Having built a little foundation, we can now start to look at some of the operations of sed. The commands will be supplied with most Linux systems and are core commands.

We will dive directly into some simple examples:

$ sed 'p' /etc/passwd  

The p operator will print the matched pattern. In this case, we have not specified a pattern so we will match everything. Printing the matched lines without suppressing STDOUT will duplicate lines. The result of this operation is to print all the lines in the passwd file twice. To print the modified lines only, we use the -n option:

$ sed -n 'p' /etc/passwd  

Brilliant!! We have just reinvented the cat command. We can now specifically work with just a range of lines:

$ sed -n '1,3 p ' /etc/passwd  

Now we have reinvented the head command, but we can also specify the range in a regex pattern to recreate the grep command:

$ sed -n '/^root/ p' /etc/passwd  

We can see this demonstrated in the following screenshot:

Note that the caret character (^) means the beginning of the line, which means the line must start with the word root. Don't worry; we will explain all these regex characters in a separate chapter.