Master Linux Fundamentals: Directory Structure, Permissions, Users, and Essential Commands

Linux System Basics

Linux Directory Structure

The Linux root directory contains many standard sub‑directories, each serving a specific purpose:

/bin : Frequently used executable commands.

/boot : Kernel images and boot loader files.

/dev : Device files representing hardware and virtual devices.

/etc : System configuration files and sub‑directories.

/home : Home directories for regular users.

/lib : Essential shared libraries (similar to Windows DLLs).

/lost+found : Recovered files after an improper shutdown.

/media : Auto‑mounted removable media such as USB drives.

/mnt : Temporary mount points for manual mounting.

/opt : Optional third‑party software packages.

/proc : Virtual filesystem exposing kernel and process information.

/root : Home directory of the super‑user.

/sbin : System administration binaries for the super‑user.

/selinux : Security‑Enhanced Linux configuration files.

/srv : Data for services provided by the system.

/sys : Sysfs virtual filesystem representing kernel objects.

/tmp : Temporary files.

/usr : Secondary hierarchy for user applications and data.

/usr/bin : General user commands.

/usr/sbin : Advanced system administration commands.

/usr/src : Kernel source tree.

/var : Variable data such as logs and caches.

Linux File Basic Attributes

Linux is a multi‑user system; each file has an owner, a group, and permission bits that control read (r), write (w), and execute (x) access for the owner, the group, and others. The first character indicates the file type: "d" for directories, "-" for regular files, "l" for symbolic links, "b" for block devices, and "c" for character devices.

# Example of ls -l output
[root@host /]# ls -l
total 64
dr-xr-xr-x 2 root root 4096 Dec 14 2012 bin
... (additional entries omitted for brevity)

Permission bits are displayed as a ten‑character string (e.g., "-rwxr-xr--"). The three groups of three characters represent owner, group, and others respectively. Numeric notation converts the bits to a three‑digit octal number (e.g., 754 = rwx r-x r--).

Changing File Ownership and Permissions

Common commands: chgrp: Change the group ownership of a file or directory. chown: Change the owner (and optionally the group) of a file or directory. chmod: Modify the nine permission bits using symbolic (u,g,o) or numeric (e.g., 777) notation.

# Change group to "users"
chgrp users install.log
# Change owner to "bin"
chown bin install.log
# Set permissions to 777
chmod 777 .bashrc
# Set permissions to 754 using symbolic mode
chmod u=rwx,g=rx,o=r .bashrc

User and Group Management

Linux manages users and groups through files such as /etc/passwd, /etc/shadow, and /etc/group. Key commands include: useradd: Add a new user account. userdel: Delete an existing user (option -r also removes the home directory). usermod: Modify user attributes (e.g., login shell, home directory, primary group). passwd: Set or change a user's password. groupadd, groupdel, groupmod: Manage groups. newusers: Bulk import of users from a formatted file. chpasswd: Set passwords for many users from a simple "user:password" list.

Disk Management Utilities

Key tools for monitoring and managing storage: df: Show overall filesystem usage (options -h, -a, -T, etc.). du: Estimate file and directory space usage (options -h, -s, -a, etc.). fdisk: Partition table manipulation (option -l lists partitions). mkfs: Create a filesystem on a partition (e.g., mkfs -t ext3 /dev/sda1). fsck: Check and repair filesystem consistency (use on unmounted filesystems). mount / umount: Mount and unmount filesystems. resize2fs: Resize an ext2/3/4 filesystem after expanding a partition.

Common File Operations

Basic commands for handling files and directories: ls, cd, pwd, mkdir, rmdir, rm, mv, cp – standard navigation and manipulation. cat, tac, nl, more, less, head, tail – view file contents with various options. scp: Securely copy files between hosts over SSH. tar: Archive and compress files (e.g., tar -czvf archive.tar.gz dir/). zip / unzip: Create and extract ZIP archives. chattr: Change extended file attributes (e.g., immutable flag).

Shell Scripting Basics

A simple Bash script starts with a shebang ( #!/bin/bash) and can use variables, conditionals ( if … then … elif … else … fi), loops ( for, while), and functions. Parameters are accessed as $1, $2, …, with special variables like $# (argument count) and $@ (all arguments).

#!/bin/bash
# Example: greet a user
if [ "$#" -eq 0 ]; then
  echo "Usage: $0 name"
  exit 1
fi
name=$1
echo "Hello, $name!"

Additional Utilities

Other useful commands covered include chattr (file attribute manipulation), zipinfo (inspect ZIP contents), and various options for chmod, chown, and chgrp to fine‑tune permissions.