Let's say that foo.txt contains a value of Hello World.

foo="Hello World"

And foo.txt is owned by john.doe.

~]$ ll foo.txt
-rw-rw-r--. 1 john.doe users 224 May  1 21:40 foo.txt

The sed command with the s (substitue) option can be used to replace a pattern in a file. When the -i or --in-place flag is NOT used, the change will not actually be made. In this way, this is a sort of test that can be used to see the change, but to not actually make the change.

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

When the -i or --in-place flag is used, the replacement will actually be made.

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

Be aware that sometimes this may change the owner of the file, perhaps from john.doe to nobody.

~]$ ll foo.txt
-rw-rw-r--. 1 nobody users 224 May  1 21:40 foo.txt

The -c or --copy flag can be used to perserve the owner of the file.

sed --copy -i 's|Hello|Hi|' foo.txt

This is not just limited to files. sed can also be used with strings and variables. In this example, "Hello" will be replaced with the text "Hi" in the $foo variable. Again, when the -i or --in-place flag is NOT used, the change will not actually be made.

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

Multiple different replacements

In this example, Hello becomes Hi and World becomes Earth, in a single inline command.

echo $foo | sed 's|Hello|Hi|; s|World|Earth|'
. . .
Hi Earth