DEV Community: yuna song

Python Basics: Strings

yuna song — Mon, 20 Jul 2026 12:32:45 +0000

1. String Basics

A string is a sequence of characters enclosed in single (') or double (") quotes.

Common Operations

text = "Python"

text + " Basics"      # Concatenate strings
text * 3              # Repeat a string
len(text)             # String length

2. Escape Sequences

Escape sequences represent special characters inside strings.

Escape	Description
`\n`	New line
`\t`	Horizontal tab
`\\`	Backslash (`\`)
`\'`	Single quote
`\"`	Double quote
`\r`	Carriage return
`\f`	Form feed
`\a`	Bell (alert)
`\b`	Backspace
`\0`	Null character

Example:

print("Hello\nWorld")

3. String Slicing

Extract part of a string using indexes.

Syntax

string[start:end]

start is inclusive.
end is exclusive.

Example:

text = "Python"

text[0:2]      # "Py"
text[:4]       # "Pyth"
text[2:]       # "thon"
text[-1]       # "n"

4. String Formatting

`%` Formatting (Legacy)

"%s" % "Python"
"%d" % 10
"%.2f" % 3.14159

Common format specifiers:

Code	Description
`%s`	String
`%c`	Character
`%d`	Integer
`%f`	Float
`%o`	Octal
`%x`	Hexadecimal
`%%`	Percent sign

`str.format()`

"Hello, {}".format("Python")
"{0} {1}".format("Hello", "World")
"{name}".format(name="Python")

Alignment:

"{:<10}".format("Hi")     # Left
"{:>10}".format("Hi")     # Right
"{:^10}".format("Hi")     # Center
"{:*^10}".format("Hi")    # Fill with *

Floating-point precision:

"{:.2f}".format(3.14159)

Escape braces:

"{{Python}}"

f-Strings (Recommended)

Available in Python 3.6+.

name = "Python"
version = 3.13

f"{name} {version}"
f"{version:.1f}"
f"{name:>10}"

Supports:

Variables
Expressions
Dictionary access
Alignment
Number formatting

Example:

x = 10
y = 20

f"{x} + {y} = {x + y}"

5. Common String Methods

Search

text.count("a")
text.find("a")
text.index("a")

count() : Count occurrences.
find() : Return index, or -1 if not found.
index() : Return index, or raise ValueError.

Case Conversion

text.upper()
text.lower()

Remove Whitespace

text.strip()
text.lstrip()
text.rstrip()

Replace

text.replace("Python", "Java")

Split

text.split()
text.split(",")

Returns a list of substrings.

Join

",".join(["A", "B", "C"])

Output:

A,B,C

Notes

Strings are immutable, meaning their contents cannot be changed directly.
Most string methods return a new string instead of modifying the original.

Note: AI was used to assist in the drafting and formatting of this post.

Linux Basics: Search, History, and Aliases

yuna song — Wed, 15 Jul 2026 12:21:31 +0000

1. `grep` — Search Text by Pattern

Searches for lines matching a string or regular expression in files or command output.

Syntax

grep [options] "<pattern>" <file>

Common Usage

grep "error" log.txt
grep -i "error" log.txt          # Ignore case
grep -r "ERROR" ./logs/          # Search recursively
grep -v "success" log.txt        # Show non-matching lines
grep -n "WARNING" log.txt        # Show line numbers
grep -l "main()" *.c             # Show matching file names only

Practical Examples

tail -f app.log | grep "ERROR"
ps aux | grep nginx
history | grep ssh

Notes

Supports regular expressions for advanced pattern matching.
Frequently combined with pipes (|) to filter command output.

2. `find` — Search Files and Directories

Searches for files or directories based on specified conditions.

Syntax

find <path> <conditions> <actions>

Common Usage

find . -name "*.txt"
find . -iname "*.txt"
find /var/log -type d
find . -size +100M
find . -mtime -7

Actions

find . -name "*.tmp" -delete
find ./logs -name "*.log" -exec grep "error" {} \;

Notes

-name : Case-sensitive filename search.
-iname : Case-insensitive filename search.
-type f : Files only.
-type d : Directories only.

3. `history` — Command History

Displays previously executed commands.

Syntax

history

Common Usage

history
history 10
history | grep ssh
!145
!!

History Management

history -c
cat /dev/null > ~/.bash_history

Notes

!<number> reruns a command by its history number.
!! reruns the previous command.
Command history is typically stored in ~/.bash_history.

4. `alias` — Create Command Shortcuts

Creates custom shortcuts for frequently used commands.

Syntax

alias <name>='<command>'
unalias <name>

Common Usage

alias ll='ls -la'
alias rm='rm -i'
alias c='clear'
alias logwatch='tail -f /var/log/nginx/access.log'

View existing aliases:

alias

Remove an alias:

unalias ll

Make Aliases Persistent

Edit your shell configuration:

nano ~/.bashrc

Add your aliases, then reload the configuration:

source ~/.bashrc

Notes

Aliases created in the terminal are temporary.
Add them to ~/.bashrc (or ~/.zshrc if using Zsh) to make them permanent.

Note: AI was used to assist in the drafting and formatting of this post.

Linux Basics: Permissions and Process Management

yuna song — Thu, 09 Jul 2026 12:31:47 +0000

1. `chmod` — Change File Permissions

Changes the permissions of files and directories.

Syntax

chmod [options] <mode> <file>

Numeric (Octal) Mode

Permissions are represented by the sum of:

4 = Read (r)
2 = Write (w)
1 = Execute (x)

Mode	Permission	Description
`755`	`rwxr-xr-x`	Owner: full access; Group/Others: read & execute
`644`	`rw-r--r--`	Owner: read & write; Group/Others: read only
`777`	`rwxrwxrwx`	Full access for everyone (not recommended)

Symbolic Mode

chmod u+x file.sh      # Add execute permission for owner
chmod g-w file.txt     # Remove write permission from group
chmod o+r file.txt     # Add read permission for others
chmod a+x script.sh    # Add execute permission for everyone

Notes

u = owner
g = group
o = others
a = all users

2. `chown` — Change File Ownership

Changes the owner and/or group of files and directories.

Syntax

chown [options] <owner>[:group] <file>

Common Usage

chown user1 file.txt
chown user1:group1 file.txt
chown :group1 file.txt

Useful Options

chown -R user1:group1 /var/www/html

-R : Change ownership recursively.

Notes

Changing ownership usually requires sudo.

sudo chown webuser:webgroup config.php

3. `ps` — Display Running Processes

Displays information about currently running processes.

Common Usage

ps -ef
ps aux

Practical Examples

ps aux | grep nginx          # Find nginx processes
ps aux --sort=-%mem          # Sort by memory usage
ps aux --sort=-%cpu          # Sort by CPU usage

Common Output Fields

Field	Description
`PID`	Process ID
`TTY`	Associated terminal
`STAT`	Process state
`TIME`	Total CPU time used
`COMMAND`	Executed command

4. `top` — Monitor System Processes

Displays CPU, memory, and process information in real time.

Syntax

top

The display refreshes every 3 seconds by default.

Useful Shortcuts

Key	Action
`P`	Sort by CPU usage
`M`	Sort by memory usage
`T`	Sort by CPU time
`k`	Kill a process (enter PID)
`q`	Quit `top`

Useful Options

top -d 1              # Refresh every second
top -u username       # Show processes for a specific user

Notes

The top section shows system-wide CPU, memory, swap, and task information, while the bottom section lists running processes.

5. `kill` — Send Signals to Processes

Sends signals to running processes, most commonly to terminate them.

Syntax

kill [signal] <PID>

Common Usage

kill 1234
kill -15 1234
kill -9 1234

Common Signals

Signal	Description
`15` (`SIGTERM`)	Gracefully terminate a process (default)
`9` (`SIGKILL`)	Forcefully terminate a process

Kill by Process Name

pkill nginx
killall chrome

Notes

Use SIGTERM (kill or kill -15) whenever possible.
Use SIGKILL (kill -9) only if the process does not respond.
Find a process ID with:

ps aux | grep <process_name>

Administrative privileges may be required:

sudo kill -9 1234

Note: AI was used to assist in the drafting and formatting of this post.

Linux Basics: File Viewing and Management Commands

yuna song — Tue, 07 Jul 2026 14:05:26 +0000

1. `touch` — Create Empty Files or Update Timestamps

Creates an empty file or updates a file's timestamps.

Syntax

touch [options] <file>

Common Usage

touch file.txt                    # Create an empty file
touch file1.txt file2.txt         # Create multiple files
touch existing.txt                # Update timestamps
touch file{1..5}.txt              # Create file1.txt ~ file5.txt

Useful Options

touch -a file.txt                 # Update access time (atime)
touch -m file.txt                 # Update modification time (mtime)
touch -c file.txt                 # Do not create the file if it doesn't exist
touch -d "2026-07-04 12:30:00" file.txt
touch -t 202607041230 file.txt
touch -r source.txt target.txt    # Copy timestamps from another file

Timestamp Types

atime : Last access time
mtime : Last modification time
ctime : Last metadata (status) change

View timestamps:

stat file.txt

2. `cat` — Display and Concatenate Files

Displays file contents or combines multiple files.

Syntax

cat [options] <file>

Common Usage

cat file.txt
cat file1.txt file2.txt
cat file1.txt file2.txt > combined.txt

Create or Append Files

cat > new_file.txt        # Create or overwrite
cat >> file.txt           # Append to an existing file

Press Ctrl + D to save and exit.

Useful Options

cat -n file.txt           # Show line numbers
cat -b file.txt           # Number non-empty lines only
cat -s file.txt           # Squeeze multiple blank lines
cat -E file.txt           # Show end-of-line markers ($)
cat -A file.txt           # Show all non-printing characters

Notes

Avoid using cat for very large files. Instead, use:

head file.txt
tail file.txt
less file.txt

3. `less` — View Large Files Efficiently

Displays large files one screen at a time without loading the entire file into memory.

Syntax

less file.txt

Exit with:

Navigation

Key	Action
`j` / ↓	Next line
`k` / ↑	Previous line
`Space` / `PgDn`	Next page
`b` / `PgUp`	Previous page
`g`	First line
`G`	Last line

Search

/keyword      Search forward
?keyword      Search backward
n             Next match
N             Previous match

Useful Options

less -N app.log           # Show line numbers
less -i app.log           # Case-insensitive search
less +F app.log           # Follow file like tail -f
ps -ef | less             # View command output page by page

Notes

Press Ctrl + C to exit follow mode (+F).
Press Shift + F to resume following.

4. `head` — Display the Beginning of a File

Prints the first part of a file.

By default, head displays the first 10 lines.

Syntax

head [options] <file>

Common Usage

head file.txt
head -n 5 file.txt
head -5 file.txt
head -c 20 file.txt

Useful Options

head -q file1.txt file2.txt     # Hide file headers
head -v file.txt                # Always show file header

Practical Examples

Display lines 11–15:

head -n 15 system.log | tail -n 5

Show the five most recently modified files:

ls -lt | head -n 6

5. `tail` — Display the End of a File

Prints the last part of a file.

By default, tail displays the last 10 lines.

Syntax

tail [options] <file>

Common Usage

tail file.txt
tail -n 5 file.txt
tail -5 file.txt
tail -n +20 file.txt

Real-Time Monitoring

tail -f app.log              # Follow file updates
tail -F app.log              # Continue following after log rotation
tail -f app.log --pid=1234   # Stop when process exits

Stop monitoring with Ctrl + C.

Useful Options

tail -n 30 file.txt
tail -c 100 file.txt

Practical Examples

Monitor only error messages:

tail -f app.log | grep "ERROR"

Notes

-f follows a file as it grows.
-F is preferred for log files because it handles log rotation automatically.

Note: AI was used to assist in the drafting and formatting of this post.

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.

Note: AI was used to assist in the drafting and formatting of this post.

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.

Note: AI was used to assist in the drafting and formatting of this post.

DEV Community: yuna song

Python Basics: Strings

1. String Basics

Common Operations

2. Escape Sequences

3. String Slicing

Syntax

4. String Formatting

% Formatting (Legacy)

str.format()

f-Strings (Recommended)

5. Common String Methods

Search

Case Conversion

Remove Whitespace

Replace

Split

Join

Notes

Linux Basics: Search, History, and Aliases

1. grep — Search Text by Pattern

Syntax

Common Usage

Practical Examples

Notes

2. find — Search Files and Directories

Syntax

Common Usage

Actions

Notes

3. history — Command History

Syntax

Common Usage

History Management

Notes

4. alias — Create Command Shortcuts

Syntax

Common Usage

Make Aliases Persistent

Notes

Linux Basics: Permissions and Process Management

1. chmod — Change File Permissions

Syntax

Numeric (Octal) Mode

Symbolic Mode

Notes

2. chown — Change File Ownership

Syntax

Common Usage

Useful Options

Notes

3. ps — Display Running Processes

Common Usage

Practical Examples

Common Output Fields

4. top — Monitor System Processes

Syntax

Useful Shortcuts

Useful Options

Notes

5. kill — Send Signals to Processes

Syntax

Common Usage

Common Signals

Kill by Process Name

Notes

Linux Basics: File Viewing and Management Commands

1. touch — Create Empty Files or Update Timestamps

Syntax

Common Usage

Useful Options

Timestamp Types

2. cat — Display and Concatenate Files

Syntax

Common Usage

Create or Append Files

Useful Options

Notes

3. less — View Large Files Efficiently

Syntax

`%` Formatting (Legacy)

`str.format()`

1. `grep` — Search Text by Pattern

2. `find` — Search Files and Directories

3. `history` — Command History

4. `alias` — Create Command Shortcuts

1. `chmod` — Change File Permissions

2. `chown` — Change File Ownership

3. `ps` — Display Running Processes

4. `top` — Monitor System Processes

5. `kill` — Send Signals to Processes

1. `touch` — Create Empty Files or Update Timestamps

2. `cat` — Display and Concatenate Files

3. `less` — View Large Files Efficiently

4. `head` — Display the Beginning of a File

5. `tail` — Display the End of a File

`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