Master Linux System Basics: Time, Hostname, Users, Network & Process Commands

Introduction

The purpose of this article is to give readers a complete basic understanding of Linux systems, covering common information and configuration tasks for network, system, CPU, memory, disks, and processes.

Linux System Time

View the current system time: date Set the system time: date -s "2016-02-09 15:15:00" Synchronize network time using NTP: ntpdate -u ntp.api.bz Note: NTP synchronizes the local clock with remote servers such as 210.72.145.44, ntp.api.bz, time.nist.gov, etc. Only the root user can change the system time, and after setting it you should run clock -w to write the time to CMOS.

Hostname and OS Information

Display the system hostname: hostname To permanently change the hostname, edit /etc/hostname and /etc/sysconfig/network, then reboot.

Show kernel and OS details with uname: uname -a Key fields: kernel name, hostname, kernel version, hardware name, processor type, hardware platform, OS name.

Common uname options: -m or --machine: hardware (CPU) name -n or --nodename: network node name -r or --release: kernel release -s or --sysname: kernel name -v: kernel version -p: processor type -i: hardware platform -o: operating system

User and Group Management

User and group classification includes local accounts, remote (domain) accounts, LDAP, NIS, and system vs. regular users. Configuration files: /etc/passwd: user account information /etc/shadow: encrypted passwords /etc/group: group information /etc/gshadow: group passwords

Example of a /etc/passwd line: root:x:0:0:root:/root:/bin/bash Fields: username, password placeholder, UID, GID, description, home directory, login shell.

Example of a /etc/shadow line:

root:$6$zVDR2oO1Yg7alTbs$.70PPMYxg70k9BvLtjHdm94CDA1YWDRDI5NFzSfcmZF5WMESgIbLUdnqRjuVycg481Ny9rl//YzAFnXhurQi//::0:99999:7:::

Fields include encrypted password, last password change date, minimum days before change, maximum days before expiration, warning period, etc.

Adding a user: useradd zhanglonghao Common useradd options: -c comment -d home directory -e account expiration date -f days after expiration before disabling -g primary group -G supplementary groups -m create home directory -r create system account -s login shell -u UID

Modifying a user: usermod -u 1005 zhanglonghao Deleting a user (optionally remove home files):

userdel -rf zhanglonghao2

Basic Network Operations

Fetch a web page with curl: curl www.baidu.com Download a file with wget:

wget http://download.redis.io/releases/redis-3.0.6.tar.gz

Text‑based browsers:

w3m www.baidu.com

links www.baidu.com

Check connectivity with ping: ping www.baidu.com Trace network paths with mtr (example options shown): mtr -r -c 30 www.baidu.com DNS lookup with host:

host www.baidu.com

Network Interface Configuration

Display interfaces with ifconfig: ifconfig Modern ip commands:

ip link show

ip -s link show

ip addr show

Network Monitoring Tools

Show active connections with netstat (common options):

netstat -a

netstat -at

netstat -au

netstat -ant

netstat -tnl

netstat -nlpt

Real‑time traffic monitoring with iftop (install first). Common options include -i to select interface, -B for bytes, -n to show IPs, -N to show ports, etc.

Disk Usage and Tree View

Show filesystem usage: df -h Show directory sizes: du --max-depth=1 -h Tree view of directories (example):

tree -L 3 src

Process Monitoring

Interactive process viewer top (default refresh 3 s). Common keys: Space refresh, q quit, M sort by memory, P sort by CPU.

List processes with ps (BSD style): ps -aux Tree view of processes:

pstree -p

Memory Information

Show memory summary: free -h Detailed memory info from /proc/meminfo:

cat /proc/meminfo