Intro

Intro#

This section is focused in options of using/configuring linux.

Check The Linux Kernel documentaiton.

Shell#

To works effectively with Linux, you must undestand capabilities and properties of the Linux shell (command line interface). There is shell section. It considers following topics:

  • Each command in Linux has sources that provide information to the source shell. There are special utilities that manipulate the outputs.

  • Environment varialbes as a way to manipulate the behaviour of the utilities and entire shell.

  • How does shell understand what has to be executed.

  • Bash is the most common shell environment that allows to build scripts that automate typical tasks.

  • Access privileges are regulated through the users/groups and files relationships.

Check more in the shell page.

Tiny commands#

There are some commands that are really primitive to use. They literally have 1-3 options. It doesn’t make sense to have a separate section for all such commands, so we’ll cover them all in this section.

Print the number of processing units available to the current process, which may be less than the number of online processors.

nproc

32

Print how long system have been running.

uptime

 16:29:57 up  3:13,  1 user,  load average: 1,13, 1,39, 1,19

Shows or sets the system’s hostname.

hostname

MBD843AE246AC7

Print working directory.

pwd

/home/f.kobak@maxbit.local/Documents/knowledge/other

Pause the execution flow for a certain amount of time. The following example shows that the second execution of uptime is delayed by sleep when compared to the first execution of uptime.

uptime
sleep 3
uptime

 18:54:35 up 42 min,  1 user,  load average: 1.02, 0.86, 0.57

 18:54:39 up 42 min,  1 user,  load average: 1.02, 0.86, 0.57

Filesystem#

In this section considered set of tools that allows to work with filesystem in Linux.

The following table shows groups of commands and their descriptions. Check more specific description for some of them in the corresponding page.

Category

Utility

Description

File and Directory Management

ls

List files and directories.

cd

Change directory.

mkdir

Create directories.

rmdir

Remove empty directories.

rm

Remove files and directories.

mv

Move or rename files and directories.

cp

Copy files and directories.

touch

Create empty files or update timestamps.

find

Search for files and directories.

locate

Quickly find files by name (uses a prebuilt database).

tree

Display directories as a tree structure.

Viewing File Contents

cat

View file contents.

less

View file contents interactively, with scrolling.

more

View file contents page by page.

head

Display the first few lines of a file.

tail

Display the last few lines of a file.

nano, vim, emacs

Text editors for viewing and editing files.

File Information

stat

Display detailed information about a file.

file

Determine the type of a file.

du

Display disk usage for files and directories.

df

Show disk space usage of filesystems.

lsblk

List information about block devices (disks and partitions).

mount/umount

Mount/unmount filesystems.

blkid

Display block device attributes (UUID, filesystem type).

test

Check file types and compare values

File Permissions and Ownership

chmod

Change file permissions.

chown

Change file ownership.

chgrp

Change file group ownership.

umask

Set default permissions for new files.

File Archiving and Compression

tar

Archive files into tarballs.

gzip, bzip2, xz, zip

Compress files.

gunzip, bunzip2, unxz, unzip

Decompress files.

Filesystem Checks and Maintenance

fsck

Filesystem check and repair.

mkfs

Create a new filesystem.

resize2fs

Resize ext2/ext3/ext4 filesystems.

tune2fs

Adjust tunable parameters on ext2/ext3/ext4 filesystems.

e2fsck

Filesystem check specifically for ext filesystems.

Symbolic and Hard Links

ln

Create hard or symbolic links.

readlink

Display the target of a symbolic link.

Advanced Utilities

rsync

Synchronize files and directories across locations.

scp/sftp

Securely copy files between systems.

lsof

List open files by processes.

inotifywait

Monitor filesystem events in real time.

fstrim

Trim unused space on filesystems for SSDs.

df

Display free disk space on mounted filesystems.

File and Directory Management

ls

List files and directories.

cd

Change directory.

mkdir

Create directories.

rmdir

Remove empty directories.

rm

Remove files and directories.

mv

Move or rename files and directories.

cp

Copy files and directories.

touch

Create empty files or update timestamps.

find

Search for files and directories.

locate

Quickly find files by name (uses a prebuilt database).

tree

Display directories as a tree structure.

Viewing File Contents

cat

View file contents.

less

View file contents interactively, with scrolling.

more

View file contents page by page.

head

Display the first few lines of a file.

tail

Display the last few lines of a file.

nano, vim, emacs

Text editors for viewing and editing files.

File Information

stat

Display detailed information about a file.

file

Determine the type of a file.

du

Display disk usage for files and directories.

df

Show disk space usage of filesystems.

lsblk

List information about block devices (disks and partitions).

mount/umount

Mount/unmount filesystems.

blkid

Display block device attributes (UUID, filesystem type).

File Permissions and Ownership

chmod

Change file permissions.

chown

Change file ownership.

chgrp

Change file group ownership.

umask

Set default permissions for new files.

File Archiving and Compression

tar

Archive files into tarballs.

gzip, bzip2, xz, zip

Compress files.

gunzip, bunzip2, unxz, unzip

Decompress files.

Filesystem Checks and Maintenance

fsck

Filesystem check and repair.

mkfs

Create a new filesystem.

resize2fs

Resize ext2/ext3/ext4 filesystems.

tune2fs

Adjust tunable parameters on ext2/ext3/ext4 filesystems.

e2fsck

Filesystem check specifically for ext filesystems.

Symbolic and Hard Links

ln

Create hard or symbolic links.

readlink

Display the target of a symbolic link.

Advanced Utilities

rsync

Synchronize files and directories across locations.

scp/sftp

Securely copy files between systems.

lsof

List open files by processes.

inotifywait

Monitor filesystem events in real time.

fstrim

Trim unused space on filesystems for SSDs.

df

Display free disk space on mounted filesystems.

System information#

This section looks at a number of utilities that can be used to check system status information.

  • Check the /etc/os-release file to find out more about system’s release.

  • The ps (process status) command allows you to view processes in your system and some information about them.

  • Check resource limit with the ulimit command.

There is more detailed description on the special page.

Network#

There are tons of utilies in linux related to network configuration core utilities are listed in the table below:

Utility

Description

ping

Test the reachability of a host on a network and measure round-trip time.

traceroute

Trace the route packets take to a network host.

netstat

Display network connections, routing tables, interface statistics, and more.

ss

Display detailed information on network connections (modern replacement for netstat).

ifconfig

Configure and display network interfaces (deprecated, replaced by ip).

ip

Show/manipulate routing, devices, policy routing, and tunnels.

nslookup

Query Internet domain name servers (DNS).

dig

A flexible tool for querying DNS servers.

host

Simple utility for performing DNS lookups.

arp

Manipulate the system’s ARP cache (deprecated, replaced by ip neigh).

ip neigh

Manage neighbor objects (ARP and NDP tables).

ethtool

Display or change Ethernet device settings.

iw

Configure and display wireless devices and settings.

iwconfig

Configure wireless network interfaces (deprecated, replaced by iw).

curl

Transfer data from or to a server using protocols like HTTP, FTP, etc.

wget

Non-interactive network downloader.

tcpdump

Network packet analyzer.

nmap

Network discovery and security auditing tool.

whois

Query databases for domain ownership and registration details.

nc

Versatile networking utility for debugging and exploration (Netcat).

telnet

Basic network communication tool (insecure, often replaced by SSH).

ssh

Secure Shell for remote login and command execution.

scp

Securely copy files between hosts.

rsync

Synchronize files and directories over SSH or local systems.

ftp

File Transfer Protocol client (deprecated, replaced by modern secure options).

iptables

Configure Linux kernel firewall (replaced by nftables in many distributions).

nft

Configure and manage firewall rules and NAT.

nmcli

NetworkManager command-line interface.

wpa_cli

Command-line utility for interacting with wpa_supplicant.

mtr

Combines traceroute and ping for real-time network path monitoring.

sshd

OpenSSH daemon for allowing secure connections.

Find out more accurate description in the special page.

Here is a simple example of using the most popular command for me - curl. With it you can send any request to the given URL.

Here is request to google.com, -L option says to follow redirections.

curl -L google.com

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Finally you just got the html code of the goole main page.

Package management#

This section covers package management techniques, focusing specifically on the apt (Advanced Packaging Tool) package manager. However, various other tools exist for managing packages. Check more in official ubuntu documentation. For advanced overview of some features visit correspongind page in this website.

Here’s a brief example demonstrating how package management works in Linux using the apt package manager. It covers basic operations such as listing available packages, installing a package, and removing it.

docker run -itd --rm --name package_management ubuntu

5c58fb6219b16d0f13403b1d031a5048368737003927c391ab31f2461e9502d0

You can list installed packages with apt list --installed.

docker exec package_management apt list --installed | head -n 10

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

Listing...
apt/now 2.7.14build2 amd64 [installed,local]
base-files/now 13ubuntu10.1 amd64 [installed,local]
base-passwd/now 3.6.3build1 amd64 [installed,local]
bash/now 5.2.21-2ubuntu4 amd64 [installed,local]
bsdutils/now 1:2.39.3-9ubuntu6.1 amd64 [installed,local]
coreutils/now 9.4-3ubuntu6 amd64 [installed,local]
dash/now 0.5.12-6ubuntu5 amd64 [installed,local]
debconf/now 1.5.86ubuntu1 all [installed,local]
debianutils/now 5.17build1 amd64 [installed,local]

The following cell shows that there are no installed vim package by default.

docker exec package_management apt list --installed | grep vim;
true

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

With apt install <package_name>, you can install a package.
Note: apt needs up-to-date information about available packages before installation. Therefore, it’s often necessary to run apt update to refresh the package index.

The following code installs vim and shows that appropriate lines appear in the list of installed packages.

docker exec package_management bash -c "apt update && apt install -y vim" &> /dev/null
docker exec package_management apt list --installed | grep vim 

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

vim-common/noble-updates,noble-security,now 2:9.1.0016-1ubuntu7.5 all [installed,automatic]
vim-runtime/noble-updates,noble-security,now 2:9.1.0016-1ubuntu7.5 all [installed,automatic]
vim/noble-updates,noble-security,now 2:9.1.0016-1ubuntu7.5 amd64 [installed]

Note: Several additional packages were installed because the vim package depends on other packages. These dependencies are necessary for vim to function correctly.

To remove installed package use apt remove <package_name>. The following cell shows the process.

docker exec package_management apt remove -y vim &> /dev/null
docker exec package_management apt list --installed | grep vim 

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

vim-common/noble-updates,noble-security,now 2:9.1.0016-1ubuntu7.5 all [installed,auto-removable]
vim-runtime/noble-updates,noble-security,now 2:9.1.0016-1ubuntu7.5 all [installed,auto-removable]

The vim package has been removed, but it’s requirements are still in the system.

docker stop package_management

package_management

Cron#

Cron is a utility that allows you to schedule commands in a Linux system. You can manage cron jobs using crontab, a special utility that handles the file defining jobs for cron.

To set up a cron job, type crontab -e to open the editor where you can define jobs using the following syntax:

minute hour day month weekday command

Each line represents a new cron job.

Find out more at the particular page.

The following cell shows the default cron schedule for Alpine Linux, allowing you to learn the typical format of a cron schedule.

docker run --rm -it alpine crontab -l

# do daily/weekly/monthly maintenance
# min	hour	day	month	weekday	command
*/15	*	*	*	*	run-parts /etc/periodic/15min
0	*	*	*	*	run-parts /etc/periodic/hourly
0	2	*	*	*	run-parts /etc/periodic/daily
0	3	*	*	6	run-parts /etc/periodic/weekly
0	5	1	*	*	run-parts /etc/periodic/monthly

GnuPG#

GnuPG (GPG) is a tool that implements the PGP protocol in Linux. Here, we will explore how to use it. Find out more in the specific page.

To avoid displaying GPG information from my local machine, we will demonstrate an example using Docker. The Docker container has GPG pre-installed and includes a pre-generated key.

Note: The sample key for the purpose has been created without a password, which you will generally need to enter when interacting with the secret key.

docker run -itd --name gpg_example --rm fedorkobak/gpg-example:2

1485149a6b2309a0af2003dacfaa460218cb2f90297c5fd0bbe6f0a24fca1421

Use the --generate-key flag to initiate generation of private/public keys. It’s impossible to display it in text form - it requires tty. Just use the command: docker exec gpg_example gpg --generate-key and follow the instructions.

After creating a key, you can list it with the --list-keys flag. Example container already contains generated gpg key.

docker exec gpg_example gpg --list-keys

[keyboxd]
---------
pub   ed25519 2024-12-15 [SC] [expires: 2027-12-15]
      2218FDFB1F24C0511F22AD226D5C29F6233CA6B7
uid           [ultimate] fed <fakeemail@wow.com>
sub   cv25519 2024-12-15 [E] [expires: 2027-12-15]

Don’t forget to stop the container afterall.

docker stop gpg_example

gpg_example
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