Day 6 - Printing specific lines

– You’ve been targeted for termination.

Terminator (1984)

Today I want to introduce you to a friend of mine, its name is sed. It’s full name is actually Stream

EDitor, but few remember it and it doesn’t care that much. Actually it answers only when you call

it with the nickname. Anyway, it is an incredibly powerful command, but for the moment I can’t

show you exactly what it can do, as you might not get how useful it will be for your work. We will

thus explore just one of the many things it can do, and probably the less important one, which is to

print lines.

The problem we often have is that of printing a specific line of a text. With head and tail we learned

how to print a certain number of lines starting from the beginning or the end, but what if we want

a line somewhere in the middle of the file? One possible solution comes from pipes. If you run

$ head -n 5 slices.txt | tail -n 1

you are asking head to show you the first 5 lines of the file, and then filtering this output with tail,

getting the last line. And this ultimately gives you exactly the line number 5.

So far so good, but this is a pretty convoluted way to just print line 5, isn’t it? Indeed, sed thinks the

same, and so it provides a simpler way to do it.

$ sed -n 5p slices.txt

I know, I know. You think that sed -n 5p is not the first thing that comes to your mind when

simplicity is the subject of the conversation. At any rate, I hope you at least agree on the fact that

having only one command is better than having to pipe two of them. Your new friend sed accepts

input from pipes as well, so whenever you want to see a specific line you can pipe the current

command into it. We will see examples of this later in the book.

I said sed is extremely powerful, so is it possible it can only print a single line? Not at all, it can

definitely print a sequence of lines, you just need to provide the first and the last line separating

them with a comma.

$ sed -n 3,8p slices.txt

This prints lines 3-8 of the given file. Are you not satisfied yet? OK, sed can also print a certain

number of lines starting from a given one. This often saves me from the shame of showing the

world that I am terrible at math.