# Linux

## 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)

{% hint style="info" %}
**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 
{% endhint %}

## CLI

<details>

<summary>Basic commands</summary>

#### the ./

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

#### mkdir

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

#### rm

{% code overflow="wrap" %}

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

{% endcode %}

#### mv

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

#### cat

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

# number the output lines
cat -n [filename]
```

#### less

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

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

#### grep

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

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

#### apropos

```bash
# 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

```bash
# to view all the history commands
history
```

</details>

<details>

<summary>pipe</summary>

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

</details>

<details>

<summary>curl</summary>

```bash
# curl 

```

</details>

<details>

<summary>Vi editor</summary>

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

#### open VI editor

```sh
vi index.html
```

#### switch to INSERT mode

* using <mark style="background-color:blue;">i key</mark> to enter insert mode.&#x20;
* using <mark style="background-color:blue;">esc key</mark> 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)

![](https://286463586-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F4PHaS9B0QkS2WKcFwT0s%2Fuploads%2FXA5czZtocf0hVHJTZDAu%2Fimage.png?alt=media\&token=91a27940-ee36-41fa-98c6-1ce7f055b8b2)

#### find stuff

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

#### copy, paste, delete

<img src="https://286463586-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F4PHaS9B0QkS2WKcFwT0s%2Fuploads%2FvhRibfyRTSRcAOenBpno%2Fimage.png?alt=media&#x26;token=844a3279-705b-4224-ba40-9760e277bb7c" alt="" data-size="original">

#### save, quit

![](https://286463586-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F4PHaS9B0QkS2WKcFwT0s%2Fuploads%2FNUAfCK2dr59oEut4NZXv%2Fimage.png?alt=media\&token=cf8bb561-f201-4324-957b-e3fb4b6f1c2c)

#### do not save, quit

Esc > `:q!`

</details>

<details>

<summary>User accounts</summary>

```sh
# 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
```

</details>

<details>

<summary>Download files</summary>

```sh
# 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
```

</details>

<details>

<summary>Tips</summary>

#### check OS version

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

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

#### check details of OS version

```sh
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
```

</details>

#### SSH

{% embed url="<https://quickref.me/ssh>" %}

<details>

<summary>Connecting</summary>

```sh
# 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
```

</details>

<details>

<summary>ping</summary>

command to send network ICMP (Internet Control Message Protocol) echo requests to a specific IP addres&#x73;**.**

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

</details>

### Package manager

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

#### .rpm

<details>

<summary>RPM (Redhat Package Manager) - Redhat</summary>

```sh
# 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
```

</details>

<details>

<summary>YUM (<strong>Yellowdog Updater Modified) - CentOS</strong></summary>

```sh
# 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
```

</details>

#### .deb

<details>

<summary>dpkg - Debiban</summary>

</details>

<details>

<summary>APT (<strong>Advanced Package Tool) - Ubuntu/Debian</strong></summary>

**APT** is a tool used with *Debian-derived* distributions.

```sh
# install app: suricata
sudo apt install suricata

# remove app
sudo apt remove suricata

# list all installed app
apt list --installed
```

</details>

### Services

```sh
# 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

```sh
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
```


---

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