In UNIX, files are divided into categories called types. The most common available file types are:

1. f : regular files
2. d : directory files

Type f files contain a program or text and are created with an editor such as vi or a compiler such as cc. The file named “letter” in your shell account is an example of a regular file. If you use the ls –1 command to show a full listing of the file named “letter,” you will see a dash as the very first character of the output.
The dash indicates this is a regular file as shown in the example below:

When searching for a regular file using the find command, use the –type f option.
Type d files are directories, which are folders that contain other files. The course-project file in your shell account is an example of a directory file. Use the –ld option with the ls command to show a full listing of the course-project directory file. A d is the very first character of the output, which indicates that this is a directory file, as shown in the examples below:
When searching for a directory file using the find command, use the –type d option.