How to view the number of files opened in linux

How to view the number of files opened in linux-general Linux technology-Linux technology and application information. For details, refer to the following section. Check the number of files opened in linux on the Internet in two ways, but the results are different. in linux, check that the number of files opened is based on the file or command?

I searched some usage of the ulimit command, including
Ulimit-n 4096
That is, limit the maximum number of files opened by a user to 4096.

There are roughly two statements about how to view the number of opened files on the Internet:

/Proc/sys/fs/file-nr
This file is related to file-max. It has three values:

Number of allocated file handles
Number of file handles used
Maximum number of file handles
This file is read-only and only used to display information.

View the number of files opened by all processes
Lsof | wc-l

View the number of files opened by a process
Lsof-p pid | wc-l