DEV Community: yuna song

Python Basics: Variables, Conditionals, and Loops

yuna song — Sat, 27 Jun 2026 08:13:27 +0000

1. Variables

A variable stores a value that can be referenced and reused throughout a program.

Syntax

name = "Python"
age = 20
pi = 3.14
is_active = True

Naming Rules

Use letters, numbers, and underscores (_).
Variable names cannot start with a number.
Do not use spaces (use snake_case instead).
Do not use Python keywords (e.g. if, for, while, class).

Common Data Types

age = 20              # int
pi = 3.14             # float
name = "Python"       # str
is_active = True      # bool

Notes

Variables can be reassigned at any time.
Python automatically determines the data type.

2. Conditional Statements

Conditional statements execute different code depending on whether a condition is True or False.

Syntax

score = 85

if score >= 90:
    print("Grade A")
elif score >= 80:
    print("Grade B")
else:
    print("Grade C")

Keywords

if : Checks the first condition.
elif : Checks additional conditions if previous ones are False.
else : Executes when none of the above conditions are met.

Comparison Operators

>    <    >=    <=    ==    !=

Logical Operators

and    or    not

Notes

A colon (:) is required after each condition.
Proper indentation defines the code block.

3. Loops

Loops execute a block of code repeatedly.

`for` Loop

Used when iterating over a sequence or a fixed range.

for i in range(5):
    print(i)

Output:

`while` Loop

Repeats as long as a condition remains True.

count = 0

while count < 3:
    print(count)
    count += 1

Output:

0
1
2

Loop Control

break      # Exit the loop immediately
continue   # Skip the current iteration
pass       # Placeholder; does nothing

Notes

Use for when the number of iterations is known.
Use while when repetition depends on a condition.
Ensure a while loop eventually becomes False to avoid an infinite loop.

Linux Basics: Essential File System Commands

yuna song — Sat, 27 Jun 2026 05:02:13 +0000

1. `cd` — Change Directory

Changes the current working directory.

Syntax

cd <path>

Common Usage

cd /              # Go to the root directory
cd ~              # Go to the current user's home directory
cd ..             # Move up one directory
cd -              # Return to the previous directory
cd ./Documents    # Relative path
cd /var/log       # Absolute path

Path Types

Absolute path: Starts with / and specifies the full path from the root directory.
Relative path: Starts from the current directory (e.g. ./, ../, or a directory name).

Tips

Press Tab for auto-completion.
Use quotes for directory names containing spaces.

cd "My Folder"

2. `ls` — List Files and Directories

Displays the contents of a directory.

Syntax

ls [options] [path]

Common Options

ls          # List files
ls -l       # Long format
ls -a       # Include hidden files
ls -la      # Long format + hidden files
ls -lh      # Human-readable file sizes
ls -t       # Sort by modification time
ls -S       # Sort by file size
ls -r       # Reverse sort order
ls -R       # Recursive listing
ls -d */    # Show directories only

Useful Combinations

ls -altr    # Hidden files, long format, time sort, reverse order
ls -alF     # Append indicators (/, *, etc.)
ls -lthr    # Human-readable, time sort, newest at the bottom

3. `pwd` — Print Working Directory

Displays the absolute path of the current directory.

Syntax

pwd

Options

pwd -L      # Logical path (default)
pwd -P      # Physical path (resolve symbolic links)

Example:

/home/user/workspace

4. `mkdir` — Make Directory

Creates one or more directories.

Syntax

mkdir [options] <directory>

Common Usage

mkdir project
mkdir dir1 dir2 dir3
mkdir -p parent/child/grandchild
mkdir -v new_folder
mkdir -m 755 secure_folder

Useful Options

-p : Create parent directories if needed.
-v : Show created directories.
-m : Set permissions when creating the directory.

5. `rm` — Remove Files and Directories

Deletes files or directories permanently.

Warning: Deleted files cannot be recovered from the Trash.

Syntax

rm [options] <file>

Common Usage

rm file.txt
rm -i file.txt
rm -r directory
rm -f file.txt
rm -rf directory

Multiple Files & Wildcards

rm file1.txt file2.txt
rm *.log

Important Options

-i : Ask for confirmation.
-r : Remove directories recursively.
-f : Force deletion without prompts.

Never run
sudo rm -rf /
This command can destroy the entire operating system.

6. `cp` — Copy Files and Directories

Copies files or directories while keeping the original intact.

Syntax

cp [options] <source> <destination>

Common Usage

cp file1.txt file2.txt
cp -r dir1 dir2
cp file1.txt file2.txt ~/backup/
cp -a /etc/config.conf ~/backup/

Useful Options

-r / -R : Copy directories recursively.
-a : Archive mode (preserve attributes, permissions, timestamps, and symbolic links).
-p : Preserve file metadata.
-i : Confirm before overwriting.
-f : Force overwrite.

7. `mv` — Move or Rename Files

Moves files/directories or renames them.

Syntax

mv [options] <source> <destination>

Common Usage

mv file1.txt ~/Documents/
mv file1.txt file2.txt
mv folder1 ~/backup/
mv file1.txt file2.txt ~/backup/

Useful Options

-i : Confirm before overwriting.
-f : Force overwrite.
-n : Never overwrite existing files.
-v : Show each move operation.

DEV Community: yuna song

Python Basics: Variables, Conditionals, and Loops

1. Variables

Syntax

Naming Rules

Common Data Types

Notes

2. Conditional Statements

Syntax

Keywords

Comparison Operators

Logical Operators

Notes

3. Loops

for Loop

while Loop

Loop Control

Notes

Linux Basics: Essential File System Commands

1. cd — Change Directory

Syntax

Common Usage

Path Types

Tips

2. ls — List Files and Directories

Syntax

Common Options

Useful Combinations

3. pwd — Print Working Directory

Syntax

Options

4. mkdir — Make Directory

Syntax

Common Usage

Useful Options

5. rm — Remove Files and Directories

Syntax

Common Usage

Multiple Files & Wildcards

Important Options

6. cp — Copy Files and Directories

Syntax

Common Usage

Useful Options

7. mv — Move or Rename Files

Syntax

Common Usage

Useful Options

`for` Loop

`while` Loop

1. `cd` — Change Directory

2. `ls` — List Files and Directories

3. `pwd` — Print Working Directory

4. `mkdir` — Make Directory

5. `rm` — Remove Files and Directories

6. `cp` — Copy Files and Directories

7. `mv` — Move or Rename Files