Linux Operating System
When we talk about Linux we first must understand its origins and the UNIX
operating system. Unix was born in 1969.
AT&T's Bell Laboratories in the early 1970s.
The Unix operating system was conceived and implemented by Ken Thompson and Dennis Ritchie (both of AT&T Bell Laboratories) in 1969 and first released in 1970.
Programming language, C, to make it portable.
In 1977, the Berkeley Software Distribution (BSD) was developed by the Computer Systems Research Group (CSRG) from UC Berkeley, based on the 6th edition of Unix from AT&T.
The 1960's "saw" people organizing groups and actively working for change in the social order along with the government.
UNIX system code all began developing their own different versions.
Universities, research institutes, government bodies and computer companies are using the UNIX system to develop many of the technologies.
Vendors concerned about encroachment into their markets and control of system interfaces, developed the concept of "open systems.".
In 1987, AT&T announced a pact with Sun Microsystems(Berkeley UNIX).
Industry clubbed together to develop their own "new" open systems operating system, the Open Software Foundation (OSF).
What is an Operating System?
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also include accounting software for cost allocation of processor time, mass storage, printing, and other resources.
Linux began in 1991 as a personal project by Finnish student Linus Torvalds: to create a new free operating system kernel.
In 1983, Richard Stallman started the GNU project with the goal of creating a free UNIX-like operating system.
https://www.gnu.org/home.en.html
GNU's not Unix
As part of this work, he wrote the GNU General Public License (GPL).
The GNU General Public License is a free, copyleft license for software and other kinds of works.
GNU General Public License is intended to guarantee your freedom to share and change all versions of a program--to make sure it remains free software for all its users.
In 1985, Intel released the 80386, the first x86 microprocessor with a 32-bit instruction set and a memory management unit with paging.
The Operating System boots or brings up a kernel.
Linux is an operating system that allows users to control computers.
Kernel - Kernel manages hardware resources and acts as a bridge between other software components.
The kernel is the interface between hardware and software.
The user transmits commands to the kernel through the shell.
The Linux kernel is the core (hence the name "kernel") of the Linux operating system.
It’s the bridge between your computer’s hardware (CPU, memory, storage, devices) and software (apps, commands, services).
Open source - Denoting software for which the original source code is made freely available and may be redistributed and modified.
Distribution
A Linux distribution is a combination of software that comes with the Linux kernel and allows users to manage the operating system.
Ubuntu
RedHat
Debian
Shell - A shell is a program in which the user instructs the operating system by entering commands.
File - A file is the unit in a computer system where data is stored.
Directory (Folder) - A directory is the unit in which files and subdirectories are organized. A directory can contain other files and subdirectories.
Tree Structure - Files and directories form a tree structure in computer systems. At the top level, there is a root directory.
Path - Addressing system used to specify the location of files or directories. The file path specifies the exact location of the file or directory.
Free - (for the most part)
Ctrl +c will always get prompt back
All Linux/Unix systems have a superuser account called "root".
This account can change passwords, delete files and do everything on the system. Full permissions.
Linux is case sensitive and does not use spaces in file names.
A file system is necessary to store files on a computer.
The Linux file system starts with the root directory (like a tree)
and branches into subdirectories from the root.
Each sub-directory can have move subdirectories underneath so it is like an upside down tree.
Linux File System Directories.
/bin: Where Linux core commands reside like ls, mv.
/boot: Where boot loader and boot files are located.
/dev: Where all physical drives are mounted like USBs DVDs.
/etc: Contains configurations for the installed packages.
/home: Where every user will have a personal folder to put his folders with his name like /home/john_iacovacci1 (taken from my gmail login)
/lib: Where the libraries of the installed packages located since libraries shared among all packages
/root: The home folder for the root user.
/sbin: Like /bin, but binaries here are for root user only.
/tmp: Contains the temporary files.
/usr: Where the utilities and files shared between users on Linux.
/var: Contains system logs and other variable data.
The cron command-line utility is a job scheduler on Unix-like operating systems. Users who set up and maintain software environments use cron to schedule jobs, also known as cron jobs, to run periodically at fixed times, dates, or intervals.
How it Flows
Applications make requests via system calls (e.g., read(), write()).
System libraries provide convenient programming interfaces.
The System Call Interface hands requests to the kernel.
The kernel manages:
CPU time (process scheduler)
Memory allocation
File operations
Device communication
Network transfers
The kernel uses device drivers to talk directly to hardware.
The default directory you are positioned under would be /home/email
The pwd command (print working directory) will display your full path name on the disk location you are working on.
$ pwd
/home/john_iacovacci1
To create a directory you use the mkdir command
$ mkdir assign
The navigation commands
The cd command allows you to move into that directory
$ cd assign
cd .. brings you up none level to previous directory
$ cd ..
you can change directory to an absolute path e.g.
cd $HOME will bring you to your default home directory
$ cd $HOME
you can change directory to an absolute path e.g.
$ cd /home/john_iacovacci1/assign
Absolute path always begins with /
Relative path allows you to change to a directory relative to where you currently are set to.
Note: cd $HOME
Will take you to your home directory.
$ cd $HOME
The touch command allows you to create a file
$ cd assign
$ touch file1
ls - lists all files and directories within the working directory (or path you provided)
$ ls
File1
Adding the -lt option for (for ownership and time stamp)
$ ls -lt
total 0
-rw-rw-r-- 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:17 file1
The ls -lt command is a powerful combination used to list files in a directory while sorting them by time. It helps you quickly see which files were recently created or modified.
To understand it, let’s break down the command into its individual parts:
ls: The base command, short for "list." It displays the contents of a directory.
-l: The "long format" flag. Instead of just showing filenames, it provides detailed information like permissions, owner, file size, and the last modified date.
-t: The "time" flag. This tells Linux to sort the list by the modification time, placing the newest files at the top.
What the Output Looks Like
When you run ls -lt, the output is organized into columns. Here is a breakdown of what each column represents:
Linux files have 3 sets of permission attributes
read(r), write(w) and execute(x) for owner, group and everyone
See how the file is permissioned.
chmod command can change the permissions of a file or directory
$ chmod +x file1
$ ls -lt
total 0
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:17 file1
The file mask has added execution permission which will allow us to create linux shell script later on. Also, files with execution permissions display on the screen in green.
To copy a file use the cp command
$ cp file1 file2
$ ls -lt
total 0
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:46 file2
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:17 file1
echo is a command to repeat what you type
$ echo "Hello World"
Hello World
By default the output of commands go to the standard output or display screen
Redirection is a feature in Linux such that when executing a command, you can change the standard input/output devices.
Redirection allows you to redirect output to a file
The > symbol allows you to redirect output to a file
$ ls -lt
total 4
-rw-rw-r-- 1 john_iacovacci1 john_iacovacci1 12 Jan 15 23:49 file3
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:46 file2
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:17 file1
cat is a command that display contents to a file
$ cat file3
Hello World
A wildcard allows you to use any command to pattern match to a file.
? - one character
* - all characters
[] - range of characters
ls -lt file?
ls -lt f*
First lets create a file called filtest
$ touch filtest
$ ls -lt
total 4
-rw-rw-r-- 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:54 filtest
-rw-rw-r-- 1 john_iacovacci1 john_iacovacci1 12 Jan 15 23:49 file3
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:46 file2
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:17 file1
Now lets look for all files that match the pattern file?
$ ls -lt file?
-rw-rw-r-- 1 john_iacovacci1 john_iacovacci1 12 Jan 15 23:49 file3
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:46 file2
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:17 file1
We can now use the wildcard * to match the pattern fil*
$ ls -lt fil*
-rw-rw-r-- 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:54 filtest
-rw-rw-r-- 1 john_iacovacci1 john_iacovacci1 12 Jan 15 23:49 file3
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:46 file2
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:17 file1
other file commands are
Linux commands are the primary way to interact with the system's kernel. While there are thousands of commands, they generally fall into a few core categories: file management, system information, and networking.
File and Directory Management
These are the most common commands used to navigate the "tree" structure of the Linux file system.
ls (List): Shows the contents of a directory.
Tip: Use ls -la to see hidden files and detailed information like permissions.
cd (Change Directory): Moves you between folders.
Example: cd /home/user/Documents
pwd (Print Working Directory): Displays the full path of the folder you are currently in.
mkdir (Make Directory): Creates a new folder.
rm (Remove): Deletes files or directories.
Warning: rm -rf deletes folders and their contents permanently without a trash bin.
cp (Copy): Copies files or directories from one location to another.
mv (Move): Moves files, but is also used to rename files.
Viewing and Editing Files
These commands allow you to look inside files or modify their text without leaving the terminal.
cat (Concatenate): Displays the entire content of a file on your screen.
less: Similar to cat, but allows you to scroll through long files one page at a time.
head / tail: Displays the first 10 or last 10 lines of a file, respectively.
nano / vi: These are text editors built into the terminal. Nano is beginner-friendly, while Vi (or Vim) is powerful but has a steeper learning curve.
grep (Global Regular Expression Print): Searches for specific text within a file or output.
Example: grep "error" logfile.txt
System Permissions and Ownership
Linux is built on a strict security model where every file has an owner and specific permissions.
sudo (SuperUser Do): Runs a command with administrative (root) privileges.
chmod (Change Mode): Changes the read, write, and execute permissions of a file.
chown (Change Owner): Changes which user or group owns a file.
System Information and Management
Use these to check how your computer is performing or to manage software.
top / htop: Displays real-time information about running processes and CPU/RAM usage.
df (Disk Free): Shows how much space is left on your hard drives.
free: Displays the amount of free and used memory (RAM).
ps (Process Status): Lists the processes currently running on your system.
apt / yum / pacman: These are Package Managers used to install, update, or remove software (the specific command depends on your Linux distribution).
Networking
ping: Checks the connection between your machine and a domain/IP address.
ip addr: Shows your network interfaces and IP addresses.
ssh (Secure Shell): Allows you to securely log into a remote Linux computer.
TAB can be used to fill up the terminal command.
Example I can partially enter a file name or directory and use tab to fill it up as long as no other matching pattern exists. Can’t partially enter fil because too many choices but fil only has one.
$ ls -lt filt
Hit tab and it finishes the command
$ ls -lt filtest
Ctrl+C can be used to stop any comma
The UP Arrow calls up the last linux command so the user can execute it again by hitting return.
Linux keeps a history of entered commands so you can hit UP until you find the previous command entered you want to execute.
exit terminal by using the exit command.
A shell script is a computer program designed to be run by a Unix shell, a command-line interpreter.
Click on New file and enter hw.sh
First line provides path to the shell
Use echo command to display “Hello UCONN World”
#!/bin/bash
echo "Hello UCONN World"
Go back to Open Terminal
Make the file executable
$ chmod +x hw.sh
$ ls -lt
total 8
-rwxr-xr-x 1 john_iacovacci1 john_iacovacci1 37 Jan 16 00:19 hw.sh
To execute the shell command you need to use the ./hw.sh which instruct linux to execute the command in the current directory.
$ ./hw.sh
Hello UCONN World
Now let's talk about variables.
A variable allows programs to store values and assign those values to a name.
We can use the shell read command to read the input from the terminal and assign the value inputted to a variable. Then we can use the echo command to display the value of that variable.
====================================================
### **Basic Arithmetic Operators**
# Addition
`echo $((5 + 3)) # 8`
|
| `-` | Subtraction | `echo $((10 - 4)) # 6` |
| `*` | Multiplication | `echo $((7 * 6)) # 42` |
| `/` | Division (integer) | `echo $((20 / 3)) # 6` |
| `%` | Modulus (remainder) | `echo $((20 % 3)) # 2` |
| `**` | Exponentiation | `echo $((2 ** 3)) # 8` |
### **Assignment Operators**
| Operator | Meaning | Example |
| -------- | ------------------- | ------------------------------- |
| `=` | Assign value | `x=10` |
| `+=` | Add and assign | `x=5; ((x+=3)); echo $x # 8` |
| `-=` | Subtract and assign | `x=10; ((x-=4)); echo $x # 6` |
| `*=` | Multiply and assign | `x=4; ((x*=2)); echo $x # 8` |
| `/=` | Divide and assign | `x=20; ((x/=5)); echo $x # 4` |
| `%=` | Modulus and assign | `x=7; ((x%=3)); echo $x # 1` |
---
### **Increment and Decrement Operators**
| Operator | Meaning | Example |
| -------- | --------- | ----------------------------- |
| `++` | Increment | `x=5; ((x++)); echo $x # 6` |
| `--` | Decrement | `x=5; ((x--)); echo $x # 4` |
—
================================================
#!/bin/bash
# Simple addition script
num1=10
num2=5
# Perform addition
sum=$((num1 + num2))
# Display result
echo "The sum of $num1 and $num2 is: $sum"
# Perform subtraction
result=$((num1 - num2))
# Display result
echo "The result of $num1 - $num2 = $result"
# Perform Multiplication
# Using let
let result=num1*num2
echo "Using let : $num1 * $num2 = $result"
==================================================
$ chmod +x ao.sh
$ ./ao.sh
The sum of 10 and 5 is: 15
The result of 10 - 5 = 5
Using let : 10 * 5 = 5
===========================================
#!/bin/bash
#String Variables
greeting="Hello, World"
echo $greeting
x=10
y=20
# Arithmetic expansion
sum=$((x + y))
echo "Sum is $sum"
# Indexed array
fruits=("apple" "banana" "cherry")
echo "First fruit: ${fruits[0]}"
# Loop through array
for fruit in "${fruits[@]}"; do
echo $fruit
done
# Associative Arrays
declare -A capitals
capitals[France]="Paris"
capitals[Germany]="Berlin"
capitals[Italy]="Rome"
echo "Capital of Germany is ${capitals[Germany]}"
# Check if variable is set
if [ -z "$name" ]; then
echo "Name is not set"
else
echo "Name is $name"
fi
# Declare integer type explicitly
declare -i count=5
count+=2
echo "Count is $count"
====================================================
$ chmod +x dt.sh
$ ./dt.sh
Hello, World
Sum is 30
First fruit: apple
apple
banana
cherry
Capital of Germany is Berlin
Name is not set
Count is 7
Validation
================================================
#!/bin/bash
echo "Enter your Age : "
read AGE
# Validate age: must be an integer between 1 and 120
if [[ ! "$AGE" =~ ^[0-9]+$ ]]; then
echo "Error: Age must be a number."
exit 1
elif (( AGE < 1 || AGE > 120 )); then
echo "Error: Age must be between 1 and 120."
exit 1
fi
echo "Age: $AGE"
====================================================
$ chmod +x vd.sh
$ ./vd.sh
Enter your Age :
25
Age: 25
$ ./vd.sh
Enter your Age :
300
Error: Age must be between 1 and 120.
name.sh terminal input
====================================================
#!/bin/bash
echo "Enter your full name : "
read my_name
echo "My Name is : " $my_name
====================================================
$ chmod +x name.sh
$ ./name.sh
Enter your full name :
John Iacovacci
My Name is : John Iacovacci
Now add redirection to name.sh
====================================================
#!/bin/bash
echo "Enter your full name : "
read my_name
echo "My Name is : " $my_name > my_info.txt
====================================================
$ ./name.sh
Enter your full name :
John Iacovacci
john_iacovacci1@cloudshell:~/assign (vertex-uconn)$ ls -lt
total 28
-rw-rw-r-- 1 john_iacovacci1 john_iacovacci1 29 Jan 16 01:03 my_info.txt
-rwxr-xr-x 1 john_iacovacci1 john_iacovacci1 99 Jan 16 01:02 name.sh
-rwxr-xr-x 1 john_iacovacci1 john_iacovacci1 310 Jan 16 00:33 vd.sh
-rwxr-xr-x 1 john_iacovacci1 john_iacovacci1 670 Jan 16 00:31 dt.sh
-rwxr-xr-x 1 john_iacovacci1 john_iacovacci1 375 Jan 16 00:25 ao.sh
-rwxr-xr-x 1 john_iacovacci1 john_iacovacci1 37 Jan 16 00:19 hw.sh
-rw-rw-r-- 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:54 filtest
-rw-rw-r-- 1 john_iacovacci1 john_iacovacci1 12 Jan 15 23:49 file3
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:46 file2
-rwxrwxr-x 1 john_iacovacci1 john_iacovacci1 0 Jan 15 23:17 file1
$ cat my_info.txt
My Name is : John Iacovacci
Environmental variables and PATH
In Linux, environment variables are dynamic "placeholders" that store information about the system environment. They are used by the shell and other processes to determine how the system should behave and where to find specific files.
Think of them as a global dictionary of settings that any program can look up when it starts.
How They Work
Every time you open a terminal session, the system initializes a set of variables. These variables exist in the background and guide the operating system's interaction with the user.
Common Examples
$PATH: Perhaps the most important variable. It contains a list of directories where the system looks for executable files (commands).
$USER: Stores the username of the current logged-in user.
$HOME: Points to the path of the current user's home directory.
$SHELL: Identifies which shell program you are currently using (e.g., /bin/bash or /bin/zsh).
$PWD: Stores the current working directory path.
Key Commands
You can interact with environment variables using these basic commands:
Why They Matter
Environment variables allow for portability and automation. Instead of hard-coding a specific file path into a script, a developer can use $HOME. This ensures the script works for every user on the system, regardless of their specific username or directory structure.
By putting the directory in the PATH, you do not have to use the ./ that tells linux the script is in the local directory. If it is not in the PATH it you need the ./
$ export PATH=$PATH:/home/john_iacovacci1/assign
$ hw.sh
Hello UCONN World
No comments:
Post a Comment