021 大小写转换

将一个文件的所有内容转换为大写：

 cat employee.txt 

100 Jason Smith
200 John Doe
300 Sanjay Gupta
400 Ashok Sharma

tr a-z A-Z < employee.txt 

100 JASON SMITH
200 JOHN DOE
300 SANJAY GUPTA
400 ASHOK SHARMA

将一个文件的内容转换为小写

 cat department.txt 

100 FINANCE
200 MARKETING
300 PRODUCT DEVELOPMENT
400 SALES

tr A-Z a-z < department.txt 

100 finance
200 marketing
300 product development
400 sales