actually be understood as the redirection of handles, but this time the target of redirection is no longer a device or a file, but another handle.

To give an example, the device or file is a reservoir, the handle is its pipe, the command is the water pump or filter on the pipe, and cmd is a self-contained water cycle system. The pipes of the reservoir are numbered from 0-9, there are 10 in total. The 0# pipe is the inlet pipe, the 1# pipe is the drain pipe, the 2# pipe is the fault test pipe, and the 3-9# pipes are unused backup pipes. And 0< is actually opening the valve of the inlet pipe, 1> is opening the valve of the drain pipe, and 1>&2 is opening the connection valve from the 1# pipe to the 2# pipe. And >nul is using a water pump to empty the water.

Re Willsort:
Through trial and error, one can guess the system's handle usage strategy. For example, in the sentence "

cnd 2>nul 3>nul changes the output, and it can be restored with cnd 2>&3 4>&3", the number 4 is the result of the guess, which is based on the relatively clear situation of handle usage just when entering cmd.
If handle redirection is repeatedly used in a batch script, it is difficult to judge the situation of the handle space. So I said "the system decides, and users can hardly control".
I agree with your concept of "unused handles", and the test has confirmed this concept. In fact, I feel that clear concepts are crucial; otherwise, it will cause great harm. Just like the ambiguous explanations in Windows help, which will only make people more confused.

Classic, and it broadens the thinking! Handle redirection!

Can't understand~ Working hard...

The two are really admirable, but in the end, it still gives the feeling that the principle of handle redirection is not clearly explained, although several examples are given.

Just know that such a thing exists. Those who don't understand can refer to "Introduction to Linux Shell"

After reading this article, I have gained a lot. However, I still hope that experts can come to guide everyone. Give it a thumbs up.

Wow! Where did you learn these profound knowledge? It's extremely deep!