Master Linux: Essential OS Concepts, Commands, and Permissions Explained

Operating System Overview

Operating System (OS) is a basic system software that manages and controls computer hardware and software resources, running directly on bare metal; all other software requires OS support.

Linux File System

/var

: files changed during normal operation, spooling, logs, lock files, temporary files, page cache, etc. /home: user files, configuration, documents, data, mail, cache; each user gets a directory under /home. /proc: virtual files representing kernel and process information, not stored on disk. /bin: essential binary executables needed at system startup, usable by all users. /etc: system configuration files (firewall, startup scripts, etc.). /root: home directory of the root (superuser). /dev: device files, abstracting hardware as files.

Common Linux Commands

pwd : display current directory.

cd : change directory (e.g., cd /path, cd ~, cd .., cd -).

ls : list directory contents ( ls -l, ls -a, ls -la).

mkdir : create directories ( mkdir dir, mkdir -p dir/subdir).

rmdir : remove empty directories ( rmdir dir, rmdir -p dir).

rm : delete files or directories ( rm -rf path, rm -ri path).

cp : copy files or directories ( cp src dest, cp -r src dest, cp -ri src dest).

mv : move or rename files ( mv old new).

touch : create an empty file ( touch filename).

vi : powerful text editor with command, insert, and last-line modes.

cat : display file content or create a new file ( cat file, cat > file).

head : show the beginning of a file ( head -n N file).

tail : show the end of a file or follow updates ( tail -f file, tail -n N file).

chmod : change file permissions using symbolic or numeric modes.

User and Group Management

Each user has a unique UID; each group has a GID. The root user (UID 0) has all privileges. Adding a user creates a corresponding group by default.

su : switch user ( su username or su - username).

whoami : display current user.

groups : list groups of current user.

id : show UID and GID.

useradd : add a new user ( useradd name, useradd -u UID name).

passwd : change user password.

userdel : delete a user ( userdel name, userdel -r name).

usermod : modify user information ( usermod -l new old, usermod -g group name).

groupadd : create a new group ( groupadd group, groupadd -g GID group).

Permission Operations

Linux defines read (r), write (w), and execute (x) permissions for owner, group, and others. Use ls -l to view. drwxr-x-x. 2 root root 4096 Sep 23 2011 bin Permission bits can be set symbolically ( chmod u+rwx,g+rw,o+rw file) or numerically ( chmod 753 file, chmod 777 file).

Process Management

ps : view running processes ( ps -ef | grep keyword).

kill : terminate a process ( kill -9 PID).

Other Useful Commands

clear

: clear the terminal. man: view command manual. mnt: mount filesystems. service sshd start|restart|stop: manage SSH service. nohup java -jar app.jar &: run a Java JAR in background.

Software Installation

Common methods: tar (extract archives), rpm (Red Hat package manager), yum (automated rpm installation). tar -zxvf file.tar.gz, tar -jxvf file.tar.bz2, tar -xvf file.tar. rpm -ivh package.rpm, rpm -qa | grep name, rpm -e package. yum install package (automatically resolves dependencies).