>
Input the previous content of> as input into the following file, which is equivalent to
the'w ' operation (write operation). The cat command is to view the content in the file. You can see that the result of the ls docker command will be input into a.txt
>>
Input the content in front of >> as input into the following file, but at this time it is equivalent to the file'a' operation (add operation), appended to the end of the file