Linux Commands

 

Linux Commands

1. OS-Release

-cat /etc/os-release

-cat /etc/redhat-release

show os //kernal information

-uname 

show kernal middleware

It is intermediator between hardware and software.

-uname  -r

what is process architect.

-uname -p

To show all information

-uname -a

2. Date-CAL

-date

-cal

3. Booting in Linux (Run-Levels)

Shutdown/Close pc

-init 0 

Single user mode

-init 1

Multiple user mode

-init 2

Multiple user mode with network plus full support

Not use

-init 4

Graphical mode

init 5

Reboot the system

-init 6

4. Target command in Linux (systemctl)

With the help of target we can manage system specific as well as user specific task.

Target command is system Control (systemctl). Basically it is utility, which build to replace 'init' command.

What systemctl can do ? 

We can find its all commands with the help of single command.

write systemctl enter twice TAB button.

//it will list all its commands.

Show current system mode

- systemctl  get-default

Change  current system mode to multi-user mode.

-systemctl isolate multi-user.target

To set permanent mode

systemctl set-default multiuser.target

Note: systemctl only work on redhat + CentOS operating system.

5. systemctl mask

When you don't want to start again the stopped service.

exp: You might have two conflicting services in your system and you don't want to start both at once than you have to restrict one service with masking.

Masking service

-systemctl mask <servicename>

Show list of masked services

-systemctl list-unit-files --type=service | grep mask

To unmask service

systemctl unmask <servicename>

6. Disk and partitioning in OS

Show list disk allocation information

-fdisk -l

Show list RAM storage with swap

-free

-free -m

-free -g

7. File types in OS

Normal(-), Directory(d),

Link a) soft link(l) b) hard link(No symbolic symbol)

Device file 

a) block file(b)

b) character file(c)

Pip file(p)

Socket file(s)

Show list file with their types

-ls -asl

-ll

-ls -l

8. ls commands in linux

Show hidden files and diretories

-ls -a

Show present directory

-ls .

Show parent directroy

-ls ..

Show list in recursive order

-ls -r

Show list according to timestamps

-ls -t

Show list with disk size

-ls -s

Show all ls commands

-man ls or ls --help

show i node value on files and directory

-ls -i

What is i node value ?

These are memory addresses. It means every file placed on specific memory address in disk storage.  

Show list of specific directory

-ls -dl

find speacific directory

-ls -d nginx

show name is  file or direction. If it is file than display its type. If it is directory than diosplay all file with their types

-ls -f nginx

9. Softlinks in linux

Soft links are shortcuts for specific file path (program/service path).

exp: In window we create shoert cust and place on desktop.Instead go long inside directories to run that program, we run from desktop.

Soft links basically shoert name for long paths,

How to create softlink?

1) Go to directory where you want to create that link.

2) Make directory -mkdir mysoftlinks

3) -cd mysoftlinks

4) -ln -s /mywork/file1

10. Editors in linux

1) vi 2) vim 3) gedit 4)cat

Create file in writeable mode.

-cat > filename

//Hello world

ctrl+d // save and close

Open and view file

-cat filename

Concatenate to file content

-cat >> filename

Find specific keyword in file

-cat filename | grep adnan

Create directory

mkdir <directoryname>

Create nested directories (directory inside another directory)

mk dir -p a/b/c/

//a,b,c are directories

show heirarchy of directories.

-ls -R 

11. cp commands in linux

How to copy files?

cp command use for copy the files.

There many options we use cp command are mentioned below ?

1) -v // viewable form 

2) -r recursively // everything includes in particular file or directory

3) -f forcely

4) -p assign same permission to file in particular directory where you are pasting that file.

Copy file

-cp -v file1 /root/desktop

Copy directory (nested recursively)

Here we are coping test directory to/root/desktop destination

cp -vr test/ /root/desktop/

Copy every thing in one command

-cp * /root/desktop/

Copy recusrsiverly.forcely with same permissions

-cp -vrfp test/ /root/desktop

12. Delete files and directories commands in linux

Delete file

-rm -rf test/

Delete directory

rmdir test/

Delete all files and directories

-rm -rf *

13. Rename files and directories commands in linux

-mv file1 myfile //rename

mv command is use for renaming and moving files.

14. Find commands in linux (Searching)

Find particular file/directory with name.

-find /root -name adnan

Find file with permission

-find / -perm 0000

Find file with size

-find / -size 200

Display information of file

- wc file1

output

3 6 27 file1

3 are lines in file

6 are character

27 are single characters 

file1 is filename

Find specific keyword in file

-cat output.error | grep adnan

Find and store result in particular file

-find / -name file1 > file2

e.g store the calendar result in file

cal > file1

15. Standard outputs

Find and store result in particular file

-find / -name passwd > myresult

If you want to append the result to existing result file

-find / -name passwd >> myresult

Note outputs will be store/append in files and errors will be displayed.

If you want to store error in file

-find / -nmame passwd 2> errorfile

to append

-find / -nmame passwd 2>> errorfile

Store both result and error in sigle file

-find . -name &> output.error

-find / -name &>> output.error

cat output.error

16. vi editor

View file in command mode

-vi fle1

When file open type i to insert mode to write or edit text in file.

ESC button use to finish writing (Not save).

There many option or command in vi editor.

1) :w save mode

2) :q close file

3) :q! close forcely

4) :w! save forcely

5) :wq! save and forcely quit

Create and open file

vi <filename>

Find / searching in file using vi

:/aws

:/linux

:/adnan

Set line into numeric line

:set nu

Set numeric lines into normal line

:set nonu

In Command Mode other options

 type

Go to top of file

:gg

Go to bottom

sift+g

Delete single char

x

undo changes

u

delete particular string

dw

Delete files string at the same time

5dw

Delete single line

dd

Delete multiple lines

2dd

Copy line

y

Pase line

p

Copy two char

2y

Copy words

yw

Copy two lines

yy

Copy single letter

yl

Copy single three letters

y3l

More on Groups references

https://www.javatpoint.com/linux-groups

Root Password reset reference

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_basic_system_settings/changing-and-resetting-the-root-password-from-the-command-line_configuring-basic-system-settings#changing-or-resetting-the-forgotten-root-password-as-a-non-root-user_changing-and-resetting-the-root-password-from-the-command-line

User-group-other files and directories permissions reference

https://www.javatpoint.com/linux-file-permissions

17. Listening ports on services

Show ip information of machine in redhat

# ifconfig -a

Check which process listening which ports

netstat -tulpn

we can also filter results

# netstat -tulpn | grep :80

useful link

https://www.cyberciti.biz/faq/what-process-has-open-linux-port/

18. firewall

• How to check open ports on RHEL 8.
• How to check services on RHEL 8.
• How to check open ports remotely using nmap.
• How to list open ports and service with firewall-cmd.

List open ports

List firewall allowed ports

firewall-cmd --list-ports

Important useful links

https://linuxconfig.org/redhat-8-check-open-ports

All configuration of firewallD (installation and configuration)

https://oracle-base.com/articles/linux/linux-firewall-firewalld