Day 4 - Beginnings and ends

– Come, let’s begin.

– We ended in F major.

Amadeus (1984)

Many times people don’t read long messages. You should be careful when you send emails, for

example. If they are too long people tend to read the beginning and then skip the central part and

read the end. That is, if the core of the message was the final result of your 20 years long research

on time travel which shows how to build a time machine out of a dishwasher, it is lost. Gone. Sorry,

you should have been more concise.

While this might be a bad habit people have, reading the initial and the final part of some stream of

data comes in handy very often. For example, we might want to list files in order of size and then

get the top 5 ones, or we might want to print out the last 10 log messages of your system, just to

keep an eye on what is happening.

Well, Unix provides two nice commands to perform those actions, namely head and tail. You should

be able to tell which is which from the name, which is a very nice thing their authors did for us.

When you run

$ head <file>

the system will show you by default the first 10 lines of the file. You can try this and the following

examples on the slices.txt file provided in the examples repository. That file is just a sequence

of numbered lines, from 1 to 20, so that you can easily check if what you did is correct. So head

slices.txt should shows you lines 1-10. Conversely, if you run tail slices.txt you will get lines

11-20, that is the last 10 ones of the file.

Ten lines are usually a good amount of content if you want to have a quick look into a file, maybe

to just have a glimpse of how the content is structured, but you might want to print less or more

lines. This can be done in both commands with the -n switch followed by the amount of lines that

you want. That is, head -n3 slices.txt will print the first 3 lines of the file.

Please note an important thing about the Unix command line: switches and their values can be

optionally separated by spaces, so executing head -n3 slices.txt or head -n 3 slices.txt gives

you the very same result. You can also add more than one space between the switch and the value,

if this makes you happy, but when we will start dividing the text in columns you will not be that

happy anymore about multiple spaces, so maybe just don’t do it. Friendly advice.