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Operating System Lab - Commands

This Repository Contains a Collection of Linux Commands Taught in each OS Lab Class Separated by Weeks

Table of Contents

Week 1 to 6

Popular Operating Systems

  • Windows, MacOS, Linux, Android and ...

Operating Systems History

Operating systems evolved from simple mainframe setups in the 1950s to today's complex, user-friendly platforms like Windows, macOS, and Linux. Key milestones include the advent of Unix in the 1970s, Windows dominance in the PC market in the 1990s, and the rise of mobile operating systems like Android and iOS in the 2000s.

  • Operating Systems Differences
  • Operating Systems Distributions
  • Open Source and Closed Source

What is Linux ?

Linux is a family of open-source operating systems based on the Linux kernel. It's known for its flexibility, stability, and security, and it's widely used in servers, supercomputers, and embedded systems.

  • Why Linux
  • Install Linux
  • Working with Linux

Linux Commands

1. ls

List Directory Contents

ls

ls also Can be Used with Options :

  • ls -a ls --all Show All Including Hidden Items

  • ls -t Show and Sort Items by Time, Newest First

  • ls -l Show Items Information Including Size, Permissions and Creation Time

Options Also Can be Used Together

  • ls -l -t -a Separated Options

  • ls -lta Merged Options

2. cd

Change Directory

cd folder_name              # Go to any Folder You Want
cd Downloads                # Go to Download Directory of Linux if It is Available in Where You are Now At
cd ~/Downloads              # Go to Download Directory of Linux from Wherever You are
cd /home/user/Downloads     # Go to Download Directory of Linux from Root Path
cd ..                       # Go to Previous Folder
cd .                        # Go to Working Directory, Not Useful for cd Command

3. Linux Path Address Symbols

/ Root Directory, All Paths Start from This Directory Like /proc/cpuinfo is Where Your Cpu Information is

/home/ramtin/Desktop Absolute Path, Absolute Address from Only Root Directory, Address from First

Desktop/Class/OS-Lab Relative Path, Relative Address from Wherever You Want, Address between Directories

~ User Home Directory, Where User's Directories are Like : Downloads, Desktop and ... Like cd ~/Desktop Goes to Desktop

. Working Path Address Like : cp ./file ~/Desktop Copy file from Wherever You are to Desktop

.. Previous Directory Like : cd ../../../ Changes Your Directory to Previous Directory 3 Times

4. clear

Clears Your Terminal

clear

5. exit

Exits Your Terminal Shell Session

exit

6. mkdir

Create Directory

mkdir My_Directory

if You Want to Create Directory with Name that Contains Blank Character Like Space, Add \ before each Space

mkdir My\ Directory\ Number\ 3

if You Want to Create Multiple Directories at Once Write Names Next to each Other

mkdir dir1 dir2 dir3 dir4

7. touch

Updates Last Opened Time and Accessed User Info of File (Main Usage is to Create File)

touch index.html styles.css

8. cat

Show (Concatenate) Files and Print Contents in Terminal

cat index.html

9. nano

Opens File Editor and Writes into Files

nano index.html

After You Added Your Lines do ctrl + s to Save and then ctrl + x to Exit

10. pwd

Prints Working Directory

pwd

11. rm

Removes Files or Directories

rm my_file.txt            # Removes File
rm -r My_Directory        # -r Option Removes Directories by Expanding each Directory Recursively and Remove Its Contents

12. ln

Creates a Link to a File

ln folder1/file.txt folder2/linked_file.txt

13. man

Manual Page, Shows Information of Command

man ls
man cd
man man

14. cp

Copies File or Directory

cp index.html ~/Desktop    # Copy index.html File to Desktop
cp -r Directory/ ~/Desktop # -r Option Copy Directories by Expanding each Directory Recursively and Copy Its Contents

15. mv

Moves File or Directory

mv index.html ~/Desktop    # Moves index.html File to Desktop
mv Directory ~/Desktop     # Moves Directory, -r Option is Not Needed

16. locate

Find Files by Name, While Booting Up Your OS, All Your Hard Drive is being Loaded and All File Paths is being Writing to Linux Database of Paths and locate Will Find Inside this Database to Find File. So, Every Files That is Created After Booting Up is Not Detectable by Locate Unless You Update Your File Paths Database

locate my_file.txt

17. history

Displays History of Commands You have Entered

history

18. find

Searchs for Files and Directory

find . -name 'index.html'     # -name Option is Needed to Find File or Directory by Name

Remember that You Can Use any Other Path Instead of Using . Working Directory

Week 7

What is Shell ?

Shell is a command-line interface (CLI) that allows users to interact with the computer system by entering commands. Shell is gateway to the operating system's services and functions. in Unix-based operating systems like Linux, MacOS and ... there are several popular shells :

What is Bash ?

Bash (Bourne Again Shell) is a popular command-line interpreter or shell for Unix and Unix-like operating systems. It's the default shell for most Linux distributions and macOS.

What is Terminal ?

Terminal is a software interface that allows users to interact with a computer system using text-based commands (I/O). It provides a way to enter shell commands to the operating system and receive textual output.

What is Shell Script

Shell Script is a script or program written in a scripting language that is interpreted by a shell, such as Bash in Unix-like operating systems. Shell scripts are used to automate tasks, execute commands, and perform system administration tasks.

Why Everything in Linux is File ?

The principle that "everything is a file" is a fundamental concept in Unix-like operating systems, including Linux. It embodies the idea that in the Unix/Linux environment, various system resources are represented as files or file-like objects. This simplifies the design and usage of the operating system, as it allows uniform access and manipulation of different resources through a common interface. for example :

  • Your computer's webcam is a file (located at /dev/video0)
  • Your mouse and keyboard (located at /dev/input/by-id/)
  • Your ls command (located at /usr/bin/ls)
  • Your SSD (located at /dev/disk/by-id)

How to Write Shell Script ?

  1. Open your terminal and create a new file like touch my_script.sh. You can name it whatever you like, but make sure it has a .sh extension to indicate it's a shell script.
  2. Use any text editor you are comfortable with, for example use nano
  3. The first line of your script should be the shebang line. This tells the system which interpreter to use to run the script. For a Bash shell script, use :
#!/bin/bash
  1. Add the commands you want to execute. Here’s an example script that create new file named file.txt and directory named copy_here and then copy file.txt to copy_here directory :
#!/bin/bash

touch file.txt
mkdir copy_here
cp file.txt copy_here
  1. save your file Now you have created a shell script.

How to Run (Execute) Shell Script ?

Before you can run the script, you need to make it executable. Use the chmod command to do this :

chmod +x my_script.sh

but what is chmod and +x ?
In Unix-like operating systems (like Linux), every file and directory has a set of permissions that determine who can read, write, or execute the file
There are three types of permissions:

  • Read (r): Permission to read the file.
  • Write (w): Permission to modify the file.
  • Execute (x): Permission to run the file as a program. for example if you do command ls -l that shows lsit of file as detailed list, you see this line for my_script.sh :
ls -l

-rw-rw-r-- 1 qb qb 0 May 14 08:30 my_script.sh

First part -rw-rw-r--'s 3-charactered section rw- only has r and w, it means my_script.sh can be writen or read but not execute because execute part is -. to add execute permission you must do that chmod +x my_script.sh command, if you that you will see this instead :

ls -l

-rwxrwxr-x 1 qb qb 0 May 14 08:30 my_script.sh

Now you can see execute permission x has been added to file and it can be seen like rwx instead rw-, that means my_script.sh can be run in terminal like this :

./my_script.sh

that ./ at first shows current directory . explained in this part. It can be other path if you are not inside my_script.sh's directory like :

# - Home path
~/Desktop/my_script.sh
# - or full path (needed in systemd services projects)
/home/qb/Desktop/my_script.sh

Why i can not use my_script.sh to run my script instead of ./my_script.sh

When you try to run a script using my_script.sh instead of ./my_script.sh, your shell looks for the script in the directories listed in your PATH environment variable. If the current directory (.) is not in your PATH, the shell won't find the script.

Linux Commands

19. echo

The echo command is a basic command-line utility in Unix and Unix-like operating systems that prints the specified arguments to the standard output. It is commonly used to display text or variables in shell scripts or interactively in a terminal session :

echo "Hello, world!"

20. grep

The grep command is a powerful tool used in Linux and Unix-like operating systems for searching text patterns within files or streams of input.
Examples :

  1. Search text or pattern (here hello) in file
grep "hello world" file.txt

Common Options :

  • -i: Ignore case distinctions.
grep -i "hELlO WOrlD" file.txt
  • -r, -R: Recursively search subdirectories listed.
grep -r "hello world" directory/
  • -n: Display line numbers along with matched lines.
grep -n "hello world" file.txt
  • -v: Invert the match, i.e., display non-matching lines.
grep -v "bye world" file.txt
  • -c: Only count the number of matching lines. (Here shows amount of your cpu cores)
grep -c "processor" /proc/cpuinfo
  • -E: Interpret the pattern as an extended regular expression (ERE). (Here will shows lines that are mails and their extention is @gmail.com)
grep -E '\b[A-Za-z0-9._%+-]+@gmail\.com\b' emails.txt

# - \b: Matches a word boundary, ensuring that the pattern matches complete email addresses and not partial ones.
# - [A-Za-z0-9._%+-]+: Matches one or more characters that can appear in the local part of an email address. This includes letters (upper and lower case), digits, and special characters commonly allowed in email addresses.
# - @gmail\.com: Matches the domain part of a Gmail address.
# - \b: Matches another word boundary, ensuring that the pattern matches complete email addresses and not partial ones.

Week 8

Review learned lessons and do some class exercises

Week 9

Variables

Like programming languages there are variables that we can store numbers, texts and other data types to them to use in our program. in Linux, variables are like that and used extensively in scripting and system configurations.

How to Define a Variable ?

Variables are assigned values using the = operator. For example :

name="Lida"
age=22
major="computer engineering"

⚠️ You can define your variables everywhere that use bash, like your terminal or shell script file
⚠️ In shell scripting, variable assignments shouldn't have spaces around the = sign :

age = 22    # - Will error
age=22      # - Will assign 22 to age

⚠️ Variable names in Linux are case-sensitive and can consist of letters, numbers, and underscores. By convention, variable names are usually in uppercase, especially for environment variables, to distinguish them from other variables and constants :

age=22
AGE=44
# - age is not AGE

How to Access Value of Variables ?

To access the value stored in a variable, you prepend a $ to its name. For example :

name="Neda"
echo "My name is $name"

$ Usages

There are several usages for $ in bash :

  • Variable Expansion $ Accesses the value stored in a variable :
text="Hello World"
echo $text
  • Command Substitution $() Executes a command and replaces the command with its output.
file_name="emails.txt"
echo "File $file_name has $(grep -c '@gmail' $file_name) mails available inside it"
  • Arithmetic Expansion $(()) Evaluates an arithmetic expression and returns the result. Remember that inside $(()) there is no need to use $ to access value of variables, you can only write their names.
num1=20
num2=30
sum=$((num1 + num2))
echo "$num1 + $num2 = $sum"
# - or you can do this inside string :
echo "$num1 + $num2 = $((num1 + num2))"

Curly Braces ${}

The curly braces {} are used to clearly delimit the variable name from surrounding text. For instance :

name="Zhila"
echo "her university mail address is $name@gmail.com"

Without the braces, Bash might not interpret the variable correctly if it is followed by characters that are part of the variable name. Compare :

echo "Hello, $name!"
echo "Hello, $name1"
echo "Hello, ${name}1"

Curly braces can also be used for parameter expansion, which provides more advanced features :

  • Length of the Variable's Value
text="Hello World"
echo ${#text}       # - Will Show 10 (Length of text)
  • Substring Extraction
text="Hello World"
echo ${text:0:5}    # - Will Show 'Hello ' (from Index 0 to 5)

Bash Conditional Statements

In Bash, conditional statements are used to perform different actions based on whether certain conditions are true or false. The if statement is a primary construct for this purpose. Here is how you can use if statements in Bash :

  • Basic Syntax
text1="hello"
text2="hello"

if [ $text1 == $text2 ]; then
    echo "text1 and text2 are same"
fi
  • Using if-else
text1="hello"
text2="bye"

if [ $text1 == $text2 ]; then
    echo "text1 and text2 are same"
else
    echo "text1 is not equal to text2"
fi
  • Using if-elif-else
text1="how are you ?"
text2="bye"
text3="how are you ?"

if [ $text1 == $text2 ]; then
    echo "text1 and text2 are same"
elif [ $text1 == $text3 ]; then
    echo "text1 and text3 are same"
else
    echo "text1 is not equal to text2"
fi

Types of Conditions

Conditions in Bash can include comparisons, file tests, and string tests. Here are some common examples :

  • Comparison Operators (Used for Numbers)
    • -eq equal to
    • -ne not equal to
    • -lt less than
    • -le less than or equal to
    • -gt greater than
    • -ge greater than or equal to
  • String Operators
    • == equal to
    • != not equal to
    • -z string is null (has zero length)
    • -n string is not null (has non-zero length)
  • File Test Operators
    • -e file exists
    • -d file is a directory
    • -f file is a regular file
  • Logical Operators
    • && Logical AND
    • || Logical OR
    • ! Logical NOT

Examples :

  • Comparison Operators
num=7

if [ $num -gt 10 ]; then
    echo "The number is greater than 10"
elif [ $num -gt 5 ]; then
    echo "The number is greater than 5 but less than or equal to 10"
else
    echo "The number is 5 or less"
fi
  • String Operators
str="hello"

if [ "$str" = "hello" ]; then
    echo "The string is hello"
else
    echo "The string is not hello"
fi
  • File Test Operations
file="example.txt"

if [ -e "$file" ]; then
    echo "The file exists"
else
    echo "The file does not exist"
fi
  • Using (()) for Mathematical Conditions Instead of Using Brackets
num1=20
num2=30

if ((num1 < num2)); then
   echo "num1 is lower than num2"
elif ((num1 > num2)); then
   echo "num1 is higher than num2"
else
   echo "num1 is equal to num2"
fi

Advanced Conditions

For more complex conditions, you can use double brackets [[ ]] :

if [[ $num -gt 5 && $num -lt 15 ]]; then
    echo "The number is between 5 and 15"
fi

Also you can use this if you don't want to use double brackets :

if [ $num -gt 5 ] && [ $num -lt 15 ]; then
    echo "The number is between 5 and 15"
fi

Common Conditional Statements Mistakes

  • No Spaces Around Condition Be careful that there should be spaces between the square brackets and the condition like [ condition ], bash does not support [condition].
  • Using == for Numeric Comparison Use -eq, -ne, -lt, -le, -gt, -ge for numeric comparisons
# - Incorrect for Numbers
if [ $num == 10 ]; then
# - Correct for Numbers
if [ $num -eq 10 ]; then
  • Quoting Strings Always quote string variables to avoid issues with spaces or special characters
str="hello world"
# - Incorrect
if [ $str = "hello world" ]; then
# - Correct
if [ "$str" = "hello world" ]; then
  • Semicolon ; at the end of condition brackets
    Remember to use ; at close bracket like [ condition ]; then

Week 10

Special Variables

In Linux, special variables are used in scripts to handle parameters passed to the script. Here are some of the most commonly used special variables:

  • Positional Parameters

    • $0 Shell Name : This variable contains the name of the script

    • $1, $2, ... Shell Parameters : These variables represent the positional parameters passed to the script. $1 is the first argument, $2 is the second argument, and so on.

      Example

      #! /bin/bash
echo "shell parameters are : $1 $2"
echo "shell name is : $0"

      if you execute this shell with :

      ./test.sh 123 abc

      It will output :

      shell parameters are 123 abc
shell name is ./test.sh

  • Special Parameters

    • $# Number of Parameters : This variable holds the number of positional parameters passed to the script.

    • $@ List of All Parameters : This variable holds the number of positional parameters passed to the script.

    • $? Execute Status : This variable contains the exit status of the last command executed. A status of 0 usually means success, while any non-zero value indicates an error.

      Example

      #! /bin/bash
echo "amount of shell parameters is : $#"
echo "passed parameters are : $@"
ls
# - Check Status of ls Command
if [ $? -eq 0 ]; then
    echo "Success"
else
    echo "Failed"
fi

      if you execute this shell with :

      ./test.sh p1 p2 p3 p4 p5

      It will output :

      amount of shell parameters is : 5
passed parameters are : p1 p2 p3 p4 p5
test.sh # - Output of ls Command
Success

Giving Input(s) to Script

Giving input to a shell script can be done in several ways, depending on the requirements of the script :

  • Command-line Arguments

    Like this part

  • Reading Inputs During Execution

    You can prompt the user for input during the execution of the script using the read command :

    Example

    #! /bin/bash
echo "Enter your name:"
read name
echo "Hello, $name"

Types of Variable (User Defined | Environment)

There are mainly two types of variables you will encounter in a Linux environment :

  • User Defined Variable

    Like this part

  • Environment Variable

    Environment Variables are predefined variables in Linux that affect the behavior of the shell and user processes. They are used to store system-wide values that need to be accessed by multiple applications and processes :

    • Typically defined in shell initialization files like .bashrc, .bash_profile, or /etc/environment.
    • Accessible to all child processes spawned from the shell.
    • Often used to configure system settings, paths, and behavior of system applications.

    Common Environment Variables

    • PATH: Specifies directories where executable programs are located.
    • HOME: The home directory of the current user.
    • USER: The username of the current user.
    • SHELL: The path to the current user's shell.

    Define Environment Variable

    in order to define environment variable you must use export command :

    # - Define a new environment variable
export MY_ENV_VAR="This is an environment variable"

echo $MY_ENV_VAR

    Listing Environment Variables

    in order to see environment variables you can use env or printenv commands :

    printenv
# - Or
env

for Loop

For loops in shell scripts are used to iterate over a series of items, such as a list of words, a range of numbers, or the contents of a file. They are a fundamental part of shell scripting, allowing you to automate repetitive tasks efficiently.

Basic Syntax

The basic syntax of a for loop in a shell script is as follows :

for variable in list do
    commands
done

Example ( Iterate Over List of Words ):

#! /bin/bash

for fruit in apple banana cherry
do
    echo "I like $fruit"
done

Example ( Iterate Over Range of Numbers )

for i in {1..5}
do
    echo "Number $i"
done

Example ( Using C-Style Syntax )

#! /bin/bash

for (( i=1; i<=5; i++ ))
do
    echo "Number $i"
done

Example ( Iterating Over Files in a Directory )

#! /bin/bash

for file in /path/to/directory/*
do
    echo "Processing $file"
    # Do something with $file
done

Example ( Using Arrays )

#! /bin/bash

arr=("apple" "banana" "cherry")
for fruit in "${arr[@]}"
do
    echo "I like $fruit"
done

Week 11

Nested for Loops

We want to multiply 2 numbers from 1 x 1 up to 4 x 4, so we need two for loops, one to iterate first number, second to iterate second number :

#! /bin/bash

echo -n "number 1 : "; read rows
echo -n "number 2 : "; read cols

for ((i = 1; i <= rows; i++)); do
    for ((j = 1; j <= cols; j++)); do
       echo -e "i * j = $i * $j = $((i * j))"
    done
    echo "------------"
done

while Loops

while loop allows you to execute a block of code repeatedly as long as a specified condition is true. This is useful for automating repetitive tasks.

Basic Syntax

The basic syntax of a while loop in a shell script is as follows :

while [ condition ]
do
    commands
done

⚠️ The condition is usually enclosed in square brackets [ ].

Example ( Simple )

#!/bin/bash

count=1

while [ $count -le 5 ]
do
    echo "Count: $count"
    ((count++))
done

Example ( Infinite Loop )

#!/bin/bash

while true
do
    echo "This is an infinite loop. Press [CTRL+C] to stop."
    sleep 1
done

Nested while Loop

Like that multiple shell script with for loop, We can use while loops instead :

#!/bin/bash

echo -n "number 1 : "; read rows
echo -n "number 2 : "; read cols

i=1
while [ $i -le $rows ]; do
    j=1
    while [ $j -le $cols ]; do
        echo -e "i * j = $i * $j = $((i * j))"
        ((j++))
    done
    echo "------------"
    ((i++))
done