🐧 Important Things to Remember in Linux¶

🔑 1. Root Account Role¶

Root = God Mode trong Linux¶

# Root user có UID = 0
root:x:0:0:root:/root:/bin/bash

Root Powers:¶

✅ Toàn quyền truy cập tất cả files/folders
✅ Cài đặt/gỡ bỏ software packages
✅ Thay đổi system configurations
✅ Tạo/xóa user accounts
✅ Kiểm soát services và processes
✅ Truy cập hardware trực tiếp

Access Root:¶

1. Switch User (su):

# Chuyển sang root user
su -
# Hoặc
su root

# Chạy 1 command với root privilege
su -c "systemctl restart nginx"

2. Sudo (Recommended):

# Chạy command với root privilege
sudo systemctl restart nginx

# Chuyển sang root shell tạm thời
sudo -i
# hoặc
sudo su -

# Chỉnh sửa file system
sudo nano /etc/hosts

3. Root Login (Không khuyến khích):

# Login trực tiếp as root
ssh root@server

Root Safety Best Practices:¶

# ❌ NGUY HIỂM - Chạy mọi thứ as root
sudo su -
rm -rf /* # 💀 NEVER DO THIS!

# ✅ AN TOÀN - Chỉ dùng sudo khi cần
sudo systemctl status nginx
sudo apt update
sudo nano /etc/nginx/nginx.conf

Sudo Configuration:¶

# Xem sudo privileges
sudo -l

# Thêm user vào sudo group
sudo usermod -aG sudo lelongc

# Chỉnh sửa sudoers file
sudo visudo

# Example sudoers entry:
lelongc ALL=(ALL:ALL) ALL
# lelongc có thể chạy tất cả commands as any user

🔤 2. Case Sensitive¶

Linux phân biệt chữ HOA/thường NGHIÊM NGẶT¶

# Đây là 4 files KHÁC NHAU:
touch file.txt
touch File.txt  
touch FILE.txt
touch FiLe.TxT

ls -la
# -rw-rw-r-- 1 lelongc lelongc 0 Aug 12 05:57 FILE.txt
# -rw-rw-r-- 1 lelongc lelongc 0 Aug 12 05:57 FiLe.TxT
# -rw-rw-r-- 1 lelongc lelongc 0 Aug 12 05:57 File.txt
# -rw-rw-r-- 1 lelongc lelongc 0 Aug 12 05:57 file.txt

Commands cũng Case Sensitive:¶

# ✅ Đúng
ls -la
pwd
cd /home/lelongc

# ❌ Sai - Command not found
LS -la
PWD
CD /home/lelongc

Directories Case Sensitive:¶

# Đây là các thư mục KHÁC NHAU:
mkdir Documents
mkdir documents  
mkdir DOCUMENTS

# Navigate phải chính xác case
cd Documents    # ✅ 
cd documents    # ✅ nhưng khác folder
cd Documents    # ❌ nếu folder tên là documents

Environment Variables:¶

# Case sensitive
echo $PATH     # ✅ Hiển thị system PATH
echo $path     # ❌ Không có giá trị (nếu chưa set)
echo $Path     # ❌ Không có giá trị

# Set variables
export MyVar="Hello"
echo $MyVar    # ✅ "Hello"
echo $myvar    # ❌ Empty
echo $MYVAR    # ❌ Empty

Case Sensitivity Examples:¶

# File operations
cat file.txt     # ✅ Đọc file.txt
cat File.txt     # ❌ Nếu chỉ có file.txt
cat FILE.TXT     # ❌ Nếu chỉ có file.txt

# User accounts
su lelongc       # ✅ 
su LeLongC       # ❌ User không tồn tại
su LELONGC       # ❌ User không tồn tại

# Package names
sudo apt install nginx    # ✅
sudo apt install NGINX    # ❌ Package không tồn tại
sudo apt install Nginx    # ❌ Package không tồn tại

📁 3. File Name Standards¶

Allowed Characters:¶

# ✅ SAFE characters:
abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
0123456789
. (dot)
- (hyphen)  
_ (underscore)

File Naming Best Practices:¶

# ✅ GOOD file names:
config.txt
my_script.sh
backup-2025-08-12.tar.gz
user_data.json
server01.conf
nginx.conf
.bashrc
.ssh/

Avoid These Characters:¶

# ❌ PROBLEMATIC characters:
file name.txt        # Space (use _ or -)
file$name.txt        # $ (shell variable)
file&name.txt        # & (background process)
file|name.txt        # | (pipe operator)
file>name.txt        # > (redirect)
file<name.txt        # < (redirect)
file;name.txt        # ; (command separator)
file'name.txt        # ' (quote)
file"name.txt        # " (quote)
file*name.txt        # * (wildcard)
file?name.txt        # ? (wildcard)
file[name].txt       # [] (pattern matching)

Special Files:¶

# Hidden files (bắt đầu với dot)
.bashrc              # Bash configuration
.ssh/                # SSH keys directory
.gitignore           # Git ignore rules
.vimrc               # Vim configuration

# Backup files
file.txt.bak         # Manual backup
file.txt~            # Auto backup (editors)
file.txt.orig        # Original file backup

# Temporary files
temp.tmp
cache.cache
/tmp/session123

File Extensions Convention:¶

# Scripts
script.sh            # Bash script
script.py            # Python script
script.pl            # Perl script
script.rb            # Ruby script

# Configuration
nginx.conf           # Nginx config
app.config           # Application config
settings.ini         # INI format

# Data
data.json            # JSON data
data.xml             # XML data
data.csv             # CSV data
data.sql             # SQL dump

# Archives
backup.tar.gz        # Compressed archive
files.zip            # ZIP archive
project.tar          # TAR archive

Length Limitations:¶

# File name limits:
# - Maximum 255 characters for filename
# - Maximum 4096 characters for full path

# ✅ GOOD:
short_name.txt

# ❌ TOO LONG (example):
this_is_a_very_very_very_long_filename_that_might_cause_problems_in_some_systems_and_applications_especially_when_dealing_with_legacy_systems_or_certain_backup_tools_that_have_filename_length_restrictions.txt

Case Strategy:¶

# ✅ CONSISTENT naming strategies:

# 1. lowercase with underscores
my_config_file.txt
user_data_backup.json
system_status_report.log

# 2. lowercase with hyphens  
my-config-file.txt
user-data-backup.json
system-status-report.log

# 3. camelCase (ít dùng)
myConfigFile.txt
userDataBackup.json
systemStatusReport.log

Working with Special Characters:¶

# Nếu file có space hoặc special chars:

# ✅ Quote the filename:
cat "file with spaces.txt"
ls "my file.txt"

# ✅ Escape with backslash:
cat file\ with\ spaces.txt
ls my\ file.txt

# ✅ Use tab completion:
cat my<TAB>  # Auto-completes and escapes

🎯 Key Takeaways for lelongc¶

1. Root Account:¶

# Luôn nhớ:
sudo command         # Safer than su -
sudo -i             # Temporary root shell
exit                # Always exit root when done

2. Case Sensitivity:¶

# Luôn check case:
ls -la              # Xem exact filename
pwd                 # Check current directory case
which command       # Verify command exists

3. File Naming:¶

# Safe naming pattern:
project_name.extension
backup-YYYY-MM-DD.tar.gz
config_file_v1.conf
script_name.sh

💡 Common Mistakes to Avoid¶

Root Account Mistakes:¶

# ❌ DON'T:
sudo su -                    # Staying in root too long
rm -rf /important/data      # Without backup
chmod 777 /etc/passwd       # Dangerous permissions

# ✅ DO:
sudo systemctl restart nginx    # Specific task
sudo cp file.conf file.conf.bak # Backup first
sudo chmod 644 /etc/passwd      # Appropriate permissions

Case Sensitivity Mistakes:¶

# ❌ Assuming case doesn't matter:
cd /HOME/lelongc           # Should be /home/lelongc
export path=$PATH          # Should be PATH
cat CONFIG.TXT             # Should match exact filename

# ✅ Always match exact case:
cd /home/lelongc           # Correct path
export PATH=$PATH:/new/path # Correct variable
cat config.txt             # Match exact filename

File Naming Mistakes:¶

# ❌ Problematic names:
"my file.txt"              # Spaces need quotes
file$var.txt               # $ causes confusion
backup file (1).txt        # Spaces and parentheses

# ✅ Better alternatives:
my_file.txt                # Underscore instead of space
file_var.txt               # No special characters
backup_file_1.txt          # Clean and clear

🚀 Quick Reference for lelongc¶

# Root operations
sudo command               # Run single command as root
sudo -i                   # Become root temporarily
exit                      # Exit root shell

# Case-sensitive operations  
ls -la                    # List with exact case
which command             # Find exact command
echo $VARIABLE            # Exact variable name

# Safe file naming
touch my_file.txt         # Underscore for spaces
mkdir backup-$(date +%Y%m%d)  # Date in filename
cp file.txt file.txt.bak  # Backup with extension

Remember: Linux không tha thứ! Một ký tự sai có thể làm hỏng cả system. Luôn double-check trước khi chạy commands có quyền root! 🔒