Let's say foo.txt contains Hello World.

Hello World

The sed command with the s (substitue) option can be used to replace data in a file or a variable. In this example, the text "Hello" will be replaced with the text "Hi". It is also important to recognize that when no options are used, the text in the file will not be changed. Instead, the result will simply be displayed in the terminal. 

sed 's/Hello/Hi/' foo.txt

The -i or --in-place option will actually change the content of the file.

sed -i 's/Hello/Hi/' foo.txt

Replace multiple instances of a string (global)

If a file contains more than one occurence of a string, the g (global) option will need to be used to change every instance of the string. For example, let's say a file contains the following text.

Hello World
Hello World
Hello World

If the g (global) flag is not used, only the first instance of the word Hello will be changed.

~]# sed 's/Hello/Hi/' file.txt
Hi World
Hello World
Hello World

The g (global) flag will change every instance of Hello to Hi.

~]# sed 's/Hello/Hi/g' file.txt
Hi World
Hi World
Hi World

Delete

The d flag can be used to delete lines in a file.

~]# sed -i '/Hello World/d' file.txt