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Linux command line for you and me

Documentation

Release 0.1

Kushal Das

Mar 06, 2023

Contents:

1 Shell commands1

1.1 Terminal emulators

1

1.2 date command

1

1.3 cal command

2

1.4 whoami command

3

1.5 id command

3

1.6 pwd command

3

1.7 cd command

3

1.8 . directory and .. directory

4

1.9 ls command

4

1.10 mkdir command

4

1.11 rm command

5

1.12 Copying a file using cp command

5

1.13 Renaming or moving a file

5

1.14 tree command

5

1.15 wc command

6

1.16 less command

6

1.17 echo command

6

1.18 Redirecting the command output

7

1.19 Using > to redirect output to a file

7

1.20 Moving around in the command line

8

1.21 man pages

8

1.22 Counting files in a directory

9

1.23 Editing longer commands

9

2 File system11

2.1 FHS

11

3 Useful commands13

3.1 Creating soft link to a file

13

3.2 Creating hard links

13

3.3 Extracting a tar file

14

3.4 Creating a tar file

14

3.5 Vim editor

15

3.6 :q to exit vim

15

3.7 Open a new file or edit an existing file

15

3.8 Different modes of vim

15 i

3.9 :w to save a file. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.10 :q! to quit without saving

17

3.11 Becoming root user

17

3.12 Using sudo command

17

3.13 !! trick

17

3.14 Setting up hostname

18

3.15 Environment variables

18

3.16 Setting up environment variable values

18

3.17 locate command

18

3.18 Finding date/time in different timezones

19

3.19 Bash history

19

3.20 Sort files by size

19

4 Curl! For all your web needs

21

4.1 Viewing a file

21

4.2 Downloading the file

21

4.3 Download with the same name

22

4.4 Making a POST request using curl

22

4.5 Following redirection

23

4.6 Example: To view github"s pull request patch

24

4.7 Viewing more details about the transfer

25

4.8 Making multiple requests at once

25

4.9 Inspecting HTTP headers

25

4.10 Adding new HTTP headers

27

4.11 Curl book

28

5 Users and Groups29

5.1 Finding the owner of file

29

5.2 /etc/passwd file

29

5.3 Details about groups

31

5.4 wheel group

31

5.5 Becoming superuser

31

5.6 Adding a new user

31

5.7 Changing user passwords

32

5.8 Modifying existing user details

32

5.9 Deleting a user

32

5.10 Adding a new group

32

5.11 Adding new group to an user

32

6 File permissions33

6.1 chmod command

33

6.2 PATH variable

34

6.3 ~/.bash_profile file

34

6.4 .bashrc file

35

6.5 /etc/profile file

35

6.6 which command

35

6.7 Usewhichcommand to see how $PATH variable works. . . . . . . . . . . . . . . . . . . . . . . . 36

6.8 she-bang or sha-bang in executable files

36

7 Processes in Linux37

7.1 How to view all running processes?

37

7.2 How to find a particular process?

37

7.3 How to kill/stop a particular process?

38

7.4 Finding out list of open files

38

7.5 Signals

38 ii

7.6 top command. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.7 Load average

39

7.8 htop tool

39

7.9 More about Linux processes

40

7.10 /proc directory

40

7.11 /proc/cpuinfo

41

7.12 /proc/cmdline

41

7.13 /proc/meminfo

41

7.14 /proc/uptime

42

7.15 /proc/sys/ & sysctl command

42

7.16 Enabling IP forward with sysctl

42

8 Linux Services45

8.1 What is a service?

45

8.2 What is a daemon?

45

8.3 What is the init system?

45

8.4 Units in systemd

46

8.5 .service units in systemd

46

8.6 How to find all the systemd units in the system?

46

8.7 Working with a particular service

47

8.8 Enabling or disabling a service

48

8.9 Shutdown or reboot the system using systemctl

48

8.10 journalctl

48

8.11 Finding the logs of a service

48

8.12 To view only the last N entries

49

8.13 Continuous stream of logs

49

8.14 Listing of previous boots

50

8.15 Time-based log viewing

50

8.16 Total size of the journal logs

51

8.17 Writing your own service file

51

9 Securing a service using systemd

53

9.1 Installing verybad service

53

9.2 Vulnerabilities in the application

54

9.3 Directory traversal vulnerability/ LFI

55

9.4 Arbitary file write vulnerability

55

9.5 Remote code execution (RCE) vulnerability

56

9.6 Remove access to system"s tmp directory

57

9.7 Protecting home dirctories

57

9.8 Fixing directory paths

57

9.9 DynamicUser

58

9.10 Allowed Executables

60

9.11 What is next?

61

10 Package management63

10.1 dnf command

63

10.2 Searching for a package

63

10.3 Finding more information about a package

64

10.4 Installing a package

64

10.5 To list the available updates

65

10.6 To list all security updates

65

10.7 Update the packages via dnf

66

10.8 Find out the services & applications need restart after update in Fedora/CentOS/RHEL

66

10.9 Automatic updates in dnf systems

68 iii

10.10 apt command. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

10.11 apt update

69

10.12 Installing a package via apt

69

10.13 apt-cache search

69

10.14 Listing upgrades

70

10.15 Upgrading packages

70

10.16 Figuring out which services/processes need restart after package upgrade in Debian

70

10.17 Listing available security updates in Debian systems

71

10.18 Unattended upgrades in Debian systems

71

11 SELinux73

11.1 SELinux Modes

74

11.2 getenforce

74

11.3 setenforce

74

11.4 Labels/Contexts

75

11.5 Checking contexts of files/directories or processes

75

11.6 SELinux booleans

76

12 File system mounting77

12.1 Connecting USB drives to your system

78

12.2 Mounting a device

78

12.3 Unmounting

78

12.4 Encrypting drives with LUKS (for only Linux)

78

12.5 Encrypting drives for any OS using Veracrypt

79

13 Networking commands81

13.1 Finding the IP address

81

13.2 Finding ARP table

81

13.3 ping command

82

13.4 Short note about DNS

82

13.5 /etc/hosts

83

13.6 /etc/resolv.conf

83

13.7 systemd-resolved controlled name resolution

83

13.8 resolvectl command

84

13.9 host command

86

13.10 dig command

86

13.11 ss command

89

13.12 traceroute command

90

13.13 tracepath command

90

13.14 Remote login to a computer using ssh tool

91

13.15 ssh key generation

91

13.16 ssh-copy-id

92

13.17 Stop and disable the sshd service

92

13.18 Disable password based login for ssh

92

13.19 How to find active (open) network connections from your computer?

93

13.20 To know about ports

93

14 Linux Firewall95

14.1 Installation

95

14.2 Tables, chains and rules

95

14.3 filter table

96

14.4 nat table

96

14.5 iptables command

96

14.6 View the existing rules

97

14.7 Appending rules to INPUT chain

97 iv

14.8 Flushing all rules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

14.9 Example of a series of rules

98

14.10 Delete a rule based on rule number

99

14.11 Delete a rule directly

99

14.12 Saving the rules

99
quotesdbs_dbs14.pdfusesText_20

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