The sed command with the s (substitue) option can be used to change text in a file or a variable. Let's say that foo contains the text Hello World.
The sed command with the s (substitute) flag is used to replace data. In this example, the text "Hi" will be replaced with the text "Hello". It is also important to recongize that with this syntax, the text in the variable will not be changed. Instead, the result will simply be displayed in the terminal.
~]# echo $foo | sed 's|Hello|Hi|' Hi World
With this syntax, Hello will be replaced with Hi in the foo variable.
foo=$( sed "s|Hello|Hi|" <<< "$foo" )
This does the same.
foo=$( echo "$foo" | sed "s|Hello|Hi|" )
Replace multiple instances of a string (global)
If a variable 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 foo contains the following text.
foo=" 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.
~]# echo $foo | sed 's/Hello/Hi/' Hi World Hello World Hello World
The g (global) flag will change every instance of Hello to Hi.
~]# echo $foo | sed 's/Hello/Hi/g' Hi World Hi World Hi World