Linux Filesystem & Essential Linux Commands

Linux Directory Structure

• /: The root (base) directory.

• /root: The home directory for the root user.

• /home: The directory containing user home directories.

• /lib: Contains essential system libraries required to run commands and tools, used by /bin and /sbin.

• /bin: Contains binary executables. Essential command binaries are stored here.

• /sbin: Contains system binaries. These are commands that can typically only be accessed by the root user.

• /usr/bin: Contains non-essential binaries. Software installed via the package manager typically places its executables here.

• /usr/local/bin:

  • This directory is intended for locally installed software and scripts that are not managed by the system package manager.

  • You will find user-installed software and custom scripts that are intended to be accessible system-wide.

  • Software and scripts manually placed in /usr/local/bin are typically not overwritten by system updates.

• /var: Contains variable files such as logs, databases, and spool files.

• /proc: A virtual filesystem providing information about processes and system information.

• /boot: Contains boot loader files required by the OS. It is generally not modified by users.

• /dev: Contains device files (hardware interface).

• /mnt: Directory for mounting external devices.

• /media: Contains mount points for removable devices.

• /etc: Contains editable text configuration files. It contains details about the configuration of tools.

• /sys: A virtual filesystem containing information about hardware and the kernel.

The PATH Variable

The PATH variable in Linux is an environment variable that specifies a list of directories where executable programs are located. When you run a command in the terminal, the system searches through these directories to find the executable file corresponding to that command.

Format of the PATH Variable

The PATH variable consists of a colon (:) separated list of directories. Here's an example:

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games

Example of Setting the PATH Variable

To include a new directory, such as where you have installed Terraform, you can modify the PATH variable as follows:

export PATH=$PATH:/usr/local/terraform

After editing the file, you can apply the changes immediately by sourcing the file:

source ~/.bashrc

Linux Commands

Gaining Root User Access

sudo su -
logout

Finding path

which

Piping Commands

# Use the output of the first command as input for the second
|

Debugging Commands

# Print command in debug mode
set -x

Error Handling in Scripts

# Exit the script when there is an error
set -e

# Exit the script when there is a pipe failure
set -o pipefail

Disk and Memory Usage

# Disk usage in human-readable format
df -h

# Memory usage
free

# Number of processing units
nproc

Real-Time Performance Monitoring

# Monitor performance in real-time
top

# All processes running in the background
ps -ef

Filtering Output

# Basic filtering command
grep

# Powerful filtering and text processing
awk

Retrieving and Downloading Information from the Internet

# Retrieve information from the internet
curl

# Download files from the internet
wget

Searching for Files

# Search for the location of a file
find / -name test.sh

Trapping Signals in Linux

trap

Working with Processes

# List all processes
ps -ef

# Print errors from remote logs
curl ... | grep ...

String Manipulation

# Write number of occurrences of 's' in the word 'missi'
x=missi
grep -o "s" <<<"$x" | wc -l

Scheduling Tasks with Crontab

# Crontab in Linux
Alarm

Editing Files

# Open a file in read-only mode
vim -r test.txt

Soft Links vs. Hard Links

Soft Link

• Acts as a shortcut reference to the original file

• The link can be broken if the original file is deleted or moved

Hard Link

• Acts as a copy

• Deleting the original file does not affect the hard link

Network Troubleshooting Utilities

Traceroute
Tracepath

Managing Large Log Files

Logrotate

Filtering JSON Data

# Filter data from JSON
jq

Filtering YAML Data

# Filter data from YAML
yq

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