Linux Basics

Mở đầu

Server world, Linux là protagonist tuyệt đối. >90% server toàn cầu chạy Linux, từ Zalo, Tiki đến Google search, sau đều Linux. Là dev, master Linux basics = must, không optional.

Bạn sẽ học:

File system: directory structure + philosophy "everything is file"
Commands: file op, text process, process management
Permissions: user, group, permission model
Shell basics: pipe, redirect, env var
Practical: log debug, process check, network diagnosis

Chương	Nội dung
1	File system
2	Common commands
3	Permission model
4	Shell basics
5	Practical scenarios

1. File system: everything is file

Linux core philosophy: everything is file. File thường = file, directory = file, disk = file, ngay cả network connection, process info = file. Unified abstraction này cho phép dùng cùng tool (read, write, permission) op gần như mọi system resource.

📁/根目录

⚙️/bin基础命令

📋/etc配置文件

🏠/home用户目录

📊/var可变数据

🗑️/tmp临时文件

📦/usr用户程序

🔍/proc进程信息

🔌/dev设备文件

整个文件系统的起点，所有目录和文件都从这里开始。Linux 中一切皆文件，所有设备、进程信息都以文件形式存在于这棵目录树中。

Directory structure

Tưởng filesystem Linux như cây ngược:

/                    ← Root (gốc)
├── home/            ← User home (file của bạn ở đây)
├── etc/             ← Config (system "setting panel")
├── var/             ← Data thay đổi (log, cache)
├── usr/             ← Software user install
├── tmp/             ← Temp file (restart mất)
├── proc/            ← Process info (virtual, không disk)
├── dev/             ← Device file (disk, terminal)
├── bin/             ← Basic commands (ls, cp, mv)
├── sbin/            ← Admin commands (cần root)
├── opt/             ← Third-party software
└── root/            ← root user home

Path

Type	Format	Example	Note
Absolute	Bắt đầu `/`	`/home/hoang/code/app.js`	Từ root, không ambiguous
Relative	Từ current dir	`./code/app.js` hoặc `../config`	`.` = current, `..` = parent

Power "everything is file"

CPU info? Read file: cat /proc/cpuinfo Memory usage? Read file: cat /proc/meminfo Random number? Read file: cat /dev/urandom Discard output? Write file: echo "no thanks" > /dev/null

Không cần API riêng, read/write file đủ. Unix philosophy elegance.

2. Common commands

Linux command format: command [options] [args]. Vd ls -la /home: ls command, -la option, /home arg.

ls列出文件和目录

ls -la /home

cd切换目录

cd /var/log

cp复制文件

cp -r src/ backup/

mv移动/重命名

mv old.txt new.txt

rm删除文件

rm -rf dist/

mkdir创建目录

mkdir -p src/components

find查找文件

find . -name "*.js" -type f

Top 10 most-used

Command	Use	Memory
`ls`	List file	list
`cd`	Switch directory	change directory
`cat`	View file	concatenate
`grep`	Search text	global regex print
`find`	Find file	find
`ps`	View process	process status
`tail -f`	Live log	-f = follow
`chmod`	Change permission	change mode
`curl`	HTTP request	client URL
`ssh`	Remote login	secure shell

Art of command combination

Linux mạnh không ở single command, mà ở combine. Qua pipe |:

bash

# Top 5 process CPU
ps aux --sort=-%cpu | head -6

# Stats most frequent error type trong log
grep "ERROR" app.log | awk '{print $4}' | sort | uniq -c | sort -rn | head -10

# Find file >100MB
find / -size +100M -type f 2>/dev/null

# Monitor live log highlight error
tail -f /var/log/app.log | grep --color "ERROR"

Unix philosophy

"Do one thing well." Mỗi command 1 function, combine qua pipe → complex op. Đây là sao Linux command ngắn — chúng là lego, không phải Swiss army knife.

3. Permission model

Linux multi-user, permission = security foundation. Mỗi file có 3 group permission: Owner, Group, Others.

Đọc `ls -l`

bash

$ ls -l app.js
-rwxr-xr-- 1 hoang dev 2048 Jan 15 10:30 app.js
│├──┤├──┤├──┤   │     │      │
│ │   │   │     │     │      └── File size
│ │   │   │     │     └── Group
│ │   │   │     └── Owner
│ │   │   └── Others: r-- (read only)
│ │   └── Group: r-x (read + execute)
│ └── Owner: rwx (read + write + execute)
└── Type: - file, d dir, l link

3 permission ops

Permission	Letter	Number	Cho file	Cho dir
Read	`r`	4	View content	List content (ls)
Write	`w`	2	Modify content	Create/delete file trong dir
Execute	`x`	1	Run program/script	Enter dir (cd)

权限数字（如 755）

-rwxr-xr-x

常见权限组合

644普通文件（owner 读写，其他只读）

755可执行文件/目录（owner 全权限）

600私密文件（仅 owner 读写）

777完全开放（不推荐）

Numeric permission

3 số = Owner/Group/Others, mỗi số = r(4) + w(2) + x(1):

chmod 755 script.sh
  7 = rwx (4+2+1)  → Owner: read + write + execute
  5 = r-x (4+0+1)  → Group: read + execute
  5 = r-x (4+0+1)  → Others: read + execute

Common	Meaning	Use
`644`	rw-r--r--	File thường (owner write, others read)
`755`	rwxr-xr-x	Executable / directory
`600`	rw-------	Private file (SSH key)
`777`	rwxrwxrwx	Mọi người read/write/execute (nguy hiểm)

sudo: tạm root

User thường permission hạn chế, 1 số op cần root. sudo:

bash

# User thường không sửa system config
$ vim /etc/nginx/nginx.conf
# Permission denied

# sudo tạm:
$ sudo vim /etc/nginx/nginx.conf

# Switch root (cẩn thận):
$ sudo su -

Minimum permission principle

Đừng chmod 777 để giải permission issue, = tháo cửa. Hãy xác định ai cần permission nào, grant chính xác. Tương tự, đừng long-term root, chỉ sudo khi cần.

4. Shell basics

Shell = "translator" giữa bạn + Linux kernel. Bash (default Linux) + Zsh (default macOS) phổ biến nhất.

Pipe + redirect

2 feature mạnh nhất Shell:

Symbol	Name	Use	Example
`	`	Pipe	Output trước → input sau
`>`	Output redirect	Write file (overwrite)	`echo "hello" > file.txt`
`>>`	Append	Append cuối file	`echo "world" >> file.txt`
`<`	Input redirect	Read từ file	`wc -l < file.txt`
`2>`	Error redirect	Error vào file	`cmd 2> error.log`
`2>&1`	Merge output	Error + normal merge	`cmd > all.log 2>&1`

Environment variables

bash

# View hết
env

# View 1 var
echo $PATH
echo $HOME

# Set tạm (chỉ current shell)
export API_KEY="abc123"

# Set permanent (write config file)
echo 'export API_KEY="abc123"' >> ~/.bashrc
source ~/.bashrc   # Apply

Common	Meaning	Example
`$PATH`	Command search path	`/usr/local/bin:/usr/bin:/bin`
`$HOME`	User home	`/home/hoang`
`$USER`	Current username	`hoang`
`$PWD`	Current dir	`/var/log`
`$SHELL`	Current shell	`/bin/bash`

Shell script

Multi command vào file → script:

bash

#!/bin/bash
# deploy.sh

APP_DIR="/opt/myapp"
LOG_FILE="/var/log/deploy.log"

echo "$(date) - Starting deploy..." >> $LOG_FILE

# Pull latest
cd $APP_DIR && git pull origin main

# Install deps
npm install --production

# Restart service
pm2 restart myapp

echo "$(date) - Deploy done" >> $LOG_FILE

bash

chmod +x deploy.sh
./deploy.sh

Debug script

Đầu script set -ex: -e script gặp error thoát ngay, -x print mỗi command execute. Standard cho production script.

5. Practical scenarios

5.1 Log debug

Service issue → reflex đầu tiên: xem log.

bash

# 1. Live tracking (hay dùng nhất)
tail -f /var/log/app/error.log

# 2. Search time range error
grep "2024-01-15 14:" error.log | grep "ERROR"

# 3. Stats error/hour
grep "ERROR" app.log | awk '{print substr($1,1,13)}' | uniq -c

# 4. Last 100 lines
tail -100 app.log

# 5. Search multiple file
grep -r "OutOfMemory" /var/log/app/

5.2 Process check

App freeze, CPU spike, memory leak — từ process:

bash

# CPU top
ps aux --sort=-%cpu | head -10

# Memory top
ps aux --sort=-%mem | head -10

# Find specific
ps aux | grep "node"

# Detail (kèm thread)
top -Hp <PID>

# Files opened by process
lsof -p <PID>

# Graceful terminate (SIGTERM)
kill <PID>

# Force kill (SIGKILL, last resort)
kill -9 <PID>

5.3 Network diagnosis

Service connect không được? Network hay app?

bash

# Ping target
ping -c 4 google.com

# Check port open
telnet db-server 3306
# Hoặc nc
nc -zv db-server 3306

# Port machine listening
ss -tlnp
# Hoặc
netstat -tlnp

# DNS resolve
dig api.example.com
nslookup api.example.com

# Test HTTP
curl -v http://localhost:3000/health

# Network connection stats
ss -s

5.4 Disk space

Disk full = fault prod common nhất:

bash

# Partition usage
df -h

# Find biggest dir
du -sh /* 2>/dev/null | sort -rh | head -10

# Drill down
du -sh /var/log/* | sort -rh | head -10

# Find big file (>100MB)
find / -type f -size +100M 2>/dev/null | head -20

# Cleanup
# Old log
sudo journalctl --vacuum-size=500M
# Docker
docker system prune -a

Prod debug formula

"Log first, process second, network third, disk fourth". 90% prod issue locate được qua 4 step. Habit này = efficiency tăng vọt.

Tổng kết

Linux = must skill cho dev. Master basics = handle đa số daily dev + ops.

Everything is file: Linux dùng file abstract unify access hardware, process, network
Command combo: single command đơn giản, qua pipe | mới power thật
Permission: Owner/Group/Others × R/W/E, dùng số (755) set nhanh
Shell basics: pipe, redirect, env var, script = automation foundation
Debug practice: log → process → network → disk

2026 cho VN dev

Distro:
- Ubuntu: phổ biến nhất, easy
- Debian: stable, server
- Alpine: lightweight, container
- Rocky/Alma Linux: thay CentOS legacy
VN context:
- VPS giá rẻ VN: BKHost, Vietnix, AzDigi
- International: DigitalOcean, Vultr, Hetzner
Modern tools 2026:
- bat: thay cat, syntax highlight
- fd: thay find, faster + simple
- ripgrep (rg): thay grep, faster
- eza: thay ls, color + icon
- fzf: fuzzy finder magic
- zoxide: smart cd
WSL2: Linux trên Windows, mainstream cho VN dev

Linux Basics ​

1. File system: everything is file ​

Directory structure ​

Path ​

2. Common commands ​

Top 10 most-used ​

Art of command combination ​

3. Permission model ​

Đọc ls -l ​

3 permission ops ​

Numeric permission ​

sudo: tạm root ​

4. Shell basics ​

Pipe + redirect ​

Environment variables ​

Shell script ​

5. Practical scenarios ​

5.1 Log debug ​

5.2 Process check ​

5.3 Network diagnosis ​

5.4 Disk space ​

Tổng kết ​

Tài liệu ​

Linux Basics

1. File system: everything is file

Directory structure

Path

2. Common commands

Top 10 most-used

Art of command combination

3. Permission model

Đọc `ls -l`

3 permission ops

Numeric permission

sudo: tạm root

4. Shell basics

Pipe + redirect

Environment variables

Shell script

5. Practical scenarios

5.1 Log debug

5.2 Process check

5.3 Network diagnosis

5.4 Disk space

Tổng kết

Tài liệu