China DOS Union

Introduction to DOS Pipe Commands
Written from memory, please point out or supplement any errors or omissions.
The function of DOS pipe commands is to redirect the output of software. Using them, we can easily intercept the output information of DOS software.
For example, if I want to write an article about the COPY command in DOS and need to use its parameter information, I can use the command copy /? >mycopy.txt, which will input the help information of the COPY command into the text file mycopy.txt. I can modify mycopy.txt to become my article.
If I also need to write about the XCOPY command, I can use xcopy /? >>mycopy.txt to get the parameter information of the XCOPY command again. The difference between >> and > is that if the target file mycopy.txt is not empty, >> will append the new content to the original content, while > will overwrite the original content. > and >> are the special symbols of DOS pipe commands.
Since DOS treats devices as files, including the empty target NUL, pipe commands can also be used to do some other work.
For example: type autoexec.bat >prn OR type autoexec.bat >lpt1 can simply test the printer
copy command.com d:\ >nul will copy command to drive D without any information
If your autoexec.bat is written with DELTREE /Y C:\*.* >NULL, the next time you start up, you will lose all the data on drive C in a frenzy of hard disk light flashing, and there will be no prompt information on the screen.

Seems very dangerous.

The four DOS commands deltree, delete, format, and fdisk are relatively dangerous commands. Don't try them easily if you don't understand them. But if you know what you're doing, that's another matter.
Read more books, understand various commands, and then try them carefully... The first two can be tried. The latter two... If you don't understand, really don't try.
If you want to try, use a virtual machine to try. Hehe...
Other DOS commands are not dangerous. Pipeline commands are not dangerous at all.
I think the pipeline command should also include |.
| is also a pipeline command, and its function is to execute several DOS commands in one line. (This is how I understand it.)
You can give it a try...
For example:
You are in the c:\ccdos directory, and there are two files ccdos.exe and ccdos.txt in it.
The result of your dir is:
.
..
ccdos.exe
ccdos.txt
Then we type:
copy ccdos.txt eg.txt|del ccdos.exe
After execution, dir again:
It becomes:
.
..
ccdos.txt
eg.txt
What does it show? This line executed two DOS commands, hehehe...

Can I partition drive A?

It seems that < can also be a pipeline command. I remember seeing debugging techniques before. You can write the operations of DEBUG one by one into a text file, and then use DEBUB < file name. I don't know if I remembered it wrong, so I didn't mention it above.

<Is to use the following output as the input of the previous command

“>”, “>>”, a:\a.txt press Enter

After pressing Enter, the directory information in the DOS directory of drive C will be automatically saved to the a.txt file on drive A. Its content is the same as the screen information you see directly when you enter the dir command in the DOS directory of drive C.

">>" is additional content, similar to ">", but the difference is that ">" generates the result into a file, and if the target file already exists, it will be directly overwritten without prompting the user; while ">>" can append the result to the end of an existing file without overwriting the existing file.
"<" is the input redirection command. Generally, it needs to be used in conjunction with the three filter commands called filters in DOS.

What you said is redirection, and the pipe is "|".

What is “|” used for?

Thank you very much. It would be even better if you could be more detailed.

How do you use <?

I don't know what the difference is between NUL and NULL.