摘要：1. case-sensitive 2. embedded spaces 3. wildcards(which is also known as globbing) 4. 阅读全文

摘要：1. date: show current date and time. 2. cal: calendar. 3. df: to see the current amount of free space on your disk drives. 4. free: show the amount of 阅读全文

摘要：1. . period character 2. / slash 3. - dash (https://en.wikipedia.org/wiki/Dash) 4. _ underscore also called understrike, underbar, underline.   阅读全文