Master Linux Basics: Filesystem, Commands, Permissions, and User Management

Operating System Overview

Operating System (OS) is the fundamental software that manages hardware and software resources on a computer. It runs directly on the bare metal and provides the platform for all other applications.

Linux Installation

Installation steps are described in the Linux installation tutorial. URL: https://blog.csdn.net/huaijiu123/article/details/82083452

Filesystem Hierarchy

/var

: variable data such as logs, spools, temporary files. /home: each user’s personal files and configuration. /proc: virtual files exposing kernel and process information; they do not occupy disk space. /bin: essential binaries required for system boot and basic commands. /etc: system and service configuration files. /root: home directory of the superuser. /dev: device files that abstract hardware as files.

Basic Command-Line Operations

Show current directory: pwd Change directory: cd [directory] (e.g., cd ~ for the user’s home, cd .. for the parent, cd - to return to the previous directory, cd / for the root).

List files: ls (options: ls -l for long format, ls -a to include hidden files, ls -la for both).

Create directories: mkdir folder_name or recursively mkdir -p folder/subfolder.

Remove empty directories: rmdir folder_name (or rmdir -p folder_name for recursive removal).

Delete files or directories: rm -rf path (forceful removal, use with caution) or rm -ri path for interactive confirmation.

Copy files/directories: cp source destination (recursive: cp -r src dst, interactive: cp -ri src dst).

Move or rename: mv old_name new_name (also moves files across directories).

Create empty file: touch filename View or edit files:

vi filename

File Editing with vi

The vi editor has three modes:

Command mode – enter commands to manipulate text.

Insert mode – edit the file content; press Esc to return to command mode.

Ex (last‑line) mode – execute commands such as :wq! to save and quit or :q! to quit without saving.

Viewing Files

cat filename

– display file content. cat > filename – create a new file and write to it. head -n N filename – show the first N lines. tail -f filename – follow a file (useful for logs). tail -n N filename – show the last N lines.

Permission Management

Linux defines three permission types (read, write, execute) for three classes of users (owner, group, others) using the symbols r, w, x. Example output of ls -l: drwxr-xr-x 2 root root 4096 Sep 23 2011 bin Interpretation: d – directory. rwx – owner has read, write, execute. r-x – group has read and execute. r-x – others have read and execute.

Changing permissions is done with chmod. Two syntax styles exist:

Symbolic mode: chmod u+rwx,g+rw,o+rw filename – use u, g, o for owner, group, others and + / - to add or remove r, w, x.

Numeric mode: Permissions are represented by three octal digits (owner, group, others). For example, chmod 753 filename gives rwx to owner, r-x to group, and -wx to others; chmod 777 filename grants full access to everyone.

Process Management

Common commands to view and control processes:

List processes: ps -ef | grep keyword – shows user, PID, PPID, CPU usage, and command.

Terminate a process: kill -9 PID – forcefully kill the process identified by its PID.

ps -ef | grep sshd

kill -9 1234

Common Utilities

clear

– clear the terminal screen. man command – view the manual page for a command. service sshd start, service sshd restart, service sshd stop – control the SSH daemon. nohup java -jar jar-0.0.1-SNAPSHOT.jar & – run a Java JAR in the background without hanging up.

Software Installation

Three primary methods for installing software on Linux:

Tar archives: Use tar to extract packages. tar -zxvf package.tar.gz – extract gzip‑compressed archive. tar -jxvf package.tar.bz2 – extract bzip2‑compressed archive. tar -xvf package.tar – extract plain tar archive.

RPM packages: Red Hat package manager. rpm -qa | grep keyword – check if a package is installed. rpm -ivh package.rpm – install with progress. rpm -e package_name – uninstall.

YUM: Front‑end to RPM that resolves dependencies automatically. yum install package_name – installs the package and any required dependencies.

User and Group Management

Linux assigns each user a unique UID and each group a GID. The root user (UID 0) has full privileges.

Add a user: useradd username (optionally specify UID with -u UID).

Delete a user: userdel username or userdel -r username to remove the user’s home directory.

Modify a user: usermod [options] username (e.g., usermod -l newname oldname to change login name, usermod -g groupname username to change primary group).

Set password: passwd username.

Add a group: groupadd groupname (optionally specify GID with -g GID).

Delete a group: groupdel groupname.

Switch user: su username (temporary) or su - username (login shell).

Show current user: whoami.

Show groups of current user: groups.

Show UID/GID: id.

Permission Operations

Changing file permissions uses chmod in symbolic or numeric mode (see Permission Management section). Example symbolic command: chmod u+rwx,g+rw,o+r filename Example numeric command:

chmod 754 filename