Linux

open-source operation system

Why Linux?

• Ansible do not run on Windows

• Kubernete do not plan to run on Windows

Distro

KALI LINUX ™, Ubuntu, and Parrot all come from Debian.

CentOS comes from Red Hat.

Shell

The shell is the command-line interpreter.

Type of shell:

• Bourne-Again Shell (bash)

• C Shell (csh)

• Korn Shell (ksh)

• Enhanced C shell (tcsh)

• Z Shell (zsh)

ksh and bash use the dollar sign ($) to indicate where users type in their commands. Other shells, such as zsh, use the percent sign (%) for this purpose.

// or to check which type of shell, using

echo $SHELL

CLI

Basic commands

the ./

# indicate the command should be executed in the current directory
# instead of searching for it in system's path
./gcloud --version

mkdir

# -p: make entire directory hierarchy
mkdir -p /tmp/subdir/work/202304

rm

# -r: recursively
# remove all folder content & sub-folder
rm -r /tmp/subdir

mv

# default: -f
# move but do not overwrite
mv -n

cat

# add content to file
cat > contents_file.txt
This is some content text

# number the output lines
cat -n [filename]

less

# show contain with line number
less -B [filename]

# when in less, using /word to find [word]
/word

grep

# grep 'keyword' [target_file]
grep 'temporary password' /var/log/mysqld.log

# with pipe |
# list all .php file
ls -la | grep ".php"

apropos

# open manual of a command
man tail

# one-line description of the command
whatis nano

# search the manual page for specific description
# -a, --and option to search for multiple keywords.
apropos [keyword]
apropos -a change password

history

# to view all the history commands
history

pipe

# using output of 1st clause, use as input for 2nd clause
cat filename | grep "word"

curl

# curl 

Vi editor

Command mode & Insert mode. When open a VI editor, the default mode is COMMAND MODE.

open VI editor

vi index.html

switch to INSERT mode

• using i key to enter insert mode.

• using esc key to return to command mode.

navigate in COMMAND mode

there are 2 ways

• using arrow keys

• using K (up) -J (down) -H (left) -L (right)

find stuff

# for example, you want to find "of"
/of

copy, paste, delete

save, quit

do not save, quit

Esc > :q!

User accounts

# check which user
whoami

# check id
id

# using system user
# if no username was specified, it will switch to root user
su [username]
Password: *****

# using ssh
ssh [username]@192.168.1.2

# using root permission
# ex: list content of /root folder, but the current user do not have the permission
sudo ls /root

Download files

# download some-file.txt
curl http://www.example.com/some-file.txt -O

# another way is using wget
# -O: specify the name of the local file you want to store
wget http://www.example.com/some-file.txt -O some-file.txt

Tips

check OS version

# using wild-card to check OS version
ls /etc/*release*

$ ls /etc/*release*
/etc/lsb-release  /etc/os-release

check details of OS version

cat /etc/*release*

$ cat /etc/*release*
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.4 LTS"
NAME="Ubuntu"
VERSION="18.04.4 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.4 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

SSH

SSH Command Cheat Sheet & Quick ReferenceQuickRef.ME

Connecting

# connect to a server with default port 22
ssh root@192.168.1.5

# connect with specific port
ssh root@192.168.1.5 -p 6222

# connect via pem file (400 permission)
ssh -i /path/file.pem root@192.168.1.5

ping

command to send network ICMP (Internet Control Message Protocol) echo requests to a specific IP address.

# -c option: specify the number of echo request send
ping -c 3 10.154.0.2

Package manager

A package manager is a tool that helps users install, manage, and remove packages or applications. Linux uses multiple package managers.

.rpm

RPM (Redhat Package Manager) - Redhat

# install package
rpm -i telnet.rpm

# query the details of the package
rpm -q telnet.rpm

# get exact package name
rpm -qa | grep ftp

# uninstall package
rpm -e telnet.rpm

YUM (Yellowdog Updater Modified) - CentOS

# install package
yum install ansible

# install package-version
yum install ansible-2.8.11-1.el7

# uninstall package
yum remove ansible

# list all YUM repos
yum repolist

# show all duplicated packages
yum --showduplicates list ansible

# show configuration of the repos
ls /etc/yum.repos.d/

# search for particular package
yum list ansible

.deb

dpkg - Debiban

APT (Advanced Package Tool) - Ubuntu/Debian

APT is a tool used with Debian-derived distributions.

# install app: suricata
sudo apt install suricata

# remove app
sudo apt remove suricata

# list all installed app
apt list --installed

Services

# start a service, e.g: httpd
service httpd start

# or
systemctl start httpd
systemctl stop httpd

# check status
# which file the service configured
systemclt status httpd
thor@host01 ~$ systemctl status app
● app.service - My python web application
   Loaded: loaded (/usr/lib/systemd/system/app.service; disabled; vendor preset: disabled)
   Active: inactive (dead)


# enable, disable after configuration
# when enable, the service start automatically when system boots up
systemctl enable httpd
systemctl disable httpd

[app].service

thor@host01 ~$ cat /usr/lib/systemd/system/app.service
[Unit]
Description=My python web application

[Service]
ExecStart=/usr/bin/python3 /opt/code/my_app.py
ExecStartPre=/bin/bash /opt/code/configure_db.sh
ExecStartPost=/bin/bash /opt/code/email_status.sh
Restart=always

[Install]
WantedBy=multi-user.target