The filesystem hierarchy, environment variables and shell scripts

So far you have worked inside your own home folder. But a Linux computer has thousands of other files, and they all have a tidy place to live. Knowing the map tells you which folders are yours to use, which hold settings and which you should leave alone. This lesson also shows how the shell remembers small facts such as your user name, and how you can save a few commands in a file so they run again with one line. Everything you type here stays inside your practice folder.

One tree, starting at the root

On Windows you may know separate drives such as C: and D:. Linux has just one tree. Everything hangs from a single starting point written / and called the root of the File system (not to be confused with the Root user, who just happens to share the name). Other disks and USB sticks are attached to a folder somewhere in that tree. This agreed layout is called the filesystem hierarchy, and nearly every Linux system follows the same plan, so what you learn on Ubuntu helps on other versions too.

Think of a large library. The front desk, the staff rooms, the stockroom and the reading tables all have fixed places. A visitor can use the reading tables freely, but should not wander through the staff rooms moving things. The same idea applies here.

Which folders are yours to use?

How full is the disk?

A full disk causes odd problems: programs fail to save, updates stop and the computer slows down. Two read-only commands help. df -h shows the free space of each disk (df means disk free, -h means human readable, so sizes appear as 61G instead of a long number of bytes). du -sh ~/practice shows how much space one folder uses (du means disk usage, -s gives one total and -h is again human readable). Neither command changes anything.

Your real output will have different names and numbers, and may include extra lines. Look for the line whose last column is /. When Use% there climbs above about 90%, it is time to delete things you own, such as old downloads, or to move them to external storage.

Environment variables: the shell's memory

An Environment variable is a named piece of text that the shell keeps and hands to the programs it starts. It is like a name badge: the program can look at it to learn who you are or where your home is. By tradition the names are written in capitals. To use a variable's value you put a dollar sign in front of its name. The shell replaces $USER with the text before running the command.

echo prints text, so echo $HOME prints the value of the variable. You can also make your own variable with NAME=value, with no spaces around the equals sign. The next command sets one, then prints it. It lasts only in this terminal window and disappears when you close it, so it is safe to experiment.

echo $USER
echo $HOME
greeting=Hello
echo $greeting, $USER

amina
/home/amina
Hello, amina

PATH: how the shell finds a command

When you type ls, you do not write where the program is. The shell looks in each folder listed in the PATH variable, from left to right, and runs the first file with that name. The folders are separated by colons. If no folder has it, you get command not found.

The command which ls prints where the shell found the program, for example /usr/bin/ls. Your own home folder is not in PATH, which is a safety feature: a stray file in a folder you happen to be in cannot pretend to be a normal command. This is why you must write ./hello.sh, where the leading ./ means 'the one in this folder', to run a script you have just made.

Your first shell script

A Shell script is a plain text file containing commands that the shell runs from top to bottom, as if you had typed them. Scripts save time and avoid typing mistakes, because the same steps run the same way each time. The first line, #!/bin/bash, is called the shebang: it tells the system which program should read the file. Lines starting with # are comments, notes for humans that the shell ignores.

First create the practice folder if you do not have one yet, move into it and open a simple editor called nano. mkdir -p makes the folder and does not complain if it already exists. nano hello.sh opens a new empty file called hello.sh.

mkdir -p ~/practice
cd ~/practice
nano hello.sh

Type the following lines into nano. $(date +%A) runs the date command and inserts today's weekday name, and $(pwd) inserts the folder you are in. Quote marks keep each message together as one piece.

#!/bin/bash
# hello.sh: my first safe script. It only prints text.
name="$USER"
echo "Hello, $name!"
echo "Today is $(date +%A)."
echo "You are in: $(pwd)"

In nano, save with Ctrl+O then press Enter to confirm the file name, and leave with Ctrl+X. Then try to run the script. It will fail first, on purpose: a new file is not allowed to run as a program until you say so. That is the permission system from the previous lesson at work. The command chmod u+x hello.sh adds the 'execute' permission for you, the owner, only.