Terminal survival and the directory tree¶

Practical 4.1 Where is my file? PDF

Think of the last photo you took. Where is it right now? You point at your phone. Fine — but where inside the phone? Could you find it with a microscope?

Nobody can answer that on the first try. This unit answers it. And to walk down to where files really live, you need a tool that every server, every developer and every system administrator uses daily: the terminal. Ten short words, and you can do what takes forty clicks.

Where does a file live?¶

When you double-click an icon, the computer walks down five layers. Each one sits inside the next:

A file is cut into blocks, usually of 4 KiB (4,096 bytes) each, and the file system writes down which blocks hold it. You can see this on deb-lab. Save a file hello.txt containing the word hello in Documents, then ask:

ls -l ~/Documents/hello.txt
du -h ~/Documents/hello.txt
df -h ~/Documents/hello.txt

-rw-rw-r-- 1 student student 6 ... hello.txt
4.0K    /home/student/Documents/hello.txt
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1        ...   ...   ...   ... /

• ls -l says the file is 6 bytes: five letters plus the end-of-line byte (0A, from Binary).
• du says it takes 4.0K on the disk. The file system hands out space in whole blocks, so 6 bytes cost one full block.
• df names the partition that holds it: /dev/vda1 (or vda2, depending on your install). The command lsblk then shows the disk vda (25G) with that partition hanging under it.

And the disk vda? On the host, virt-manager shows that the whole disk of deb-lab is one file, ending in .qcow2, usually in /var/lib/libvirt/images/. A box inside a box: your file lives in a file system, inside a disk that is itself a file on the host's real SSD. That is why in this unit you can break disks and nobody cries — the new disks you add will be just files.

Why a terminal?¶

1969, Bell Labs: Ken Thompson and Dennis Ritchie start Unix. There are no screens. You sit at a teletypewriter, a typewriter wired to the computer: you type a line, the machine prints the answer on paper. That is why Linux still calls a terminal a tty, and why commands are so short — every letter was noise and paper.

The shell is the program that reads the line you type and runs it.

Ours is Bash, written by Brian Fox for the GNU project in 1989. The name is a joke: Bourne-Again SHell, after the older shell of Stephen Bourne.

It is 2026. Why would anyone still type?

The prompt¶

Open the Terminal in deb-lab. The shell greets you with the prompt:

student@deb-lab:~$

The shell is waiting. Nothing happens until you press Enter.

Where am I? pwd, ls, cd¶

student@deb-lab:~$ pwd
/home/student
student@deb-lab:~$ ls
Desktop  Documents  Downloads  Music  Pictures  Public  Templates  Videos
student@deb-lab:~$ ls -a
.  ..  .bash_logout  .bashrc  .profile  Desktop  Documents  ...

• pwd = print working directory: the answer to "where am I?".
• ls = list. ls -a = all: names that start with a dot are hidden.
• ls -l = long: one line per entry. The first letter says what it is: d directory, - file, l link.

Now move. cd = change directory:

student@deb-lab:~$ cd /etc
student@deb-lab:/etc$ cd ..
student@deb-lab:/$ ls
bin  boot  dev  etc  home  lib  lib64  lost+found  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var
student@deb-lab:/$ cd home/student
student@deb-lab:~$ cd Documents
student@deb-lab:~/Documents$ cd
student@deb-lab:~$

Watch the prompt: the part before $ changes every time you move.

Paths: the address of a file¶

A path is the address of a file or directory in the tree.

Think of a postal address — the one analogy of this page. There are two ways to give one:

The short forms you will use every day:

Three keys that make typing bearable¶

• Tab completes names. Type cd /usr/sh and press Tab: the line becomes cd /usr/share/. Press it twice when there are several choices. If Tab does nothing, the name does not exist — a free spell-check.
• Up brings back the previous line. Nobody types a long command twice.
• Ctrl+C cancels the current line or stops a running command. Ctrl+L clears the screen.

How to read a command¶

Every command line has up to three parts, separated by spaces:

ls        -l        /etc
command   option    argument
what      how       on what

• The command is the program to run.
• Options change how it works and start with -. They can be joined: ls -la.
• Arguments say what it works on. With no argument, most commands use the current directory.

When a command answers with an error, the error is the answer. Read it:

student@deb-lab:~$ cd /nope
bash: cd: /nope: No such file or directory
student@deb-lab:~$ Ls
bash: Ls: command not found
student@deb-lab:~$ cat /etc/shadow
cat: /etc/shadow: Permission denied

Make, copy, move, delete¶

A short session in your home, one line at a time:

student@deb-lab:~$ mkdir lab
student@deb-lab:~$ cd lab
student@deb-lab:~/lab$ touch notes.txt
student@deb-lab:~/lab$ mv notes.txt todo.txt
student@deb-lab:~/lab$ mkdir old
student@deb-lab:~/lab$ mv todo.txt old/
student@deb-lab:~/lab$ rm old
rm: cannot remove 'old': Is a directory
student@deb-lab:~/lab$ rm -r old
student@deb-lab:~/lab$ cd ..
student@deb-lab:~$ rmdir lab

The tree: FHS¶

Type ls /. Who decided those names? They come from Unix, and since the 1990s they are written down in a standard: the Filesystem Hierarchy Standard (FHS). Version 3.0 was published by the Linux Foundation on 3 June 2015. That is why what you learn on Debian works on Ubuntu, Fedora or a web server in a data centre.

The big idea: Linux has one tree, and it starts at /. No C:, no D:. A USB stick or a second disk appears as a directory somewhere inside the tree — that is called mounting, and it has its own page.

flowchart TB
    ROOT["<b>/</b><br>the root: top of the one tree"]
    ROOT --> HOME["<b>/home</b><br>one folder per person"]
    ROOT --> ETC["<b>/etc</b><br>system configuration"]
    ROOT --> USR["<b>/usr</b><br>installed programs"]
    ROOT --> VAR["<b>/var</b><br>changing data, logs"]
    ROOT --> OTHER["<b>/tmp /boot /dev /proc</b><br>/root /media /mnt"]
    HOME --> STUDENT["<b>/home/student</b><br>your home: ~"]
    STUDENT --> DOCS["<b>Documents</b>"]
    USR --> BIN["<b>/usr/bin</b><br>the commands"]
    VAR --> LOG["<b>/var/log</b><br>the logs"]

The Windows tree in three lines¶

• Windows has one tree per drive, each with a letter: C:, D:.
• The separator is the backslash: C:\Users\marta\Documents.
• Names are not case-sensitive: Notes.txt and notes.txt are the same file there, and two different files on Linux.

Practical 4.1 · Treasure hunt¶

It is your first day at Tramuntana Apps, a 12-person app studio in Palma. Before you touch anything, get to know the machine. The Practical 4.1 sheet (the button at the top) has ten questions to answer with today's commands only — the Debian version in /etc, the kernel in /boot, the processors in /proc/cpuinfo — and then a small tree to build for the three departments, dev, design and admin. You hand it in on Classroom as practical-4.1-surname.md.

Key takeaways¶

• A file lives in a chain: file → directory → file system → partition → disk. Space is given in whole blocks (4 KiB), and a VM's whole disk is one .qcow2 file on the host.
• The shell (Bash) reads your line; the prompt says who, where and which machine. A command line is command + options + arguments.
• pwd, ls, cd move you around. An absolute path starts at / and works from anywhere; a relative path depends on where you stand. .. is the parent, ~ is home.
• cat, less, mkdir, touch, cp, mv, rm. mv also renames; rm has no recycle bin.
• Linux has one tree described by the FHS: /home people, /etc configuration, /usr programs, /var logs, /dev devices, /proc the live kernel. Windows has one tree per drive letter.

Quick check¶

Now you can move around the tree. But what if the file you need is one among thousands? Next: Finding information.