I saw the man page of test.
where the below is mentioned.
-e pathname
True if pathname resolves to a file that exists. False if pathname cannot be resolved.
-f pathname
True if pathname resolves to a file that exists and is a regular file. False if pathname cannot be resolved, or if pathname resolves to a file that exists but is not a regular file.
the -f flag says
True if pathname resolves to a file that exists and is a regular file
Could anybody please tell what is a regular file and what is not a regular file.
Non-regular files are devices, pipes, sockets... try [ -f /dev/tty0 ], for example. Symlinks are also non-regular, but they're resolved by test -f.
They're text or binary data, called 'regular files' to distinguish them from other types like directories, symbolic links, sockets etc.
Take a look at http://en.wikipedia.org/wiki/Unix_file_types
It is not a directory, device file, block device, FIFO or socket. If you do an ls -l. The ones which start with a "-" are the files. Actually what most people to refer as just a file.
来源：https://stackoverflow.com/questions/6858452/what-is-a-regular-file-on-unix