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linux-administration

Linux Administration: The Complete Linux Bootcamp for 2022 by Andrei Dumitrescu

Details

Click to Contract/Expend

Section 2: Setting Up the Environment

5. Linux Distributions

.rpm: Redhat Package Manager
- Redhat: commercial
- CentOS: based on redhat but free
- yum
.deb: Debian package manager
- Ubuntu
- apt-get

Linux Distributions

7. Things to Do After Installing Ubuntu

update and upgrade apt packages

sudo apt update
apt list --upgradable
sudo apt full-upgrade

Insert Guest Addition or install it

sudo bash /media/kimn/VBox_GAs_6.1.24/VBoxLinuxAdditions.run
# if I get an error run first:
# sudo apt-get install build-essential gcc make perl dkms
reboot

and eject the Guest Addition image

Take a snapshot(back up) at this moment

backup: Virtual Box menu bar -> Machine -> Take Snapshot
restore: Image -> Snapshots -> Select the target -> Restore

8. Installing CentOS in a VM

Installation
1. Installation Destination
  - Select ATA VBOX HARDDISK
  - Storage Configuration - Automatic
  - Done
2. Network & hostname
  - Ethernet -> On -> Done
  - Host name -> centos8 -> Apply
  - Done
3. Software Selection
  - O, Server with GUI
  - Server : no gui
4. User Creation
  - Make this user administrator
  - noah@sk12
Once installation is done, remove optical disk from virtual drive

Section 3: The Linux Terminal In Depth

11. Terminals, Consoles, Shells and Commands

Ctrl + Alt + T : terminal

A Terminal Emulator and is a crucial part of any Linux system because it basically allows you to access the system through a shell
A shell is a program that takes commands from the user and gives them to the operating system's kernel to execute. It's also called the command interpreter. The shell gets started when the user logs in or starts the terminal
Linux is a case-sensitive operating system (windows is not)

ps
#   PID TTY          TIME CMD
#  2469 pts/0    00:00:00 bash
#  4186 pts/0    00:00:00 ps

sudo apt update && sudo apt install terminator

12. Linux Command Structure

# count 1 - only one ping
ping -c 1 8.8.8.8
# PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
# 64 bytes from 8.8.8.8: icmp_seq=1 ttl=115 time=31.8 ms

# --- 8.8.8.8 ping statistics ---
# 1 packets transmitted, 1 received, 0% packet loss, time 0ms
# rtt min/avg/max/mdev = 31.750/31.750/31.750/0.000 ms
ping -c 1 127.0.0.1

# list directory contents
man ls
ls -a
ls --all
ls -l -all -h /var/
ls -alh /var/

# report file system disk space usage
man df
df -h
df -hi -all

13. Getting Help, Man Pages (man, type, help, apropos)

Press h: you will see all short-cuts
Search: /
- /pattern * Search forward for (N-th) matching line.
- ?pattern * Search backward for (N-th) matching line.
- n * Repeat previous search (for N-th occurrence).
- N * Repeat previous search in reverse direction.

man

type df
# df is /usr/bin/df
type rm
# rm is /usr/bin/rm
type apt
# apt is /usr/bin/apt

help

type cd
# cd is a shell builtin
type alias
# alias is a shell builtin
type umask
# umask is a shell builtin
help alias
# alias: alias [-p] [name[=value] ... ]

the way both work
```
rm --help
cd --help
```

# To see what's available to see
man -k ifconfig
man -k uname
man -k "copy files"

# apropos = man -k
apropos ifconfig
apropos -k uname

15. Mastering the Terminal: The TAB Key

if # double tab
# if        ifconfig

18. Mastering the Terminal: The Bash History

# command history
cat ~/.bash_history
# in the file
echo $HISTFILESIZE
# 2000

history
 # in memory
echo $HISTSIZE
# 1000

.bash_history file will be updated when logging out the user

history
! 349 # execute the specific command number
!! # execute the last command
!-7 # execute 7th previous command
!ping # execute the last ping command
!ping:p # print the last ping command with full options, not executing

Ctrl + R : search in history
Ctrl + G : leave from the search

# delete a specific command in history
history -d 361
history -c # delete entire history

19. Running Commands Without Leaving a Trace

# download google image
wget https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_272x92dp.png

# add white space not to add this command to the history in Ubuntu as default
 wget https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_272x92dp.png
history

# in CentOS, as default $HISCONTROL is 'ignoredups' not 'ignoreboth'
 wget https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_272x92dp.png
history

HISTCONTROL=ignorespace
 wget https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_272x92dp.png
history

# append HISTCONTROL option to .bashrc file
echo "HISTCONTROL=ignoreboth" >> ~/.bashrc

$HISTCONTROL
- echo $HISTCONTROL
- HISTCONTROL=ignorespace
- ignorespace :
- ignoredups
- ignoreboth

20. Recording the Date and Time for Each Line in History

HISTTIMEFORMAT="%d/%m/%y %T "
echo "HISTTIMEFORMAT=\"%d/%m/%y %T \"" >> .bashrc

Section 5: The Linux File System

25. Intro to The Linux Files System

/bin: contains binaries or user executable files which are available to all users.
/sbin: contains applications that only the superuser (hence the initial s) will need.
- for e.g) ifconfig
/boot: contains files required for starting your system.
/home: is where you will find your users' home directories. Under this directory there is another directory for each user, if that particular user has a home directory.
/root: root has its home directory separated from the rest of the users' home directories
/dev: contains device files.
/etc: contains most, if not all system-wide configuration files.
/lib: contains shared library files used by different applications.
/media: is used for external storage will be automatically mounted.
/mnt: is like /media but it's not very often used these days. (cd-rom or floppy disk)
/tmp: contains temporary files, usually saved there by applications that are running.
- Non-priviledged users may also store files here temporarily.
/proc: is a virtual directory. It contains information about your compouter hardware,
- such as information about your CPU, RAM memory or Kernel.
- The files and directories are generated when your computer starts,
- or on the fly, as your system is running and things change.
/sys: contains information about devices, drivers, and some kernel features.
/srv: contains data for servers.
/run: is a temporary file system which runs in RAM
/usr: contains many other subdirectories binaries files, shared libraries and so on.
- On some distributions like Cent OS many commands are saved in /usr/bin and /usr/sbin instead of /bin and /sbin
/var: typically contains variable-length files such as logs which are files that register events that happen on the system.

# file system disk space usage
df -h

# first partition of hard drive
ls -l /dev/sda1

# cpu information and ram information
cat /proc/cpuinfo
cat /proc/meminfo

# see logs
cat /var/log/auth.log

27. Absolute vs. Relative Paths. Walking through the File System (pwd, cd, tree)

sudo tail /var/log/boot.log
ls /etc/cron.daily

# tree command
sudo apt install tree
tree /etc       # => Ex: tree .
tree -d /etc    # => prints only directories
tree -df /etc   # => prints absolute paths

29. The LS Command In Depth (ls)

ls /etc/ /var/ .
ls -ld /etc # only directories
ls -lh /etc # human readable
ls -lSh /etc # order by file size
ls -lX /etc # order by extension
ls --hide=*.conf /etc # hide .conf file
ls -lR /etc/  # recursively display under the path

# size indicates differently
ls -lh /
# drwxr-xr-x 128 root root  12K Feb  3 08:19 etc
du -sh /etc
# 11M	/etc

# default ls alias
type ls
# ls is aliased to `ls --color=auto'

31. Understanding File Timestamps: atime, mtime, ctime (stat, touch, date)

atime: (ls -lu) The access timestamp is the last time the file was read
mtime: (ls -l, ls -lt) The modified timestamp is the last time the contents of the file was modified
ctime: (ls -lc) The changed timestamp ctime is the last time when some metadata related to the file was changed

stat /etc/passwd
#   File: /etc/passwd
#   Size: 2780      	Blocks: 8          IO Block: 4096   regular file
# Device: 805h/2053d	Inode: 1574499     Links: 1
# Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
# Access: 2022-02-04 08:15:34.812933718 +1100
# Modify: 2021-07-26 16:12:32.396190867 +1000
# Change: 2021-07-26 16:12:32.396190867 +1000
#  Birth: -

ls -l --full-time /etc/
# change timestamp
touch ~/linux.txt
# stat ~/linux.txt
touch -a ~/linux.txt
# stat ~/linux.txt
touch -m ~/linux.txt
# stat ~/linux.txt
touch -m -t 202202040823.00 ~/linux.txt # specific modified timestamp
touch -d "2020-02-04 08:30:33" ~/linux.txt  # access timestamp

touch ubuntu.txt
touch linux.txt -r ubuntu.txt

# date
date
# Fri 04 Feb 2022 08:32:02 AEDT
date -u
# Thu 03 Feb 2022 21:32:05 UTC

32. Sorting Files by Timestamp

ls -lt  # sorted by and show modification time
ls -lu  # sorted by and show the access time
ls -lu -r # reverse sorted
ls -ltur
ls -ltu --reverse

33. File Types in Linux (ls -F, file)

# files with no extensions
ls /etc/passwd
ls /etc/passwd
ls -l /etc/group
ls /var/log/syslog
ls -l /bin

# file details
file ~/Desktop/linux.jpeg
mv ~/Desktop/linux.jpeg ~/Desktop/linux.png
file ~/Desktop/linux.png
# the results are the same

### file types
ls -l
# 1. block devices
ls -l /dev/sda1
# brw-rw---- 1 root disk 8, 1 Feb  7 17:23 /dev/sda1
# 2. char type
ls -l /dev/
# crw-------  1 root    root     10, 249 Feb  7 17:23 zfs

# 3. socket type
ls -l /run/
# srw-rw-rw-  1 root              root     0 Feb  7 17:23 snapd.socket
# 4. pipe type
# prw-------  1 root              root     0 Feb  7 17:23 initctl

# -F option : display indicator
ls -lF
ls -lF /run/

# symbolic link
ls -F /usr/bin/ls

# all file information
file /run/*

35. Viewing Files - Part 1 (cat)

cat /var/log/auth.log
cat -n /etc/passwd
cat /etc/hosts /etc/host.conf > my_host.txt

36. Viewing Files - Part 2 (less)

less /var/log/dmesg
# Press 'h' to see short cuts

man command is using less as default

37. Viewing Files - Part 3 (tail, head, watch)

# display last 10 lins
tail /etc/passwd
tail -n 2 /etc/group
# from line 20 to the end
tail -n +20 /var/log/syslog
# watching - attached mode to see your login
tail -f /var/log/auth.log
sudo su # put wrong/correct passwords to see the log changing

head /etc/passwd
head -n 3 /etc/group
head -7 /etc/group

# it runs 'ls' every 2 seconds
watch ls
# highlight the changed line
watch -n 3 -d ls -l
watch -n 1 -d ifconfig  # if run firefox browser, we can see the changed packets

39. Creating Files and Directories (touch, mkdir)

touch report.txt logs
stat report.txt logs
touch rePort.txt
touch reporT.txt
# case sensitive

mkdir -v dir2   # showing message
mkdir -p first/second/third # it will create all directories
three first

40. Copying Files and Directories (cp)

cp -v /etc/group ./users.txt  # display message
cp -i /etc/group ./users.txt  # ask if duplicated on prompt
cp learning_linux.txt logs users.txt first/ # copy 3 files to the first folder
cp -i learning_linux.txt logs users.txt first/
sudo cp -r /etc/ ~/Desktop/ # copy all files in the etc to Desktop
sudo cp users.txt u.txt # the owner changes to the root user
sudo cp -p users.txt u1.txt # the owner remain same

41. Moving and Renaming Files and Directories (mv)

mv -i dir1/dir2/a.txt dir1/ # ask if duplicated on prompt
mv -n dir1/dir2/a.txt dir1/ # do not overwrite an existing file
mv -u dir1/b.txt dir1/dir2/ # move  only  when  the  SOURCE file is newer than the destination file or when the destination file is missing

# rename
mv dir1/a.txt dir/abc.txt
mv dir1/ dir10

# move and rename at the same time
mv dir10/c.txt dir10/dir2/cc.txt

42. Removing Files and Directories (rm, shred)

rm -i dir10/dir2/b.txt  # interact
rm -v dir10/b.txt # message
rm -r dir10 # recursive
rm -rf Music/ Pictures/ # -f is force
rm -ri file dir/  # remove file/directory recursive with prompting

echo *.txt  # to see what files are matching

# secure removal of a file (verdose with 100 rounds of overwriting)
shred -vu -n 100 passwd

44. Working With Pipes in Linux (|, wc)

STDIN (0) - Standard Input
STDOUT (1) - Standard Output
STDERR (2) - Standard Error

ls -lSh /etc/
ls -lSh /etc/ | head  # display top 10 list
ls -lSh /etc/ | head -n 20 | tail -n 1

cat -n /var/log/auth.log  # with line number
cat -n /var/log/auth.log | grep -a "authentication failure"
cat -n /var/log/auth.log | grep -a "authentication failure" | wc -l # print line count

wc /etc/passwd
wc -l /etc/group  # display only line count
wc -w /usr/share/dict/american-english  # word count
wc -c /usr/share/dict/american-english  # character count

45. Command Redirection (>, >>, 2> &>, cut, tee)

ls -l > ls.txt
ifconfig > ls.txt # overwrite

ls -l >> output.txt
ifconfig >> output.txt  # append

tty
# /dev/pts/0

# open the new terminal
ifconfig > /dev/pts/0 # dispaly to the first terminal

tail -n 3 /etc/shadow # Permission denied
tail -n 3 /etc/shadow 2> error.txt
tail -n 3 /etc/shadow 2>> error.txt # append
tail -n 2 /etc/passwd /etc/shadow > output.txt 2> error.txt # output goes to output.txt and error goes to the error.txt
tail -n 2 /etc/passwd /etc/shadow > output.txt 2>&1 # consider STDERR (2) as STDOUT (1)

ifconfig | grep ether > mac.txt
ifconfig | grep ether | cut -d" " -f10 > mac.txt # cut only mac address, split with " " and take the 10th field
cat /etc/passwd
cut -d":" -f1 /etc/passwd # split with ":" and take the 1st field
cut -d":" -f3 /etc/passwd # split with ":" and take the 3rd field

ifconfig | grep ether
ifconfig | grep ether > mac.txt
ifconfig | grep ether | tee mac.txt # combine those two command above. display and save
who | tee -a m.txt  # append
uname -r  # display kernel version
uname -r | tee -a mac.txt kernel.txt  # append to two files

47. Finding Files and Directories - Part 1 (locate, which)

sudo apt install mlocate
sudo updatedb
ls /var/lib/mlocate

locate passwords
locate eahorse
locate -b eahorse
locate -b *eahorse*
locate -b '\eahorse'  # back slush matches the exact name

touch myfile123x
locate myfile123x  # can't find anything
sudo updatedb
locate myfile123x # found!

rm myfile123x
locate myfile123x # hm.. still there
locate -e myfile123x

locate Rainshadow # nothing found
locate -i Rainshadow

locate -b -r '^shadow\.[0-9]'

which ls
# /usr/bin/ls
which rm
# /usr/bin/rm
which -a find
# /usr/bin/find
# /bin/find
ls -l /bin/find
# -rwxr-xr-x 1 root root 320160 Feb 18  2020 /bin/find
ls -ld /bin
# lrwxrwxrwx 1 root root 7 Jul 26  2021 /bin -> usr/bin

which pwd ifconfig find grep firefox
# /usr/bin/pwd
# /usr/sbin/ifconfig
# /usr/bin/find
# /usr/bin/grep
# /usr/bin/firefox

49. Finding Files and Directories - Part 2 (find)

man find
mkdir projects
touch projects/report.txt
touch projects/todo.txt
tree projects
find . -name todo.txt
find . -iname todo.txt  # case insensitive
find . -name todo  # found nothing
find . -name todo*
find . -name todo.txt -delete

find /etc/ -name passwd
sudo find /etc/ -name passwd -ls
find /etc/ -type d  # list up directories recursively
find /etc/ -type d -maxdepth 2 # depth 2
find /etc/ -type d -maxdepth 2 -perm 755 | wc -l

sudo find /var/ -type f -size 100k -ls  # around 100k
sudo find /var/ -type f -size +10M -ls  # larger than 10m
sudo find /var/ -type f -size -10k -ls  # smaller than 10k
sudo find /var/ -type f -size +5M -size -10M -ls

sudo find /var -type f -mtime 0 -ls # files modified less than 24 hours
sudo find /var -type f -mtime 1 -ls # files modified less than 48 hours
sudo find /var -type f -mtime +1 -ls # files modified at least 2 days ago
sudo find /var -type f -mmin -50 -ls # modificatino time is less than 50 minutes

sudo find /var/ -type f -user gdm -ls # search by the owner

sudo find /etc/ -type f -not -group root -ls # files "not beloned" to the root group

50. Find and Exec

sudo find /etc -type f -mtime 0
sudo find /etc -type f -mtime 0 -exec cat {} \; # display contents of the results found
sudo mkdir /root/backup
sudo find /etc/ -mtime -7 -type f -exec cp {} /root/backup \;   # copy files changed in the last 7 days to the /root/backup

51. Searching for String Patterns in Text Files (grep)

grep user /etc/ssh/ssh_config
grep "command line" /etc/ssh/ssh_config
grep "SSH" /etc/ssh/ssh_config  # no result
grep -i "SSH" /etc/ssh/ssh_config  # case insensitive
grep -i -n "SSH" /etc/ssh/ssh_config  # line number

grep body /etc/passwd   # find partial
grep -w body /etc/passwd  # whole word

grep kernel /var/log/dmesg
grep -v kernel /var/log/dmesg # invert : not including kernel

grep root /var/log/auth.log # it works, but in the lecture the auth.log file is data not ASCII text
file /var/log/auth.log
# /var/log/auth.log: ASCII text
grep -a root /var/log/auth.log  # to search binary file

sudo grep -R 127.0.0.1 /etc  # search recursively in the /etc folder
grep -s -R 127.0.0.1 /etc  # suppress nonexist messages

grep -c error /var/log/syslog # count
grep error /var/log/syslog | wc -l

dmesg # print or control the kernel ring buffer
dmesg | grep error
dmesg | grep -A 3 -B 4 error  # print after 3 lines, before 4 lines together
dmesg | grep -C 3 error  # print before/after 3 lines together

sudo netstat -tupan # list all open ports
sudo netstat -tupan | grep 53
sudo netstat -tupan | grep 80

ls -RF /etc/  # all file recursively
ls -RF /etc/ | grep / # only directories
ls -RF /etc/ | grep -v / | grep -v "^$" # invert directories and invert empty strings
sudo ls -RF /etc/ | grep -v / | grep -v "^$" | wc -l  # count
sudo ls -RF /etc/ | grep -v / | grep -v "^$" | sort -r  # desc sort by file name
sudo ls -RF /etc/ | grep -v / | grep -v "^$" | sort -r | less # display less
sudo ls -RF /etc/ | grep -v / | grep -v "^$" | sort | less # ascending order
sudo ls -RF /etc/ | grep -v / | grep -v "^$" | sort > result.txt # save it into the file

53. Searching for Strings in Binary Files (strings)

which ls
sudo apt install binutils
strings /usr/bin/ls
strings /usr/bin/ls | less

# my partition
df -h
# /dev/sda5 : my main partition
sudo strings -a /dev/sda5

# look into computer memory
ls -l /dev/mem
man mem # for more information
sudo strings /dev/mem | less

54. Comparing Files (cmp, diff, sha256)

man cmp # compare binary file

ifconfig > a
ping 8.8.8.8
ifconfig > b
cmp a b # it displays the first different line

which ls
cp /usr/bin/ls .
cmp /usr/bin/ls ./ls  # they are the same
sha256sum /usr/bin/ls ./ls  # the hashes are the same

echo "a" > ls # change ls in local
cmp /usr/bin/ls ./ls  # different
sha256sum /usr/bin/ls ./ls  # different

man diff # compare only text file
diff a b

sudo apt install ssh  # install ssh
cp /etc/ssh/sshd_config .
vi sshd_config # and modify port to 29
diff /etc/ssh/sshd_config ./sshd_config

man patch
diff -B a b   # ignore blank line
diff -w /etc/ssh/sshd_config ./sshd_config # ignore spaces
diff -i /etc/ssh/sshd_config ./sshd_config # ignore case differences
diff -c /etc/ssh/sshd_config ./sshd_config # display a few more line before/after
diff -y /etc/ssh/sshd_config ./sshd_config # display two files in two panels
diff -y /etc/ssh/sshd_config ./sshd_config | less

55. The Basics of VIM Text Editor

sudo apt install vim
sudo dnf install vim # on CentOS

vi a.txt  # vi is an alias of vim on CentOS
cp /etc/ssh/sshd_config .
vim sshd_config

vim has 3 mods

command (via ESC key)
insert (i,I,a,A,o,O)
last line (:)

vim shortcuts

x : delete r : replace a character u : undo r : redo i, I, a, A, o, O : change to insert mode ":" : change to last mode

ZZ : save and quit

vimtutor  # to practice vim shortcuts - it's read only
:help   # to see manual

56. The VIM Editor In Depth - Part 1

:ifconfig : shell command

/ssh : search n : next search N : previous search

G : jump to the end ?ssh : search backward

: search the word on the current cursor

:%s/no/XXX/g : replace all 'no' to 'XXX' :e! : undo to the last saved file u : undo r : redo

dd : cut the line press number + dd : cut as many lines as we put p : paste

Selection v : from the cursor Shift + v : line Ctrl + v : rectenglar selection y : copy p: paste after the cursor Shift + p : paste before the cursor

:set nu :set nonu

:syntax on :syntax on

customise configure file

vim ~/.vimrc set nu syntax on

57. The VIM Editor In Depth - Part 2

:100 : move to line 100 Shift + g : move to the last line gg : move to the first line

ifconfig > a
who -a >b
vim a b

type n or next in the last line mode: move to the next file

vim -o a b

Ctrl + w : move to the next file

this is useful to copy files

vim -d /etc/ssh/sshd_config sshd_config
vimdiff /etc/ssh/sshd_config sshd_config

vim sshd_config
Ctrl + z
# [1]+  Stopped                 vim sshd_config
# [O]pen Read-Only, (E)dit anyway, (R)ecover, (Q)uit, (A)bort:
R # recover
rm .sshd_config.swp

58. Commands - VIM

##########################
## VIM
##########################

Modes of operation: Command, Insert, and Last Line Modes.
VIM Config File: ~/.vimrc

# Entering the Insert Mode from the Command Mode
i  => insert before the cursor
I  => insert at the beginning of the line
a  => insert after the cursor
A  => insert at the end of the line
o  => insert on the next line

# Entering the Last Line Mode from the Command Mode
:

# Returning to Command Mode from Insert or Last Line Mode
ESC

# Shortcuts in Last Line Mode
w!  => write/save the file
q!  => quit the file without saving
wq! => save/write and quit
e!  => undo to the last saved version of the file
set nu => set line numbers
set nonu  => unset line numbers
syntax on|off
%s/search_string/replace_string/g

# Shortcuts in Command Mode
x   => remove char under the cursor
dd  => cut the current line
5dd => cut 5 lines
ZZ  => save and quit
u   => undo
G   => move to the end of file
$   => move to the end of line
0 or ^  => move to the beginning of file
:n (Ex :10) => move to line n
Shift+v     => select the current line
y           => yank/copy to clipboard
p           => paste after the cursor
P           => paste before the cursor
/string     => search for string forward
?string     => search for string backward
n           => next occurrence
N           => previous occurrence

# Opening more files in stacked windows
vim -o file1 file2

# Opening more files and highlighting the differences
vim -d file1 file2
Ctrl+w => move between files

59. Compressing and Archiving Files and Directories (tar, gzip)

Archive : combine and compress
tar - archive file (can compress with specific options)

useful tar options c : create an archive x : extract the archive t : display the contents of the archive z : use gzip compression f : specify file name (for tar option file name must be after this -f option, -fczv won't work) j : use bz2 zip? (more compressed and slower than gzip but nowadays computer is fast enough)

# gzip
sudo tar -czvf etc.tar.gz /etc/
# bzip2
sudo tar -cjvf etc.tar.bz2 /etc/
# multiple
sudo tar -czvf archive.tar.gz /etc/passwd /etc/group /var/log/dmesg /etc/ssh
# exclude
tar --exclude='*.mkv' --exclude='.config' --exclude='.cache' --exclude='node_modules' -czvf myhome.tar.gz ~

# extract zip file to target destination
tar -xjvf etc.tar.bz2 -C my_back/

ls -lh
# -rw-r--r-- 1 root root 1.1M Mar  4 07:43 etc.tar.bz2
# -rw-r--r-- 1 root root 1.3M Mar  4 07:38 etc.tar.gz
tar -xzvf etc.tar.bz2 -C my_back/ # this won't work. cannot use -z option for bzip2 file
tar -xvf etc.tar.bz2 -C my_back/  # works

tar -tf etc.tar.bz2 | grep sshd_config
tar -tf etc.tar.bz2 | grep apache2

tar -cjvf etc-$(date +%F).tar.bz2 /etc/ # specify file name including date string of today
# etc-2022-03-04.tar.bz2

gzip --help
gunzip --help
bzip2 --help
bunzip2 --help

GNU core util - all core linux commands are here https://github.com/coreutils/coreutils

60. Hard Links and the Inode Structure

touch a.txt
la -li # inode
# 6160408 -rw-rw-r-- 2 kimn kimn     0 Mar  5 19:32 a.txt
stat a.txt
ln a.txt b.txt
# 6160408 -rw-rw-r-- 2 kimn kimn     0 Mar  5 19:32 a.txt
# 6160408 -rw-rw-r-- 2 kimn kimn     0 Mar  5 19:32 b.txt
ifconfig > a.txt
cat b.txt
# they are the same file now with just different names
mkdir my_backup
ln b.txt my_backup/c.txt

stat b.txt  # links - 3
rm a.txt
stat b.txt  # links - 2

find . -inum 6160408

ln /etc/ dir1

find /usr/ -type f -links +1 -ls

61. Working With Symlinks. Symlinks vs. Hard Links

ln -s /etc/passwd ./pswd
ls -l pswd

ps aux > processes.txt
ln processes.txt p.txt # hard link
ln -s processes.txt symlink_p.txt # sym link

mv p.txt ~/Desktop  # no problem
mv symlink_p.txt ~/Desktop  # the symlink_p.txt is unlinked

Section 7: User Account Management

66. Understanding passwd and shadow files

/etc/passwd : all users
/etc/shadow : secret information

less /etc/passwd
# nm-openvpn:x:118:124:NetworkManager OpenVPN,,,:/var/lib/openvpn/chroot:/usr/sbin/nologin
# kimn:x:1000:1000:kimn,,,:/home/kimn:/bin/bash

passwd
- Column 1 - kimn : user name
- Column 2 - x : if password has been assigned and stored in the shadow file, 'x', otherwise, blank
- Column 3 - 1000 : assigned id number
- Column 4 - 1000 : assigned id number
- Column 5 - kimn,,, : comment
- Column 6 - /home/kimn : home directory
- Column 7 - kimn : user id
- Column 8 - /bin/bash : default shell
- ... there's every explanation in man passwd

sudo /etc/shadow
# kimn:$6$NFoLRKax9SEqzB7P$4EVvUtkHlGqEQUh2KhElIrgyApQS70RNsrr1cbzp97w5EXOw2DG4..Ti4NnC2qvhjJsrMktXs2a2iAJcaAWEb/:18834:0:99999:7:::

man shadow

shadow - encrypted format
- Column 1 - kimn : user name
- Column 2 - encrypted password
- Column 3
- ... there's every explanation in man shadow
password structure $typ$salt$hash
- 1 : MD5
- 2a : Blowfish
- 2y : Eksblowfish
- 5 : SHA-256
- 6 : SHA-512

sudo useradd user1
sudo passwd user1 # 1234
sudo useradd user2
sudo passwd user2 # 1234
tail /etc/shadow
# user1:$6$jm9D3xXpj8BM.B2C$ULfORyth7ZbTRaox/v2Ivor/56oqhhjrYcBTVr5ELxpna8pwoxgZBLi.rI4t7spzi1QDDvW.ON1z5LmdSHgr.0:19059:0:99999:7:::
# user2:$6$AFm8dWJdnX7HBOtr$n.0unMz95.KpIpsn8DB1pK7zZtLvNqpbVJkYYcYV0JMYm.3ByDMa1hIQKtTZ0uvBzlDr4YT/.NUvJ8UM6L.tH1:19059:0:99999:7:::

they are using unique salt

67. Understanding Linux Groups (groups, id)

The primary group: the id is stored in /etc/passwd and the group name in /etc/group
Secondary groups : stored in /etc/group

tail -n 3 /etc/group

touch abs.txt
ls -l abs.txt

grep kimn /etc/passwd
# kimn:x:1000:1000:kimn,,,:/home/kimn:/bin/bash
# first 1000 - user id, second 1000 - group id
grep 1000 /etc/group
# kimn:x:1000:
less /etc/group
# /kimn   to search
# adm:x:4:syslog,kimn
# syslog, kimn users are under adm group
groups
# kimn adm cdrom sudo dip plugdev lpadmin lxd sambashare
groups root
groups kimn
# kimn : kimn adm cdrom sudo dip plugdev lpadmin lxd sambashare
id
# uid=1000(kimn) gid=1000(kimn) groups=1000(kimn),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),120(lpadmin),131(lxd),132(sambashare)

68. Creating User Accounts (useradd)

sudo useradd u1
tail -n 5 /etc/passwd
less /etc/default/useradd # to see some explanation of useradd

groups u1
less /etc/login.defs # document
sudo passwd u1
sudo useradd -m -d /home/james -c "C++ Developer" -s /bin/bash -G sudo,adm,mail james
man useradd
sudo useradd -e 2022-03-13 u2
sudo chage -l james  # change user password expiry information
man chage
less /etc/passwd
# pulse:x:123:128:PulseAudio daemon,,,:/var/run/pulse:/usr/sbin/nologin
# gnome-initial-setup:x:124:65534::/run/gnome-initial-setup/:/bin/false
# not much difference between `nologin` and `false`
man adduser
# useradd - native binary
# adduser - more user friendly comands based on useradd?

69. Changing and Removing User Accounts (usermod, userdel)

cat /etc/passwd
cat /etc/shadow
cat /etc/group
cat /etc/gshadow
cat /etc/login.defs

sudo usermod -c "Golang developer" james
grep james /etc/passwd
# james:x:1004:1004:Golang developer:/home/james:/bin/bash
sudo usermod -g daemon james
groups james
# james : daemon adm mail sudo
sudo groupadd developers
sudo groupadd managers
sudo usermod -G developers,managers james
groups james
# james : daemon developers managers
sudo usermod -aG sudo james
groups james

sudo userdel u1
sudo userdel -r james
man deluser

70. Creating Admin Users

# Create a user
sudo useradd -m -s /bin/bash toor
sudo passwd toor
su toor
id

cat /etc/shadow
# cat: /etc/shadow: Permission denied
sudo usermod -aG sudo toor  # I should login my acc (kimn) which is already sudo, to give the permission
# -a : append, without -a all group will be replaced

71. Group Management (groupadd, groupdel, groupmod)

sudo groupadd engineering
tail -3 /etc/group
sudo useradd u1
sudo useradd -G engineering u2
groups u1
groups u2
sudo usermod -aG engineering u1 # -a : append, without -a all group will be replace
sudo groupmod -n engineers engineering  # change group name to 'engineers'
groups u1
sudo groupdel engineers
# however, the primary group 'u1' cannot be deleted until the user is deleted

72. User Account Monitoring (whoami, who am i, who, id, w, uptime, last)

RUID : user who initially logs in (never change)
EUID : (Effective uid) current user in the shell

whoami  # EUID
# root
id -un
# root
who   # RUID
# kimn     :0           2022-03-18 08:08 (:0)

Files related to login

sudo cat /var/run/utmp
cat /var/log/wtmp

# to connect my ssh from CentOS with
sudo apt install openssh-server
sudo systemctl status ssh

# Have to change the network to 'Bridged Adapter', not 'NAT'
ifconfig # check my IP
# 192.168.8.141

# On CentOS
ssh [email protected]
ssh [email protected]

# On Ubuntu
who
# kimn     :0           2022-03-18 08:08 (:0)
# kimn     pts/2        2022-03-18 08:30 (192.168.8.142)
# toor     pts/3        2022-03-18 08:32 (192.168.8.142)
who -a
w
uptime
ls -l /var/log/wtmp # login information
last
last root
last kimn
last toor
last kimn | head -n 5

Section 9: Linux File Permissions

75. Understanding File Permissions

Each file or directory has an owner and a group. By default, the owner is the user who creates the file and the group is the primary group of that user
chown, chgrp
For each file the permissions are assigned to three different categories of users
- the file owner
- the group owner
- others

who -a > users.txt
ls -l users.txt
# -rw-rw-r-- 1 kimn kimn 320 Mar 19 08:14 users.txt
chmod u-r users.txt
ls -l users.txt
# --w-rw-r-- 1 kimn kimn 320 Mar 19 08:14 users.txt
cat users.txt
# cat: users.txt: Permission denied
id > users.txt
chmod u-w users.txt
ls -l users.txt

sudo su
ifconfig > interfaces.txt
chmod 000 interfaces.txt
ls -l interfaces.txt
# ---------- 1 root root 881 Mar 19 08:17 interfaces.txt

# However, root user can still read/write the file.
# the permission is only for non-priviliged users

76. Octal (Numeric) Notation of File Permissions

stat /etc/passwd

0755 = 755 (rwx r-x r-x)
0644 = 644 (rw- r-- r--)

77. Changing File Permissions (chmod)

chmod [who][operation][permissions] filename

who
- u: the user that owns the file.
- g: the group that the fille belongs to.
- o: the other users.
- a: all
OPERATION
- -: remove the spicified permissions.
- +: add the specified permission.
- =: change the current permissions to the specified permissions.
permissions: r, w, x

man chmod
who > user.txt
cat user.txt
ls -l user.txt
# -rw-rw-r-- 1 kimn kimn 44 Mar 22 07:36 user.txt
chmod u-w user.txt
# -r--rw-r-- 1 kimn kimn 44 Mar 22 07:36 user.txt
chmod u+rwx user.txt
# -rwxrw-r-- 1 kimn kimn 44 Mar 22 07:36 user.txt
chmod u-x,g+w,o-rwx user.txt
# -rw-rw---- 1 kimn kimn 44 Mar 22 07:36 user.txt
chmod ug-r,u+x,o-rwx user.txt
# --wx-w---- 1 kimn kimn 44 Mar 22 07:36 user.txt
chmod a+r,a-wx user.txt
# -r--r--r-- 1 kimn kimn 44 Mar 22 07:36 user.txt
chmod ug=rw,o= user.txt
# -rw-rw---- 1 kimn kimn 44 Mar 22 07:36 user.txt
chmod 644 user.txt
# -rw-r--r-- 1 kimn kimn 44 Mar 22 07:36 user.txt
chmod 400 user.txt
# -r-------- 1 kimn kimn 44 Mar 22 07:36 user.txt

mkdir -p dir1/dir2
touch dir1/a dir1/dir2/b dir1/dir2/c
tree dir1
# dir1
# ├── a
# └── dir2
#     ├── b
#     └── c
ls -lR dir1/
# dir1/:
# total 4
# -rw-rw-r-- 1 kimn kimn    0 Mar 23 09:33 a
# drwxrwxr-x 2 kimn kimn 4096 Mar 23 09:33 dir2

# dir1/dir2:
# total 0
# -rw-rw-r-- 1 kimn kimn 0 Mar 23 09:33 b
# -rw-rw-r-- 1 kimn kimn 0 Mar 23 09:33 c
chmod -R 750 dir1/
# dir1/:
# total 4
# -rwxr-x--- 1 kimn kimn    0 Mar 23 09:33 a
# drwxr-x--- 2 kimn kimn 4096 Mar 23 09:33 dir2

# dir1/dir2:
# total 0
# -rwxr-x--- 1 kimn kimn 0 Mar 23 09:33 b
# -rwxr-x--- 1 kimn kimn 0 Mar 23 09:33 c

ifconfig > i.txt
ls -l i.txt user.txt
# -rw-rw-r-- 1 kimn kimn 886 Mar 23 09:36 i.txt
# -r-------- 1 kimn kimn  44 Mar 22 07:36 user.txt
chmod --reference=i.txt user.txt
# -rw-rw-r-- 1 kimn kimn 886 Mar 23 09:36 i.txt
# -rw-rw-r-- 1 kimn kimn  44 Mar 22 07:36 user.txt

78. The Effect of Permissions on Directories

mkdir -p linux/ubuntu
who -a > linux/user.txt
tree linux

chmod 400 linux/
ls -ld linux/
# dr-------- 3 kimn kimn 4096 Mar 24 07:48 linux/
ls -l linux
# ls: cannot access 'linux/user.txt': Permission denied
# ls: cannot access 'linux/ubuntu': Permission denied
# total 0
# d????????? ? ? ? ?            ? ubuntu
# -????????? ? ? ? ?            ? user.txt
cd linux/
# bash: cd: linux/: Permission denied
rm -rf linux/user.txt
# rm: cannot remove 'linux/user.txt': Permission denied

Why ls cannot access?

becuase ls alias which is ls --color=auto tries to get file type
\ls linux using the original ls command (with \) works

chmod 600 linux/
ls -ld linux/
# drw------- 3 kimn kimn 4096 Mar 24 07:48 linux/
ls -l linux
cd linux/
rm -rf linux/user.txt
# it will get all the same errors as the 400

I would still cannot access Because without execution permission, write permission is not effective

chmod 700 linux/
ls -ld linux
# drwx------ 3 kimn kimn  4096 Mar 24 08:14 linux

# now all command will work if I'm the owner
cd linux
ls > a.txt
cat a.txt

chmod 000 linux/a.txt
ls -l linux/a.txt
# ---------- 1 kimn kimn 22 Mar 24 08:02 a.txt
rm linux/a.txt
# rm: remove write-protected regular file 'linux/a.txt'? y
# still can delete.

Directory permission is more important than the file permission

79. Combining Find and Chmod Commands Together

# chmod -R 644 ~
find ~ -type f  # find all files under home directory
find ~ -type f -exec chmod 640 {} \;
find ~ -type f -exec chmod 750 {} \;  # change all files permission
find ~ -type d -exec chmod 750 {} \;  # change all directories permission

80. Changing File Ownership (chown, chgrp)

lscpu
lscpu > cpu.txt
ls -l cpu.txt
tail /etc/passwd
chown toor cpu.txt
# chown: changing ownership of 'cpu.txt': Operation not permitted
sudo chown toor cpu.txt
ls -l cpu.txt
# -rw-rw-r-- 1 toor kimn 1899 Mar 26 08:27 cpu.txt
sudo chown toor cpu.txt linux/

tail /etc/passwd
# u1:x:1002:1002::/home/u1:/bin/sh
sudo chown 1002 cpu.txt
# -rw-rw-r-- 1 u1 kimn 1899 Mar 26 08:27 cpu.txt
# but if we have the user id "1002", this will be that user. To avoid this
sudo chown +1002 cpu.txt  # + means user id, not user name

sudo chown kimn:daemon cpu.txt
# -rw-rw-r-- 1 kimn daemon 1899 Mar 26 08:27 cpu.txt

# some linux distributors use this syntax
# sudo chown kimn.daemon cpu.txt

sudo chgrp sudo cpu.txt
# -rw-rw-r-- 1 kimn sudo 1899 Mar 26 08:27 cpu.txt
sudo chown :sudo cpu.txt  # equivalent

sudo chown -R kimn:kimn ~ # recursive

81. Understanding SUID (Set User ID)

S - sticky bit

which cat
# /usr/bin/cat
ls -l /usr/bin/cat
# -rwxr-xr-x 1 root root 43416 Sep  5  2019 /usr/bin/cat

cat /etc/shadow
# cat: /etc/shadow: Permission denied
ls -l /etc/shadow
# -rw-r----- 1 root shadow 1661 Mar 17 08:06 /etc/shadow
id
stat /usr/bin/cat

sudo chmod 4755 /usr/bin/cat
ls -l /usr/bin/cat
# -rwsr-xr-x 1 root root 43416 Sep  5  2019 /usr/bin/cat
# there is 's' instead of 'x'
sudo chmod u-x /usr/bin/cat
ls -l /usr/bin/cat
# -rwSr-xr-x 1 root root 43416 Sep  5  2019 /usr/bin/cat
# 'S' has no execution permission

sudo chmod u+x /usr/bin/cat
stat /usr/bin/cat

# after this command, non-priviliged users can see any file with cat command
sudo chmod u+s /usr/bin/cat
cat /etc/shadow

# back to normal permission
sudo chmod u-s /usr/bin/cat

which passwd
ls -l /usr/bin/passwd
# -rwsr-xr-x 1 root root 68208 Jul 15  2021 /usr/bin/passwd
find /usr/bin -perm 4000
find /usr/bin -perm -4000
find /usr/bin -perm -4000 -ls

82. Understanding SGID (Set Group ID)

sudo su
groupadd programmers
useradd -s /bin/bash pr1
useradd -s /bin/bash pr2
usermod -aG programmers pr1
usermod -aG programmers pr2

mkdir /programming
ls
# cpu.txt  _initial-setup  linux  README.md  _summary
chown pr1:programmers /programming/
ls -ld /programming
# drwxr-xr-x 2 pr1 programmers 4096 Mar 29 08:41 /programming
chmod 770 /programming
# drwxrwx--- 2 pr1 programmers 4096 Mar 29 08:41 /programming/

su pr1
cd /programming
touch source1.cpp
ls -l
# total 0
# -rw-rw-r-- 1 pr1 pr1 0 Mar 29 08:43 source1.cpp
exit
chmod 2770 /programming
# or chmod g+s /programming
ls -ld /programming
# drwxrws--- 2 pr1 programmers 4096 Mar 29 08:43 /programming
stat /programming/
#   File: /programming/
#   Size: 4096      	Blocks: 8          IO Block: 4096   directory
# Device: 805h/2053d	Inode: 5505031     Links: 2
# Access: (2770/drwxrws---)  Uid: ( 1004/     pr1)   Gid: ( 1008/programmers)
# Access: 2022-03-29 08:43:29.833994937 +1100
# Modify: 2022-03-29 08:43:16.594063584 +1100
# Change: 2022-03-29 08:44:09.769792177 +1100
#  Birth: -

su pr2
touch source2.cpp
mkdir golang python
ls -l
# total 8
# drwxrwsr-x 2 pr2 programmers 4096 Mar 29 08:45 golang
# drwxrwsr-x 2 pr2 programmers 4096 Mar 29 08:45 python
# -rw-rw-r-- 1 pr1 pr1            0 Mar 29 08:43 source1.cpp
# -rw-rw-r-- 1 pr2 programmers    0 Mar 29 08:45 source2.cpp

SGUI is useful when we use shared directories

83. Understanding the Sticky Bit

The Sticky Bit is applied to directories
A user may only delete files that he owns or for which he has explicit write permission granted, even when he has write access to the directory.
The sticky bit allows you to create a directory that everyone can use as a shared file storage. The files are protected because, no one can delete anyone else's files.
Absolute mode: chmod 1XXX directory
Relative mode: chmod o+t directory

sudo su
mkdir /temp
chmod 777 /temp/
ls -ld /temp
# drwxrwxrwx 2 root root 4096 Mar 30 07:14 /temp

su kimn
cd /temp
touch file1.txt file2.txt
ls -l
# total 0
# -rw-rw-r-- 1 kimn kimn 0 Mar 30 07:15 file1.txt
# -rw-rw-r-- 1 kimn kimn 0 Mar 30 07:15 file2.txt
chmod 600 file*
# -rw------- 1 kimn kimn 0 Mar 30 07:15 file1.txt
# -rw------- 1 kimn kimn 0 Mar 30 07:15 file2.txt
ls -ld .
# drwxrwxrwx 2 root root 4096 Mar 30 07:15 .

su toor
cd /temp
rm -rf file1.txt
# removed

sudo su
chmod 1777 /temp
# chmod o+t /temp
ls -ld /temp
# drwxrwxrwt 2 root root 4096 Mar 30 07:17 /temp

su toor
cd /temp
rm -rf file2.txt
# rm: cannot remove 'file2.txt': Operation not permitted

# these directories are set to sticky bit
ls -ld /var/tmp/
# drwxrwxrwt 10 root root 4096 Mar 30 07:09 /var/tmp/
ls -ld /tmp/
# drwxrwxrwt 20 root root 4096 Mar 30 07:20 /tmp/

84. Umask

mkdir mydir
touch myfile
ls -l
# drwxrwxr-x 2 kimn kimn  4096 Mar 31 07:52 mydir
# -rw-rw-r-- 1 kimn kimn     0 Mar 31 07:52 myfile

Default permission

0777 for directories
0666 for files

umask
# 0002

subtract with umask

0777 - 0002 = 0775
0666 - 0002 = 0664

umask 0022
mkdir mydir1
touch myfile1
ls -l
# drwxrwxr-x 2 kimn kimn  4096 Mar 31 07:52 mydir
# drwxr-xr-x 2 kimn kimn  4096 Mar 31 07:57 mydir1
# -rw-rw-r-- 1 kimn kimn     0 Mar 31 07:52 myfile
# -rw-r--r-- 1 kimn kimn     0 Mar 31 07:57 myfile1

85. Understanding Files Attributes (lsattr, chattr)

lsattr
man chattr
# 'aAcCdDeFijPsStTu'

who > user.txt
sudo chattr +a user.txt
lsattr user.txt
# -----a--------e----- ./user.txt
# Now only 'append' is permitted. not even edit. not even root user.
sudo ls > user.txt
# bash: user.txt: Operation not permitted
ls >> user.txt

stat user.txt
# Access: 2022-04-04 07:50:56.630770265 +1000
sudo chattr +A user.txt  # Access time
lsattr user.txt
-----a-A------e----- user.txt

# to remove user.txt
sudo chattr -a -A user.txt
rm user.txt

# When you want to avoid accidental delete or change
# i : immutable, the file become frozen
ifconfig > i.txt
cat i.txt
sudo chattr +i i.txt
lsattr i.txt
# ----i---------e----- i.txt
sudo rm -rf i.txt
# rm: cannot remove 'i.txt': Operation not permitted
sudo ls > i.txt
sudo ls >> i.txt
sudo chmod 700 i.txt
# Operation not permitted

sudo chattr -R +i dir1/
lsattr -R dir1

sudo chattr -R -i dir1
# now we can modify or delete

Section 11: Linux Process Management

88. Processes and The Linux Security Model

2 types of commands

executable file
shell built-in

type ls
# ls is aliased to `ls --color=auto`
type rm
# rm is /usr/bin/rm
type cp
# cp is /usr/bin/cp
type type
# type is a shell builtin
type cd
# cd is a shell builtin
type umask
# umask is a shell builtin

When executable file runs, process will be created. But when shell builtin runs process will not be created

Process properties:
- PID (Process ID) - a unique positive integer number
- User
- Group
- Priority / Nice
Type of Processes
- Parent
- Child
- Daemon
- Zombie (defunct) : data is not collected. When it will be removed quickly from the memory
- Orphan : child process that the parent process is quit earlier than the child

ps -ef | less
# UID          PID    PPID  C STIME TTY          TIME CMD
# root           1       0  0 07:26 ?        00:00:02 /sbin/init splash
who
# bash is the parent process and who is the child process

Thread : kind of sub process within one process
Task : synonym of process

89. Listing Processes (ps, pstree)

# most common process related commands
ps
pgrep
pstree
top

# -e : full list
# -f : including detailed information
ps -ef
ps -ef | wc -l

# TTY is '?', probably that process is a daemon
man sshd

ps -aux
ps aux
ps aux | less
ps aux --sort=%mem | less
ps aux --sort=-%mem | less
ps -f -u kimn

ps -ef | grep sshd
ps -ef | grep xyz123 # `grep xyz123` command will be selected itself
pgrep sshd
pgrep -l sshd
pgrep -l cups
pgrep -u root sshd
pstree | less
pstree -c | less
man pstree

91. Getting a Dynamic Real-Time View of the Running System (top, htop)

top
# top - 07:24:51 up 1 min,  1 user,  load average: 2.32, 0.82, 0.29
# Tasks: 228 total,   1 running, 227 sleeping,   0 stopped,   0 zombie
# %Cpu(s):  1.7 us,  3.3 sy,  0.0 ni, 92.8 id,  0.3 wa,  0.0 hi,  1.7 si,  0.0 st
# MiB Mem :   7952.0 total,   5381.2 free,   1136.8 used,   1434.0 buff/cache
# MiB Swap:   2048.0 total,   2048.0 free,      0.0 used.   6541.1 avail Mem

man top
# search `DESCRIPTIONS of Fields`

top - 07:24:51 up 1 min, 1 user, load average: 2.32, 0.82, 0.29
Tasks: 228 total, 1 running, 227 sleeping, 0 stopped, 0 zombie
%Cpu(s): 1.7 us, 3.3 sy, 0.0 ni, 92.8 id, 0.3 wa, 0.0 hi, 1.7 si, 0.0 st
- man top search CPU States
MiB Mem : 7952.0 total, 5381.2 free, 1136.8 used, 1434.0 buff/cache
MiB Swap: 2048.0 total, 2048.0 free, 0.0 used. 6541.1 avail Mem

top
h # help
2 # press number 1-?, change cpus when multi core cpus
m # toggle summary
d # set update interval
space # refresh
y # toggle highlight running task
b # 'b' bold/reverse (only if 'x' or 'y')
<, > # move columns
R # Toggle: 'R' Sort
e # 'E'/'e' summary/task memory scale

F # Fields Management, # when quit the top, setting will be gone
W # saving top settings

top -d 1 -n 3 -b > top_processes.txt

sudo apt update && sudo apt install htop
htop

93. Signals and Killing Processes (kill, pkill, killall, pidof)

kill # sending a signal
# default signal is `15` or `SEGTERM`
kill -l
# list of signals
#  1) SIGHUP	 2) SIGINT	 3) SIGQUIT	 4) SIGILL	 5) SIGTRAP
#  ...
# 63) SIGRTMAX-1	64) SIGRTMAX

# open the text editor
pgrep -l gedit
# 4056 gedit
kill -2 4056  # 2) SIGINT

# open the firefox browser
pidof firefox
4332 4309 4300 4241 4185 4092
kill -INT 4332 4309 4300 4241 4185 4092  # 2) SIGINT

# open the firefox browser
kill -SIGINT $(pidof firefox)

# they are all equivalent commands to kill processes

# to check if the service is running
sudo systemctl status ssh
# sudo systemctl start ssh
# sudo apt install ssh

tail -f /var/log/auth.log

# open a new tab and
cat /var/run/sshd.pid
# 684
pgrep -l sshd
# 684 sshd

# restart sshd service
sudo kill -1 684
sudo kill -1 $(cat /var/run/sshd.pid)

15) SIGTERM : "soft kill" there would possibly be some processes remains
9) SIGKILL : "hard kill"

killall

sleep 123&
sleep 311&
pgrep -l sleep
# 4046 sleep
# 4048 sleep
killall -15 sleep
# [1]-  Terminated              sleep 123
# [2]+  Terminated              sleep 311

sleep 311&
sleep 331&
pgrep -l sleep
4079 sleep
4082 sleep
killall slee  # partial name won't work
pkill slee  # partial name works
# [1]-  Terminated              sleep 311
# [2]+  Terminated              sleep 331

Warning: SIGTERM and SIGKILL are extrememly strong commands. Be careful when using.

94. Foreground and Background Processes

sleep 15
sleep 20 &

ifconfig > output.txt 2> errors.txt &
# `2` represents standard error

ping -c 1 google.com # sending one packet
ping -c 1 google.com > /dev/null 2>&1
# https://stackoverflow.com/questions/10508843/what-is-dev-null-21#:~:text=%2Fdev%2Fnull%20is%20a%20special,stream%20gets%20redirected%20as%20well.
# /dev/null is a special filesystem object that discards everything written into it. Redirecting a stream into it means hiding your program's output.
# The 2>&1 part means "redirect the error stream into the output stream", so when you redirect the output stream, error stream gets redirected as well.

95. Job Control (jobs, fg, bg)

ping -c 1 google.com > /dev/null 2>&1 &
# [1] 5769
# [1]+  Done                    ping -c 1 google.com > /dev/null 2>&1

Job ID : [1]
Process ID : 5769

sleep 15&
# [1] 5823
sleep 20&
# [2] 5824
jobs
# [1]-  Running                 sleep 15 &
# [2]+  Running                 sleep 20 &

fg: bring the process to the foreground
bg: take the process to the background

sleep 10&
# [1] 5837
jobs
# [1]+  Running                 sleep 10 &
fg %1
# sleep 10

# suspend the process
# Ctrl + z

sleep 10 # run the process in foreground
Ctrl + z
# [1]+  Stopped                 sleep 10
pgrep -l sleep
# 5889 sleep

# resume it in the background
jobs
# [1]+  Stopped                 sleep 10
bg %1
# [1]+ sleep 10 &
# [1]+  Done                    sleep 10

sleep 432
Ctrl + z
jobs
fg %1
Ctrl + c  # quit the process

# in the foreground
sleep 123
# close the terminal and open it again
pgrep -l sleep
# nothing - the process was quit

# in the background
sleep 123&
# [1] 6265
# close the terminal and open it again
pgrep -l sleep
# nothing - the process was quit

kill -l

When the terminal is closed, it sends 1) SIGHUP. so the process will be quit
It would be problematic when connected through ssh

nohub sleep 123 &
# [1] 3861
# nohup: ignoring input and appending output to 'nohup.out'

# close the terminal and open it again
pgrep sleep
# 3861

nohup ifconfig &
# [2] 3902
# nohup: ignoring input and appending output to 'nohup.out'
# [2]+  Done                    nohup ifconfig
cat nohup.out

Not to terminate the process when closing the terminal, these can be good tools
GNU screen: https://www.gnu.org/software/screen/
tmux : https://github.com/tmux/tmux/wiki

Section 13: Networking on Linux

98. Getting Information about the Network Interfaces (ip, ifconfig)

ifconfig  # from net-tools
ip a  # new one from ip route package

sudo apt install net-tools  # to use ifconfig

# Split the view
ifconfig -a
ip address show
ip addr show  # equivalent
ip a show  # equivalent
ip a  # equivalent

ip -4 address
ip -6 address

enp0s3
- en : ethernet, wl : wireless lan
- p: port number
- s: slot number in PCI(Peripheral Component Interconnect) cards

ifconfig enp0s3
ip addr show dev enp0s3
ip a s dev enp0s3

route -n
# Kernel IP routing table
# Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
# 0.0.0.0         10.0.2.2        0.0.0.0         UG    100    0        0 enp0s3
# 10.0.2.0        0.0.0.0         255.255.255.0   U     100    0        0 enp0s3
# 169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 enp0s3

ip route show
# default via 10.0.2.2 dev enp0s3 proto dhcp metric 100
# 10.0.2.0/24 dev enp0s3 proto kernel scope link src 10.0.2.15 metric 100
# 169.254.0.0/16 dev enp0s3 scope link metric 1000

systemd-resolve --status
# ...
# Current DNS Server: 192.168.8.1
#        DNS Servers: 192.168.8.1
#         DNS Domain: ~.

101. Setting Up Static IP on Ubuntu (netplan)

ls /etc/netplan
# 01-network-manager-all.yaml

set Network to 'Bridge Adapter'
On Ubuntu, actually we can set the manual IP on the setting GUI
But we can use netplan to set it up in terminal

# 1. Stop and disable NetworkManager
sudo su
id
systemctl stop NetworkManager
systemctl disable NetworkManager
systemctl status NetworkManager

systemctl is-enabled NetworkManager
# disabled

# 2. modify netplan
vim /etc/netplan/01-netconfig.yaml
cat /etc/netplan/01-netconfig.yaml
ipconfig
# enp0s3: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
#         inet 192.168.0.20  netmask 255.255.255.0  broadcast 192.168.0.255
#         inet6 fe80::a00:27ff:fe3b:7b03  prefixlen 64  scopeid 0x20<link>

route -n
# Kernel IP routing table
# Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
# 0.0.0.0         192.168.0.1     0.0.0.0         UG    20100  0        0 enp0s3
# 192.168.0.0     0.0.0.0         255.255.255.0   U     100    0        0 enp0s3

103. Testing and Troubleshooting Network Connectivity

ping ubuntu.com
# PING ubuntu.com (185.125.190.20) 56(84) bytes of data.
# 64 bytes from website-content-cache-1.canonical.com (185.125.190.20): icmp_seq=1 ttl=45 time=289 ms
ping ubuntu.com -n
ping -c 4 ubuntu.com  # 4 times

64 bytes : packet size
icmp_seq : number of packet. ICMP (Internet Control Message Protocol)
ttl : time to live, usually number of sources between source and destination
time : response time
- less than 30ms: fast
- 30 to 50ms: average
- 50 to 100ms: somehow slow
- higher than 100ms: slow response

ping -i 0.4 -c 5 google.com  # interval - default is 1
ping -i 0.1 -c 5 google.com  # general user is not allowed to set too fast
# ping: cannot flood; minimal interval allowed for user is 200ms
sudo ping -i 0.1 -c 5 google.com  # root can do

sudo ping -q -c 5 google.com  # summary

ping -t 1 -c 3 -n ubuntu.com  # first router
ping -t 2 -c 3 -n ubuntu.com  # second router

man ping

Simple troubleshooting
1. check my gateway
  1. route -n
  2. ping to the default gateway
    - ping 10.0.2.2
  3. if it doesn't ping, my lan has a problem
2. check other website
  1. ping 8.8.8.8 - google.com
  2. ping 1.1.1.1
3. check dns server working
  1. ping google.com

104. Using SSH

The SSH protocol is used for
- Secure Remote Management of Servers, Routers, other Networking Devices
- Network File Copy: rsync, scp, sftp, winscp
- Tunneling, SSH Port Forwarding
sshd is the SSH server (daemon) and ssh or putty is the client
Installation
- Ubuntu: sudo apt update & sudo apt install openssh-server openssh-client
- CentOS: sudo dnf install openssh-server openssh-clients
Checking its status: sudo systelctl status ssh
Stop/Restart/Start: sudo systelctl [start|restart|stop] ssh
Enable, Disable auto botting: sudo systelctl [enable|disable] ssh
Server config file: /etc/ssh/sshd_config
Client config file: /etc/ssh/ssh_config

sudo apt update & sudo apt install openssh-server openssh-client

systemctl status ssh
ps -ef | grep sshd

systemctl is-enabled sshd
enabled # it means it will start when boot time

105. Troubleshooting SSH

systemctl status ssh
systemctl restart ssh
systemctl stop ssh
systemctl start ssh

sudo apt install telnet
telnet 10.135.30.xxx 22 # need to connect vpn

sudo apt install nmap
nmap -p 22 10.135.30.xxx

# if it is down
nmap -p 22 10.135.30.xxx -Pn

sudo iptables -vnL
sudo ufw status verbose # Uncomplicated Firewall
# Status: inactive
sudo ufw enable
sudo ufw app list
# Available applications:
#   CUPS
#   OpenSSH
sudo ufw allow ssh
# Rule added
# Rule added (v6)
ssh -v [email protected] # verbose : more logs
ssh -vv [email protected] # more verbose : even more logs

sudo tail -f /var/log/secure  # in CentOS

106. Securing the OpenSSH Server (sshd)

Server config file: /etc/ssh/sshd_config
Client config file: /etc/ssh/ssh_config

# /etc/ssh/sshd_config
# Port 22
Port 2278
PermitRootLogin no
AllowUsers student toor u1
ClientAliveInterval 300
ClientAliveCountMax 0

# All options can be found in the sshd_config man page
man sshd_config

# accpeting only two IP address to 2278 port which is sshd
iptables -A INPUT -p tcp --dport 2278 -s 2.2.4.5 -j ACCEPT
iptables -A INPUT -p tcp --dport 2278 -s 2.45.6.100 -j ACCEPT
iptables -A INPUT -p tcp --dport 2278 -j DROP

108. Copying Files Over the Network (scp)

# Ubuntu - [email protected] / skah4
# CentOS - [email protected] / sk12

# from CentOS
ssh [email protected]
# Ctrl + c
ip add show > ip.txt
cat ip.txt
scp -P 22 ip.txt [email protected]:~
scp -P 22 ip.txt [email protected]:~/ip_centos.txt

mkdir mydir1
my ip.txt mydir1/
ls mydir1/
scp -r -P 22 mydir1/ [email protected]:~ # copy folder
scp -rp -P 22 mydir1/ [email protected]:~  # -p : preserve modification/access time
# scp command will overwrite if the same file exists

scp user1@IP1:/path_to_source_file user2@IP2:/path_to_destination_dir

109. Synchronizing Files and Directories using rsync

man rsync

sudo rsync -av /etc/ ~/etc-backup/
# print all copying files
sudo rsync -av /etc/ ~/etc-backup/
# sending incremental file list
# sent 98,681 bytes  received 376 bytes  198,114.00 bytes/sec
# total size is 5,602,100  speedup is 56.55

at the second execution, it won't copy anything. It will check the modification time and pick to copy

sudo rsync -aq /etc/ ~/etc-backup/  # quiet: it won't print

# creating or modifying file and rsync
# It will pick up only those changed files
sudo rsync -av /etc/ ~/etc-backup/
# but not deleted file yet.
# to include deleted files (= mirroring)
sudo rsync -av --delete /etc/ ~/etc-backup/

# rsync treats the last back slash(/)
# the targets are contents inside /etc/
sudo rsync -av --delete /etc/ ~/etc-backup/
# the target is /etc/
sudo rsync -av --delete /etc ~/etc-backup/

# exclude specific folders/files
mkdir -p my_project/dir1
mkdir -p my_project/dir2
touch my_project/a.png
touch my_project/a.txt
touch my_project/dir1/a.pdf
touch my_project/dir1/a.png

touch my_project/movie1.mkv
tree my_project/

vim exclude_files.txt
cat exclude_files.txt
# movie1.mkv
# dir1/
# *.png

rsync -av --exclude-from='./exclude_files.txt' my_project/ backup/
# actually absolte path is better
# rsync -av --exclude-from='/home/kimn/exclude_files.txt' my_project/ backup/

rsync -av --exclude='*.png' my_project/ backup/
rm -rf backup/

rsync -av --exclude='*.png' --exclude='a.pdf' my_project/ backup/

110. Using rsync Over the Network

sudo rsync -av -e ssh /etc/ [email protected]:~/etc-centos/

# as the same as local, if nothing changes, it won't copy
sudo rsync -av -e ssh /etc/ [email protected]:~/etc-centos/

sudo groupadd devops
sudo touch /etc/test-file
sudo rsync -av -e ssh /etc/ [email protected]:~/etc-centos/

# mirroring
sudo rm /etc/test-file
sudo rsync -av -e ssh --delete /etc/ [email protected]:~/etc-centos/

# can use any ssh options like this
sudo rsync -av -e 'ssh -p 22' --delete /etc/ [email protected]:~/etc-centos/

# from remote to local
sudo rsync -av -e ssh [email protected]:~/my_project/ .

112. Using wget (downloading)

wget [download_url]
wget -P kali/ [download_url] # specific download path
wget --limit-rate=100k [download_url] # download speed limited to 100k/s
wget -c -P kali/ [download_url] # ro resume the the previous download (if it stops during downloading)

# Download from the list in a text file
vim images.txt
# https://www.kali.org/images/kali-everywhere-baremetal.svg
# https://www.kali.org/images/kali-everywhere-vm.svg
wget -i images.txt

# background
wget -b -c -P kali/ [download_url]
tail -f wget-log
pkill wget

# even when the terminal is closed
nohub wget -b -c -P kali/ [download_url]

# when offline (no internet connection)
wget --mirror --convert-links --adjust-extension --page-requisites --no-parent http://example.org

113. Checking for Listening Ports (netstat, ss, lsof, telnet, nmap)

sudo netstat -tupan
# -t: tcp, -u: udp
# -p: PID, -a: All, -n: numeric
sudo netstat -tupan | grep :22

sudo apt install nginx
sudo systemctl status nginx
sudo netstat -tupan | grep :80

# socket statistics: it's a newer command and faster than netstat
ss
ss -tupan

# list open files
lsof
lsof -u kimn # open files by user kimn
sudo lsof -u root
sudo lsof -u ^root  # not by root
sudo lsof -c nginx  # by nginx service
sudo lsof -iTCP -sTCP:LISTEN  # LISTEN state of TCP
sudo lsof -iTCP -sTCP:LISTEN -nP # display port number
sudo lsof -iTCP:22 -sTCP:LISTEN -nP # specific port

# turn on CentOS (e.g IP is 192.168.0.113)
# check if the specific port is open or not
telnet 192.168.0.113 22
telnet 192.168.0.113 25
telnet google.com 443
# it's open
# Trying 142.250.70.174...
# Connected to google.com.
# Escape character is '^]'.
telnet google.com 22
# it's closed
# Trying 142.250.70.174...

# nmap : professional port scanner
sudo apt install nmap
sudo nmap 192.168.0.113

nmap -p 80 linux.com
nmap -p 80 linux.com -sV # service/version detection
nmap -p 81 linux.com -sV

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