Understanding Unix

NUnix · L3 MIAGE · CM1 · 90 minutes

Louis Ledoux

ISTIC · University of Rennes

2026–2027

Linux is closer than you think

You can use it all day without opening a terminal.

Steam Deck runs SteamOS 3, based on Arch Linux. Raspberry Pi OS is based on Debian. AWS provides Amazon Linux for cloud applications. Word size does not represent usage.

Yes, even the coffee machine

Android phones

The Linux kernel manages processes, memory and hardware.

Google · platform architecture

Freebox Server Pop

A home internet box. Free publishes its Linux source code.

Free · firmware 4.9.10

MARO Model One

A coffee machine with embedded Linux, remote support and updates.

Toradex · May 2026

Linux can run behind a touchscreen or a web interface.

Free's package list includes Linux 6.13.12 for Freebox Server Pop firmware 4.9.10. MARO Model One uses Torizon OS. Toradex's May 2026 interview describes remote monitoring, updates and support for over 500 connected machines. Android uses the Linux kernel with its own application platform.

Unix and Linux usage

Mobile web use

69.17%

of observed mobile page views come from Android, built on Linux.

Statcounter · Sept 2026

Websites

92.1%

use the Unix family, including Linux, among sites whose OS W3Techs identifies.

W3Techs · 1 Oct 2026

Supercomputers

500/500

systems in the June 2026 TOP500 list use the Linux family.

TOP500 · June 2026

Statcounter measures mobile page views on participating websites. Its September figure is a snapshot from 1 October 2026 and can be revised within 45 days. Data: Statcounter Global Stats, CC BY-SA 3.0.

W3Techs counts websites with an identifiable OS; its Unix family includes Linux. TOP500 counts the 500 systems on the June 2026 list. Each percentage has a different population.

Course structure

Three lectures of 90 minutes.

Lecture	Topics	Application
CM1 · Unix	Files, paths and permissions	Find files and check access
CM2 · Commands	Processes, search, pipes and redirections	Count errors in a log
CM3 · Bash	Variables, conditions, loops and functions	Write scripts with arguments and loops

The exercises practise commands first, then scripts.

Working with files in Unix

1. Navigate directories and understand file paths.
2. Read files and quote names containing spaces.
3. Check file and directory permissions.

Why use a script?

Today: find the main error in one log.
Next week: check 200 logs every morning, including when you are away.

A saved script lets the team:

• Apply the same rule to every input.
• Review and change the procedure as text.
• Run it remotely or through a scheduler.
• Check output and failure status.

The 200-log workload is a hypothetical example. A scheduler can run a saved script each morning; SSH can run commands on a remote machine. A script repeats its mistakes too, so results and exit statuses need checking.

Which tool fits the task?

Task	A useful starting point
Explore sales figures, filter and make a chart	Excel or another graphical analysis tool
Inspect a server’s logs, processes and permissions	Remote shell and system commands
Repeat a file-processing procedure every morning	A tested script and a scheduler
Query related business tables	SQL and a database
Explain the result to the team	A spreadsheet, dashboard or report

Excel also offers Power Query and Office Scripts for repeatable work.

Power Query records transformations that can be refreshed. Office Scripts automates workbook operations. SQL suits queries across related tables. Shell commands give access to the files, processes and permissions of the machine hosting an application.

1. Linux and the shell

The operating system, the terminal and the command interpreter

What the operating system manages

Resource	Operating system responsibilities
Processes	Start, schedule and stop programs
Memory	Allocate memory and isolate processes
Files	Store data and control access
Devices	Provide interfaces to hardware

Programs request services through system calls.

Linux

• 1991: Linus Torvalds announces his kernel project.
• Linux is a Unix-like kernel, distributed as free and open source software.
• A distribution combines the kernel with tools, libraries and applications.

Our working environment: a Linux system with the Bash shell and command-line tools.

A Linux distribution combines the kernel with libraries, tools and applications. Historical source: Linux Foundation.

Unix, GNU and Linux

Name	Meaning
Unix	The operating-system family that inspired Linux
GNU	The project behind Bash and many command-line tools
Linux	The kernel: manages processes, memory and hardware

GNU = GNU’s Not Unix.
The name contains itself: a recursive acronym.

GNU aims to provide a free Unix-compatible system. The recursive acronym is explained in the GNU FAQ.

Terminal, shell, command

Name	Role
Terminal emulator	Displays text and receives keyboard input
Shell	Interprets command lines and runs commands
Command	Performs an operation, such as listing files

A script saves a sequence of shell commands in a file.

Konsole, GNOME Terminal and xterm are terminal emulators. Bash, zsh, ksh and csh are shells. The account database records the login shell, often in /etc/passwd. The examples use Bash.

A command and its arguments

ls -l /home/alice

Part	Role
ls	Command name
-l	Option: request the long listing format
/home/alice	Argument: directory to list

Arguments provide information to a command when it starts.

Getting help

man ls
info coreutils
apropos directory
help cd

• Tab completes a name or displays possible completions.
• Up recalls a previous command. history lists history.
• clear clears the display. In man, q quits.

help documents Bash builtins, including cd. man and info require the corresponding documentation to be installed. In interactive Bash, !! recalls and executes the previous command.

One directory tree

• The filesystem has a common root: /.
• Other filesystems appear at mount points in that tree.
• Each account has a home directory and belongs to groups.
• Permissions control access to files and directories.

Example home directory: /home/alice
The university may use a different path.

A USB filesystem can be mounted under /media or /mnt. Its device node is under /dev. The mount point gives access to the files stored on the device.

Where Linux stores files

Need	Typical location
Alice’s personal files	/home/alice
System configuration	/etc
Installed add-on applications	/opt
Changing application data	/var/lib
Log files	/var/log

Configuration: /etc · Logs: /var/log

These paths follow common Linux conventions. /var/lib stores persistent application state. Filesystem Hierarchy Standard.

Users, administrators and software

Directory	Typical role
/home, /root	User homes; the root account’s home
/usr	Most distribution-provided tools and libraries
/usr/local	Software installed locally by the administrator
/opt	Add-on software packages

/ is the filesystem root.
root is also the administrator account’s name.

/root is the root account's home directory, inside the filesystem root /. /usr contains software and shared resources. Ordinary user homes are usually under /home. FHS.

Temporary files and system information

Directory	Purpose / lifetime
/var	Changing data, often persistent
/tmp	Temporary files; cleanup policy varies
/run	Runtime state, cleared at boot
/dev	Device interfaces
/proc, /sys	Views of processes, kernel and devices

A path can expose live system information.
It need not refer to ordinary data stored on disk.

Files in /tmp may be removed at boot or by periodic cleanup. /proc and /sys are virtual filesystems that expose live kernel information. /proc/meminfo, for example, reports memory statistics.

Several filesystems, one tree

/
├── home/       user files
├── mnt/        a temporary mount point
└── media/      removable media mount points

Mounting makes another filesystem visible at a directory.
The device node might be /dev/sdb1; its files appear at the mount point.

/dev/sdb1 names a device partition; /media/alice/USB is an example mount point. Many distributions link /bin and /sbin into /usr. ls -ld /bin /sbin shows whether this is the case on your machine.

The current directory

Notation	Meaning
.	Current working directory
..	Parent directory
/	Root directory
~	Your home directory, expanded by the shell
$HOME	Variable containing your home path
$PWD	Variable containing the current directory path

An interactive login commonly starts in the home directory, but a terminal or program can start elsewhere. The directory used to resolve a relative path is the current working directory. Plain ~ means the current user's home; ~alice means Alice's home. ~/alice means a directory named alice inside your own home.

Absolute and relative paths

Current directory: /home/alice

Path	Interpretation
/home/alice/notes.txt	Absolute: start at /
notes.txt	Relative: start in the current directory
./notes.txt	The same relative path
../bob/notes.txt	Go to the parent, then into bob

A path tells us where to look. Permissions determine whether access is allowed.

2. Files and directories

Navigate, inspect and change the directory tree

Moving around

pwd
cd /home/alice
cd ..
cd

• pwd prints the current working directory.
• cd path changes the current directory.
• cd or cd ~ returns to your home.

Listing directory entries

ls
ls -l
ls -a
ls -la /home/alice

• -l adds details, including ownership and permissions.
• -a includes names beginning with ., such as .bashrc.
• ls directory lists that directory's contents.

Creating and removing directories

mkdir practice
rmdir practice

rmdir removes an empty directory.

rm -r practice also removes the directory's contents.
Shell deletion does not send files to a recycle bin.

Creating, copying and moving files

touch notes.txt
cp notes.txt notes-copy.txt
mv notes-copy.txt archive.txt
rm archive.txt

touch creates a missing file, or updates timestamps of an existing file.

cp and mv can overwrite an existing destination. cp file directory copies into the directory. cp -r source destination copies a directory tree; mv can also move directories.

Reading text files

Command	Result
cat notes.txt	Print the whole file
less notes.txt	Browse interactively, q to quit
more notes.txt	Browse a page at a time
head -n 5 notes.txt	Print the first five lines
tail -n 5 notes.txt	Print the last five lines
diff old.txt new.txt	Show differences between two files

Why is less called less?

Mark Nudelman was investigating errors in large logs.
He wanted to read the lines before an error.

His version of more could not move backward.
The new pager’s name joked about a backward more.

less app.log

In less, /ERROR searches for ERROR, n finds the next match, and q quits. Source: Mark Nudelman's FAQ.

Concatenating files

cat first.txt second.txt
cat

• With filenames, cat prints their contents in argument order.
• Without filenames, cat reads standard input.
• At an empty terminal input line, Ctrl+D indicates end of input.

cat first.txt - second.txt reads first.txt, then stdin, then second.txt. Ctrl+D is the terminal's end-of-file input action, not a signal. If characters are pending on a line, the first Ctrl+D submits them instead of producing an empty read. The distinction returns in the process lecture.

Filename patterns

Pattern	Matches
?	Exactly one character
*	Zero or more characters
[abc]	One character from the set

ls /etc/rc*
ls /etc/rc????
ls [abc]oo.c

The shell expands these patterns before running ls.

Examples depend on existing names. With Bash defaults, an unmatched pattern remains literal. A wildcard usually does not match an initial dot, and a slash is a path separator. [abc]oo.c may match aoo.c, boo.c and coo.c. These are filename patterns, also called globs; grep's regular expressions use different rules.

Quoting special characters

Form	Effect
\*	Treat the next * literally
'*.txt'	Preserve every character inside the quotes
"$HOME"	Expand the variable, keep the result as one word

echo *.txt
echo '*.txt'
cat "meeting notes.txt"

Quotes help preserve one argument even when its value contains spaces.

Double quotes allow variable, command and arithmetic expansion. Backslash can escape selected characters. The shell removes its quote characters before passing arguments to the command.

What does the command receive?

In a directory containing only a.txt and meeting notes.txt:

printf '<%s>\n' *.txt
printf '<%s>\n' '*.txt'

<a.txt>
<meeting notes.txt>
<*.txt>

The first glob expands into two arguments. A matched filename containing spaces stays one argument. The quoted pattern stays literal. printf repeats its format for each argument. LC_ALL=C gives the ordering shown here.

Checkpoint: files and paths

1. What does cd .. do?
2. Which ls option includes hidden entries?
3. What does cat a.txt b.txt print first?
4. Does echo '*.txt' expand the pattern?

1. Move to the parent directory.
2. -a
3. The contents of a.txt.
4. No. Single quotes preserve the literal *.

3. Who may access this file?

Identify the account, inspect the path, explain the permission check

Users, groups and ownership

whoami
id
ls -ld .
ls -l

• id shows your user identity and groups.
• ls -l shows the file’s owner, group and permission bits.
• ls -ld inspects a directory itself.

These commands show your account and the ownership of files in the current directory.

Permission classes

-rwxr-xr--
  u  g  o

Class	Applies to
u	The file's owner
g	Members of the file's group
o	Other users
a	All three classes when using chmod

The first character is the entry type: - for a regular file, d for a directory, l for a symbolic link. The next nine characters form the owner, group and other permission classes. The applicable class determines access; the bits from different classes are not added together. ACLs and privileged accounts add further rules.

What r, w and x allow

Bit	Regular file	Directory
r	Read contents	List entry names
w	Modify contents	Add or remove entries*
x	Execute	Search / traverse

*Changing directory entries also requires search permission. Other restrictions, such as the sticky bit, may apply.

Deleting a filename is an operation on its parent directory. A file's own write bit is not the permission that directly authorises deleting its name. Directory read permission alone does not grant access to the files inside it.

Changing permission bits

chmod u+x script.sh
chmod go-w notes.txt
chmod u=rw,g=r,o= notes.txt

• + adds the selected permission bits.
• - removes them.
• = sets the selected class to the listed permissions.

The owner can normally change a file's permissions. If u, g, o or a is omitted, the umask affects which bits chmod changes.

Numeric permissions

Each class adds its bit values: r = 4, w = 2, x = 1.

Owner	Group	Others
rwx = 7	rw- = 6	r-- = 4

chmod 764 notes.txt

The three digits describe owner, group, others, in that order.

Checkpoint: permissions

1. Decode 640 for a regular file.
2. Add execute permission for the owner of run.sh.
3. Which directory bit allows traversal?

1. Owner: rw-
   Group: r--
   Others: ---
2. chmod u+x run.sh
3. x

“I can see the name, but cannot read it”

# Illustrative server metadata
 drwxr----- alice students  shared/
 -rw-r----- alice students  shared/notes.txt

Bob belongs to students. He can list the names, but cannot open notes.txt. Why?

The directory’s group bits lack x: traversal.
If team access is intended, the owner can add g+x.

The directory lacks group search permission. Its owner can grant it with chmod g+x shared. Bob then has directory traversal and file read access. ls -ld shows the permissions on each directory in the path. The ownership values shown are part of the example.

CM1 summary: files and permissions

1. Locate configuration, software, data and logs.
2. List and read files; quote names containing spaces.
3. Check owner, group and other permissions.

Optional reference

Examples to revisit during TD or personal practice

Operating system, shell and GUI

Layer	What it gives us
Operating system	Processes, storage and access control
Graphical interface	Windows, menus and direct visual interaction
Shell	Commands we can combine and save in scripts
Application	A task-specific tool, such as Excel or a database

Your laptop can run Windows while the company’s application runs on Linux. You need to work with both.

Linux distributions can provide graphical desktops. Windows provides shells such as PowerShell, and WSL runs a Linux environment. The operating system and the user interface are separate choices.

Summarise a log before opening it in Excel

Quantity	Meaning
2,000,000 log lines	Example input size
1,048,576 rows	The maximum in one Excel worksheet
One row per error code	The summary the team wants to discuss

Process the raw log where it lives, save the counts, then use Excel to analyse or present the summary.

Microsoft: worksheet limits. Power Query and the Data Model offer other ways to handle larger datasets.

Two million log lines is a hypothetical input. The worksheet limit does not cover every Excel workflow: Power Query can transform data before loading, and the Data Model provides another destination. Text tools also consume resources; sorting may use temporary disk space.

Path components and word counts

dirname /home/alice/notes.txt
basename /home/alice/notes.txt
wc -w notes.txt

• dirname prints /home/alice.
• basename prints notes.txt.
• wc -w counts words. wc -l counts newline characters.

Hard links and symbolic links

Hard link

ln notes.txt other.txt

• Another name for the same file.
• Removing one name leaves the other usable.
• Same filesystem, normally no directory links.

Symbolic link

ln -s notes.txt shortcut

• Stores a path to a target.
• May cross filesystems or target a directory.
• Can become dangling if the target path disappears.

A relative symbolic-link target is resolved from the directory containing the link. Hard links are equivalent names for the same file.

Builtins and external commands

Builtin

Implemented inside the shell.

cd ..

Can change the current shell's state.

External program

Loaded from an executable file.

ls -l

The shell searches directories in PATH.

PATH commonly includes /usr/bin and other directories. An external cd could only change its own process's working directory. cd must run inside the shell to change that shell's directory. echo and pwd can have both builtin and external implementations.

Recognising a command

type cd
type ls
type -a echo
help cd

type reports how Bash resolves a name.
help documents its builtins.

Aliases and functions can change which command runs. type -a lists all available interpretations of a name.