Conceptual Architecture of UNIX/Linux Systems

Multiuser means many users can work safely on the same system

Linux was designed for shared systems. Multiple users can log in locally, over SSH, through terminal sessions, or through services. Each user has an identity, usually represented internally by a UID and one or more GIDs.

The kernel enforces who can read, write, execute, signal, or control resources. This is why one trainee can have files under /home/student1 while another has separate files under /home/student2.

Multitasking means many processes share CPU time

A Linux system may run hundreds or thousands of processes. The CPU can execute only a limited number of threads at the same instant, so the kernel scheduler rapidly switches between runnable tasks. This creates the practical effect that many programs run at once.

3. UNIX/Linux is case sensitive

In Linux, uppercase and lowercase letters are different characters in file names, directory names, commands, usernames, variables, and many configuration values.

Common mistakes trainees make

5. Linux does not depend on filename extensions

Linux does not require a file to have an extension such as .txt, .sh, .log, or .conf. Extensions are mainly conventions used by humans and applications.

How Linux decides what a file is

Everything is a file: what it really means

UNIX popularized a clean abstraction: many resources can be accessed using file-like operations such as open, read, write, and close. This does not mean every resource is a regular text file. It means many things can be represented by file descriptors or filesystem entries.

Everything active is a process

A running program is represented as a process. A process has a PID, parent PID, owner, state, memory mappings, open files, environment, and command line.

Lab 1 — Multiuser and identity

whoami
id
who
w
ps -u "$USER" -o pid,ppid,user,stat,comm | head

# Optional on a lab VM with sudo access:
sudo useradd trainee1
sudo passwd trainee1
su - trainee1
whoami
exit

Lab 2 — Multitasking and jobs

sleep 300 &
jobs
ps -o pid,ppid,stat,comm -p $!
kill $!
jobs

Lab 3 — Case sensitivity

mkdir case-lab
cd case-lab
touch Report.txt report.txt REPORT.txt
ls -l
cat > Demo <<'EOF'
This is Demo
EOF
cat > demo <<'EOF'
This is demo
EOF
cat Demo
cat demo
cd ..

Lab 4 — Filename extensions are conventions

mkdir extension-lab
cd extension-lab
cat > runme <<'EOF'
#!/bin/bash
echo "I have no .sh extension, but I can run."
EOF
chmod +x runme
./runme

cat > script.txt <<'EOF'
#!/bin/bash
echo "My extension says .txt, but I am still a shell script."
EOF
chmod +x script.txt
./script.txt
file runme script.txt
cd ..

Lab 5 — IPC using a pipe

printf "ok\nerror\nwarning\nerror\n" | grep error
ps aux | grep ssh
journalctl -n 20 2>/dev/null | grep -i error || true

Lab 6 — Named pipe / FIFO

mkdir ipc-lab
cd ipc-lab
mkfifo trainee_pipe

# Terminal 1:
cat trainee_pipe

# Terminal 2:
echo "hello from another process" > trainee_pipe

# Cleanup:
rm trainee_pipe
cd ..

Lab 7 — Everything is file-like

ls -l /dev/null
printf "discard this" > /dev/null
ls -l /proc/$$
cat /proc/$$/cmdline | tr '\0' ' '; echo
ls -l /proc/$$/fd