∙ Viewing the contents of files

Viewing files

There are two main ways of viewing the contents of files on the command line: “printing” the output to the terminal window, and viewing file contents with a “pager”.

cat

Let’s look at outputting first with a command called cat. Sadly, this has nothing to do with small furry felines. cat is short for “conCATenate”, which means “join files end to end”, but it is commonly used for looking at a single file. Move into the folder called example_files, and run this command

cat meaning_of_life.txt

which means “output the contents of of the file meaning_of_life.txt to the terminal”. This is useful for seeing what is in short files. Using cat for longer files, it is less useful.

Move into the folder called alexandre-dumas and try this and see for yourself:

cat The-Count-Of-Monte-Cristo.txt

head and tail

We might want to just see the start or the end of the file, so experiment with these commands:

head The-Count-Of-Monte-Cristo.txt
tail The-Count-Of-Monte-Cristo.txt

head and tail are common both on the command line, and in programming languages like Python and R. We can change how many lines we see using the flag -n followed by a number.

More ways of printing files

Here are some more commands for outputting file contents in a variety of ways. Try them on the example files and work out what they do. You might want to run these commands on shorter files to make it easier to understand the output, animals.txt, back in the sandpit folder might be useful here.

tac
rev
uniq
sort

These might seem trivial (maybe even useless!) but these are powerful building blocks for arranging data.

Introducing wildcards *

We will cover this in more detail later, but let’s look at using the asterisk, *, as a wildcard. Experiment with a command like these

head -n 3 numbers*
tail -n 4 w*
head -n 2 *
head -n 3 *ea*

The strangely-named less

Now let’s look at using a pager. A pager does not put the file contents to the terminal output, but instead shows it to you in a piece of dedicated software. It might not look like it, since it is still just plain text in your terminal window! But when using a pager, you are no longer on the command line. We will use the command less, which will open a pager to view a file. (Why this program is called “less” is a story for another time :)

less The-Count-Of-Monte-Cristo.txt

Programs like less are useful for viewing large files, since it does not need to load the whole file into memory to show it to you. There are many things that less can do, but for now I will mention that you can search by pressing / and typing what you are looking for, and you can start less at the bottom of the file with less +G [filename].