Basics of linux

This is a huge chapter. I could divide it up in many subchapters but I like to have it all at one place so I can just do ctr-f, and search for whatever I am looking for.

1. The Shell - Bash

The shell, or the terminal is a really useful tool. Bash is the standard shell on most Linux distros.

pwd - Print working directory

cd - Change directory

cd ~ - Change directory to your home directory

cd - - Go back to previous directory

Looking at files

ls - List files in directory

ls -ltr - Sort list by last modified. -time -reverse

file - Show info about file. What type of file it is. If it is a binary or text file for example.

cat - Output content of file.

less - Output file but just little bit at a time. Use this one. Not more.

Use /searchterm to search. It is the same command as in vim. n to scroll to next search result. Press q to quit.

more - Output file but just little bit at a time. less is better.

Working with files

touch - Create a new file.

cp - Copy

mkdir - Make directory.

# Make entire directory structure
mkdir -p new/thisonetoo/and/this/one

rm - Remove file

# Remove recursively and its content. Very dangerous command!
rm -rf ./directory

Watch the command destroy an entire machine: https://www.youtube.com/watch?v=D4fzInlyYQo

rmdir - Remove empty directory

A little bit of everything

history - Show commands history

sudo

List what rights the sudo user has.

sudo -l

Sudo config file is usually /etc/sudoers

Finding files

There are mainly three ways to find files on Linux: find, locate, and which.

Find

Find is slower than locate but a lot more thorough. You can search for files recursively and with regex and a lot of other features.

# This will send all permissions denied outputs to dev/null.
find / -name file 2>/dev/null

Locate

Locate is really fast because it relies on an internal database. So in order to have it updated you need to run:

sudo updatedb

Then you can easily find stuff like this:

locate filename

Which

Outputs the path of the binary that you are looking for. It searches through the directories that are defined in your $PATH variable.

which bash
# Usually outputs: /bin/bash

2. Editing text

First let's just clear out something about standard streams, or I/O-streams. Standard streams are the streams that are used to interact between the human computer-user and the machine. There are three standard streams: standard input (stdin), standard output (stdout), and standard error (stderr).The stdin stream can be seen as an abstractions of the real keyboard input. So when you issue a command/program that requires input the program does not read straight from the keyboard input, instead it reads from the file STDIN.

Stdin

Stdin is the data that gets inputed into the program. An example of a program that requires stdin data is cp. In order for the program to do anything it needs input data. For example cp file1 copy_of_file1. Here file1 and copy_of_file1 is the stdin.

So the default Stdin comes from the STDIN-file that is a text-file representation of the keyboard input. But often times we do not want to input stuff from the keyboard, sometimes we want to input something into a program that comes from another file. That is when we can use redirection symbol: >.

So an example could be cat < my_text_file.txt. The data from my_text_file.txt will now be used as input instead of the keyboard input.

The file descriptor for stdin is: 0

Stdout

Stdout is the data that get ouputed from the program. For example, when you use the command cat file1 that data/text that gets outputed is the stdout The same with the program ls. Not all programs have stdout. For example when you use mv or cp successfully you get no stdout back from the program.

The stdout can be redirected to another file by using these symbols > and >>. So now we can do the following:

ls > result_of_ls.txt
# now the result will be written to the file result_of_ls.txt
ls >> result_of_ls.txt
# This will append the data to the bottom of the file result_of_ls.txt

Another incredibly useful feature is the pipe feature, reprsented with this symbol |. It will take the stdout and redirect it into another program. Here is an example:

ls -la | less

This will take the stdout from ls -la and forward/redirect it into the less program. Using the pipe you can now chain different commands.

The file descriptor for stdout is: 1

Stderr

Stderr is the stream used for outputting error messages. So if a program fails for whatever reason. For example, if we try to copy a file that does not exist, this will be the stdrr output:

cp thisfiledoesnotexist aaaaaaaaaa
cp: cannot stat 'thisfiledoesnotexist': No such file or directory

This is a common way for stderr to present itself, just straight out into the terminal. But sometimes stderr gets sent to a log file.

Stderr is useful because with it we can separate between stdout and stderr. However, to the eye it might be difficult to distinguish what output is stdout and what output is stderr.

One easy way to determine is the output is stderr or stdout is to simply redirect it into a file. Because by default you only redirect stdout, and not stderr.

cp thisfiledoesnotexist aaaaaaaaaa > result.txt
cp: cannot stat 'thisfiledoesnotexist': No such file or directory
# If we now look at result.txt we will find that it is empty. Since the error-text we recieved could not be redirected into the textfile, since it is stderr and not stdout.

Filters

There are certain programs that are especially useful to use together with pipes. They can also be used as stand-alone programs but you will often see them together with pipes.

sort

sort test.txt

uniq

sort -u test.txt
sort test.txt | uniq
cat filename | sort -u > newFileName

grep

head

tail

tr

sed

Editing text

sed

Can perform basic editing on streams, that is to say, text.

Remove first line of file/stream

sed "1d"

cut

Cut by column

This is a useful command to cut in text.

Let's say that we have the following text, and we want to cut out the ip-address.

64 bytes from 192.168.0.1: icmp_req=1 ttl=255 time=4.86 ms
cut -d" " -f4

-d stands for delimiter. and -f for field.

tr - Translate

Transform all letter into capital letters

tr "[:lower:]" "[:upper:]" < file1 > file2

Example
Remove character

# Remove characters
cat file.txt | tr -d "."

# Remove and replace
# Remove all dots and replace them with underscore.
cat file.txt | tr "." "_"

http://www.thegeekstuff.com/2012/12/linux-tr-command/

awk

So awk is an advanced tool for editing text-files. It is its own programming language to it can become quite complex. Awk iterates over the whole file line by line.

This is the basic structure of an awk command

awk '/search_pattern/ { action_to_take_on_matches; another_action; }' file_to_parse

The search pattern takes regex.
You can exclude the search portion or the action portion.

This just prints every line of the file.

awk '{print}' filename

Filtering out specific ip-address:

awk '/172.16.40.10.81/' error.log

Now we want to print out the fourth column of that file, we can just pipe this to cut, but we can also use awk for it, like this:

awk '/172.16.40.10.81/ {print $4}' error.log
# Another example
awk '{print $2,$5;}' error.txt
This prints columns 2 and 5.

We can use the -F flag to add a custom delimiter.

awk -F ':' '{print $1}' test.txt

So if you are manipulating some text you might want to start the output with some info about the columns or something like that. To do that we can use the BEGIN-keyword.

awk 'BEGIN {printf "IP-address \tPort\n"} /nop/ {print $3}' test.txt | head
awk 'BEGIN{printf "IP-address \tPort\n"} /nop/ {print $3} END {printf "End of the file\n"}' test.txt | tail

Here we are printing IP-address PORT to the first line of the file.

3. User management

To add a user we do:

adduser NameOfUser

# On some machines it is
useradd nameOfUser

To add user to sudo-group:

adduser NameOfUser sudo

On some machines we might not be able to edit the sudoers file because we don't have an interactive shell, in this case can you can just redirect the text into the file, like this:

echo "username ALL=(ALL) ALL" >> /etc/sudoers

Check which users are in the sudo group:

cat /etc/group | grep sudo

Switch user in terminal:

su NameOfUser

Remove/delete user:

sudo userdel NameOfUser

4. Permissions

ls -la

Shows all the files and directories and their permission settings.

drwxrwxrwt 2 root root 4,0K ago  3 17:33 myfile

Here we have 10 letters in the beginning. The first one d shows that it is a directory.
The next three letters are for read, w for write and x for execute. The first three belong to the owner, the second three to the group, and the last three to all users.

https://linuxjourney.com/lesson/file-permissions

5. Processes

To display information regarding the systems processes you can use the ps command.

ps -aux

-a stands for all -u stands for all processes by all users -x stands for all processes that don't run a tty

If you run this command you will probably see a pretty big output. In the column for command you will see what command has been run. Every process has a Process Identification Number (PID). Something you will also see in the output.

All of theses processes can actually be found in /proc. You just go to /proc/[pid]. In /proc you can find information about the system, and you can actually change the system if you change those files! But more on that later. What I wanted to explain is that if we look at the output from ps we see that some commands are in brackets. Like this:

root        10  0.0  0.0      0     0 ?        S    ene14   0:00 [watchdog/0]
root        11  0.0  0.0      0     0 ?        S    ene14   0:00 [watchdog/1]
root        12  0.0  0.0      0     0 ?        S    ene14   0:00 [migration/1]
root        13  0.0  0.0      0     0 ?        S    ene14   0:00 [ksoftirqd/1]

Those are usually kernel processes, and you can safely assume that no user has started them.

If you want to monitor processes in real time you can use top or htop. top comes preinstalled on most distros. But htop is really a lot nicer.

For htop the F1-10 keys might trigger OS-events. So you can use the shortcuts instead.

Shortcut Key    Function Key    Description
h                    F1           Invoke htop Help
S                    F2           Htop Setup Menu
/                    F3           Search for a Process
I                    F4           Invert Sort Order
t                    F5           Tree View
>                    F6           Sort by a column
[                    F7           Nice – (change priority)
]                    F8           Nice + (change priority)
k                    F9           Kill a Process
q                    F10         Quit htop

http://www.thegeekstuff.com/2011/09/linux-htop-examples/

6. Packages

Something that difference Linux from windows is how it handles installing new software. In windows you usually have to google around and then click on random scary download buttons that might fuck up your computer, or not. It's like a constant lottery where you win by no installing malware. In Linux that is usually not really an issue. That is because distros have their own software repositories from where you can download your software. This is kind of like an app-store except everything is free.

The different major branches of teh GNU/Linux OS have their own software repositories. Ubuntu has their own, debian has their own, and so on.

Different distros also have their own package-amangers. For example, Debian and ubuntu uses apt, while Redhat uses rpm, and Arch uses pacman. You should strick to your own package-manager, because even though chaning package-manager is possible it will probably just cause you more headache than benefits.

Install package

Example of how to install something with apt:

sudo apt-get install nmap

If you only have a .deb file you do this to install from the terminal:

sudo dpkg -i /path/to/deb/file
sudo apt-get install -f

Remove packages

This can be tricky. First find the package

dpkg --list

Then you find it in your list.

sudo apt-get --purge remove nameOfProgram

When you remove some package it might have requires some other dependencies. To remove those you run

sudo apt-get autoremove

Organizing your $path variable

I am talking about debian/ubuntu here. On other systems I don't know.

You can define your path in /etc/environment. If you don't have it you can create it and add the path like this:

source /etc/environment && export PATH

If you are using zsh (which you should) you have to add it here

sudo vim /etc/zsh/zshenv

And add this line somewhere:

source /etc/environment

Adding a path

This is a non-persistent way to add binaries to your path. Might be useful if you have entered a system that has limited binaries in the path.

export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

Installing custom packages

If you download a package that is not in the official repository you can put the binary in /opt. That is good place to put your binaries.

Now you need to add that path to your path-variable. Remember how we set that in /etc/environment. So now open up that file and add /opt to it, so i looks like this.

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/opt"

I always add custom binaries last. That means that if we have two binaries with the same name the machine will first select the original binary. This way you won't have to fear screwing up, by accidentally creating a new ls binary for example.

7. Cronjobs

There are two ways to configure cronjobs. The first one is by putting scripts in the following folders.

/etc/cron.daily
/etc/cron.hourly
/etc/cron.weekly
/etc/cron.monthly

The second way is to write the command in the crontab

# list cronjobs
crontab -l

# Edit or create new cronjobs
crontab -e

8. Devices

List all devices

fdisk -l

9. The Filesystem

The Filesystem Hierarchy Standard


This image is copied from here: http://askubuntu.com/questions/138547/how-to-understand-the-ubuntu-file-system-layout/138551#138551

Difference between sbin and bin

sbin is system binaries. A normal user do not have access to these binaries. It is only root and users with sudo privileges that do.

[email protected]:/bin$ ls -la /bin
total 4092
drwxr-xr-x  2 root root   4096 2012-02-04 19:12 .
drwxr-xr-x 21 root root   4096 2012-02-06 18:41 ..

--snip--

-rwxr-xr-x  1 root root  27312 2008-04-04 02:42 cat
-rwxr-xr-x  1 root root  45824 2008-04-04 02:42 chgrp
-rwxr-xr-x  1 root root  42816 2008-04-04 02:42 chmod
-rwxr-xr-x  1 root root  47868 2008-04-04 02:42 chown
-rwxr-xr-x  1 root root  71664 2008-04-04 02:42 cp
-rwxr-xr-x  1 root root 110540 2007-11-13 05:54 cpio
-rwxr-xr-x  1 root root  79988 2009-03-09 09:03 dash
-rwxr-xr-x  1 root root  24684 2008-04-04 02:42 echo
-rwxr-xr-x  1 root root  40560 2008-02-29 02:19 ed
-rwxr-xr-x  1 root root  96440 2007-10-23 16:58 egrep
-rwxr-xr-x  1 root root  22192 2008-04-04 02:42 false
-rwxr-xr-x  1 root root   5740 2008-02-06 17:49 fgconsole
-rwxr-xr-x  1 root root  53396 2007-10-23 16:58 fgrep
-rwxr-xr-x  1 root root   8796 2007-11-15 13:01 hostname

We have echo, cp, grep. The normal stuff a user needs.

In sbin we have binaries that control the system.

ls -la /sbin
total 5884
drwxr-xr-x  2 root root     4096 2012-02-04 10:01 .
drwxr-xr-x 21 root root     4096 2012-02-06 18:41 ..
-rwxr-xr-x  3 root root    23840 2008-03-27 13:25 findfs
-rwxr-xr-x  1 root root    20020 2008-03-27 13:25 fsck
-rwxr-xr-x  1 root root    15168 2008-09-26 08:43 getty
-rwxr-xr-x  1 root root      375 2009-12-10 10:55 grub-install
lrwxrwxrwx  1 root root        6 2012-02-04 09:51 halt -> reboot
-rwxr-xr-x  1 root root    69228 2008-03-28 18:26 hdparm
-rwxr-xr-x  1 root root    31620 2008-09-26 08:43 hwclock
-rwxr-xr-x  1 root root    61808 2007-12-13 05:51 ifconfig
-rwxr-xr-x  2 root root    27372 2007-09-19 20:25 ifdown
-rwxr-xr-x  2 root root    27372 2007-09-19 20:25 ifup
-rwxr-xr-x  1 root root    89604 2008-04-11 09:50 init
-rwxr-xr-x  1 root root    47448 2008-01-28 08:49 ip6tables
-rwxr-xr-x  1 root root    51680 2008-01-28 08:49 ip6tables-restore
-rwxr-xr-x  1 root root    51644 2008-01-28 08:49 ip6tables-save
-rwxr-xr-x  1 root root    10948 2007-12-13 05:51 ipmaddr
-rwxr-xr-x  1 root root    47480 2008-01-28 08:49 iptables

Mount

So everything on the linux-filesystem belongs to some part of the filesystem-tree. So if we plug in some device we need to mount it to the filesystem. That pretty much means that we need to connect it to the filesystem. Mount is like another word for connect.

So if you want to connect a CD-rom or USB to your machine. You need to mount it to a specific path on the filesystem.

So if you plug in the usb it might be accessible at /dev/usb. But that it not enough for you to be able to browse the usb content. You need to mount it. You do this by writing

mount /dev/usb /media/usb

Or whereever you want to mount it.

So when you click on Eject or Safetly remove you are just unmounting.

umount /media/usb

Knowing how to mount and unmount might be useful if you want to get access to a remote NFS-directory. You will need to mount it to your filesystem to be able to browse it.

10. Controlling services

Systemctl

Systemctl can be used to enable and disable various services on your linux machine.
Start ssh

systemctl start ssh
systemctl status ssh
systemctl stop ssh

You can verify that the service is listening for connection by running network status.

netstat -apnt

Make ssh start upon boot

systemctl enable ssh
systemctl enable apache2

Init.d

Init.d is just a wrapper around Systemctl. I prefer it.

/etc/init.d/cron status
/etc/init.d/cron start
/etc/init.d/cron stop

rcconf

This is a tool to control services more easily, what is running upon boot and so on.

11. Kernel

The Kernel is responsible for talking between the hardware and the software, and to manage the systems resources.

The Linux Kernel is a monolithic kernel, unlike the OSX and the Windows kernels which are hybrid.

You can find the kernel file in /boot. It might look like something like thisvmlinuz-4.4.0-57-generic. In the beginning of time the kernel was simply called linux. But when Virtual Memory was introduced they changed the name to vmlinux to reflect that the kernel could handle virtual memory. When the kernel later became too big it was compressed using zlib , therefore the name was changed to vmlinuz.

The Linux Kernel differs from Windows in that it contains drivers by default. So you don't have to go around looking for drivers like you do on windows when you want to install a printer, or something like that.

It is really easy to upgrade to the latest Linux kernel, all you have to do tis this:

sudo apt-get update && sudo apt-get dist-upgrade
# or
sudo apt-get update && sudo apt-get upgrade

If you are using a distro that is Long Term Supported (LTS). You will not get the latest Kernel version, but you will get the latest Long Term Supported version.

14. Logging

Logs can be viewed here on debian distros /var/log/

16. Network basics

Netstat - Find outgoing and incoming connections

Netstat is a multiplatform tool. So it works on both mac, windows and linux.

$ netstat -antlp
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 mymachine:domain        *:*                     LISTEN      -               
tcp        0      0 localhost:ipp           *:*                     LISTEN      -               
tcp        0      0 localhost:27017         *:*                     LISTEN      -               
tcp        0      0 localhost:mysql         *:*                     LISTEN      -               
tcp        0      0 192.168.0.15:44013      ec2-54-85-27-14.c:https ESTABLISHED 6604/slack --disabl
tcp        0      0 192.168.0.15:51448      ec2-50-16-193-3.c:https ESTABLISHED 3120/chrome     
tcp        0      0 192.168.0.15:43476      104.27.152.203:https    TIME_WAIT   -               
tcp        0      0 192.168.0.15:59380      149.154.175.50:https    ESTABLISHED 5068/Telegram   
tcp        0      0 192.168.0.15:53840      149.154.175.50:http     ESTABLISHED 5068/Telegram   
tcp        0      0 192.168.0.15:47158      176.32.99.76:https      ESTABLISHED 3120/chrome     
tcp        0      0 192.168.0.15:47161      176.32.99.76:https      ESTABLISHED 3120/chrome     
tcp        0      0 localhost:27017         localhost:44196         ESTABLISHED -               
tcp        0      0 192.168.0.15:46910      a104-114-242-25.d:https ESTABLISHED 3120/chrome     
tcp        0      0 localhost:44196         localhost:27017         ESTABLISHED 6903/node       
tcp        0      0 192.168.0.15:36280      cb-in-f101.1e100.:https ESTABLISHED 3120/chrome     
tcp        0      0 192.168.0.15:47160      176.32.99.76:https      ESTABLISHED 3120/chrome     
tcp        0      1 192.168.0.15:59285      149.154.175.50:https    LAST_ACK    -               
udp        0      0 *:35733                 *:*                                 -               
udp        0      0 mymachine:domain        *:*                                 -               
udp        0      0 *:bootpc                *:*                                 -               
udp        0      0 *:33158                 *:*                                 -               
udp        0      0 *:ipp                   *:*                                 -               
udp        0      0 *:mdns                  *:*                                 3120/chrome     
udp        0      0 *:mdns                  *:*                                 3120/chrome     
udp        0      0 *:mdns                  *:*                                 -               
udp        0      0 192.168.0.15:55065      ce-in-f189.1e100.:https ESTABLISHED 3120/chrome

A few interesting things to observe here is that my machine is using any port over 1024 to connect to the outside. So it is not like just because we communicate with https and connect to port 443 that we use that port on our machine. On our machine it can be any port (over 1024) and usually any port over 10000.

Find out what services are listening for connection on your machine
Flags

-a # All
-n # show numeric addresses
-p # show port
-t # tcp
netstat -anpt

To easily check out what process is using lots of bandwidth you can use nethogs.

sudo apt-get install nethogs
nethogs

Or you can use tcpdump, or iptables.

Every listening process of course has a PID, but unless you are root you can't might not see them all.

Firewall - Iptables

Iptables is a firewall tool in linux. A firewall is basically a tool that scans incoming and/or outgoing traffic. You can add rules to the iptables to filter for certain traffic.

Types of chains

So you can filter traffic in three different ways input, forward, and output. These are called three different chains.

INPUT
This is for incoming connections. If someone wants to ssh into your machine. Or a web-server responds to your request.

FORWARD
This chain is used for traffic that is not aimed at your machine. A router for example usually just passes information on. Most connections are just passing through. As you can see this will probably not be used so much on your machine, as a normal desktop or a server doesn't router that much traffic.

OUTPUT

This chain is used for outgoing traffic.

Active rules

To view your active rules you do

iptables -L
# It will output something like this

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

So as we can see the current policy is to accept all traffic in all directions.

If you for some reason has been tampering with the iptables and maybe fucked up. This is how you return it to the default setting, accepting all connections

iptables --policy INPUT ACCEPT
iptables --policy OUTPUT ACCEPT
iptables --policy FORWARD ACCEPT

If you instead want to forbid all traffic you do

iptables --policy INPUT DROP
iptables --policy OUTPUT DROP
iptables --policy FORWARD DROP

Okay, so let's block out some connections. To do that we want to add/append a new rule. We want to block all connections from our enemy 192.168.1.30.

# A for append, and S for source. 
iptables -A INPUT -s 192.168.1.30 -j DROP
# Block an entire range
iptables -A INPUT -s 192.168.1.0/24 -j DROP

Now if we want to see our current rules we just do

iptables -L

And we can now see our new rule.

To add line-numbers for each rule, so that you can then specify which rule you want to reset or change or something you can output the rluels with line-numbers

iptables -L -v --line-numbers

Remove/delete a rule
To remove a rule you just do

# Remove one specific rule
iptables -D INPUT 2
# Remove all rules
iptables -F

Save your changes
Your changes will only be saved and therefore in action until you restart iptables. So they will disappear every time you reboot unless you save the changes. To save the changes on ubuntu you do

sudo /sbin/iptables-save

Measuring bandwidth usage

There are a few different tools in hour arsenal that we can use to measure bandwidth usage. We will start with iptables.

To view the input and output traffic we just list the rules with some verbosity.

iptables -L -v
# Stdout
Chain INPUT (policy ACCEPT 6382 packets, 1900K bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain OUTPUT (policy ACCEPT 4266 packets, 578K bytes)
 pkts bytes target     prot opt in     out     source               destination

So clean this up and reset the count we can do the following

# Restar the count
iptables -Z
# Remove all the rules, FLUSH them
iptables -F

So now we just need to add our rules. A simple script for this would be

#!/bin/bash
iptables -F
iptables -I INPUT 1 -p tcp -j ACCEPT

Then check out the traffc with

iptables -L -v --line-numbers

Examples

Block outgoing connections to a specific ip

iptables -A OUTPUT -d 198.23.253.22 -j DROP

https://www.digitalocean.com/community/tutorials/how-to-list-and-delete-iptables-firewall-rules

Troubleshooting

Have you tried turning it on and off?

I have had problems with the network-adapter not starting or something like that, on Ubuntu. You can try to restart the network manager if this happens:

sudo service network-manager restart

Magical rfkill

If for some reason the wifi is blocked you can unblock it (or block it) with rfkill.

$ rfkill list
0: phy0: Wireless LAN
    Soft blocked: no
    Hard blocked: no
2: hci0: Bluetooth
    Soft blocked: no
    Hard blocked: no

To block or unblock the phy0 from the example above you do:

# Block
rfkill block 0
# Unblock
rfkill unblock 0

If there is a hard block it means that there is a physical switch on you machine that you need to switch off.

17. Subnetting

18. Routing

21. DNS

References

https://linuxjourney.com/
https://github.com/jlevy/the-art-of-command-line

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