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Linux-Cheatsheet

User Info

1. who : It is used to get information about currently logged in user on the system. It Displays

  • Login name of the user
  • User terminal
  • Date & Time of login
  • Remote host name of the user 
    $ who
    mohit :0 2020-10-21 01:21 (:0)

2. whoami: It display the system’s username

   $ whoami
   mohit

3. users : Displays usernames of all users currently logged on the system.

   $ users
   sj

4. last or lastb : Displays a list of last logged in users on the system. You can pass user names to display their login and hostname details.

    
    $ last
    mohit       :0           :0               Fri Oct 16 01:27    gone - no logout
    reboot   system boot  5.4.0-29-generic    Thu Oct 15 01:27   still running
    mohit       :0           :0               Wed Oct 14 11:46 - crash (29+13:40)
    reboot   system boot  5.4.0-29-generic    Sat Oct 10 11:45   still running
    mohit       :0           :0               Fri Oct 09 21:04 - crash (75+14:41)
    reboot   system boot  5.4.0-29-generic Thu May 14 21:03   still running
    
    wtmp begins Thu May 14 21:03:56 2020

File Commands

1. pwd The pwd command is used to print the current Working Directory

   $ pwd
   /home/mohit/Desktop/

2. mkdir The mkdir command is used to make new directory.

   $ mkdir python
   $ ls
   python

3. rmdir: The rmdir command is used to remove directories.

   rmdir FolderName

4. rm: The rm command is used to remove objects such as files, directories, symbolic links etc from the file system.

  1. remove file:

    rm filenamename

  2. remove file forcefully:

    rm -f filename

  3. remove directory

    rm -r dirname

5. touch: The touch command is is used to create, change and modify timestamps of a file without any content.

  1. Create new file: creating new file in the dir. 
    touch filename
  2. Create multiple files: creating multiple new files in dir. 
    touch file1name file2name file3name
  3. Change access time: Used to change the access time of a file. 
    touch -a filename
  4. Change modification time: Used to change the modified time. 
    touch -m filename
  5. Use timestamp of other file: Used to get timestamp of another file. 
    touch -r file2 file1

6. cat: The cat command is used to list the contents of file

      cat file1name

File Permissions

Linux is Mutliuser Operating system so security is reqiured to prevent people from accesing each other's confidential files.

Linux Divides this authorization in 2 levels-

1. Ownership:

  • User: Owner of the file who created it.
  • Group: Group of users with the same access permissions to the file or directory.
  • Other: Applies to all other users on the system.

2. Permissions:

  • Read: Give you the authority to open and read a file and lists its content for a directory.
  • Write: Give you the authority to modify the contents of a file and add, remove and rename files stored in the directory.
  • Execute: Give you the authority to run the program in Unix/Linux.

The permissions are indicated with below characters,

r = read permission

w = write permission

x = execute permission

\- = no permission

Networking

1. Display network information: ifconfig command is used to display all network information(ip address, ports etc)

ifconfig -a

2. Test connection to a remote machine: Send an echo request to test connection of a remote machine.

    ping <ip-address> or hostname
    Example:
    ping 10.0.10.11

3. Show IP Address: Display ip address of a currennt machine

    hostname -I
    (OR)
    ip addr show

4. Active ports: Shows active or listening ports

     netstat -pnltu

5. Find information about domain: whois command is used to find out information about a domain, such as the owner of the domain, the owner’s contact information, and the nameservers used by domain.

    whois [domain]
    Example:
    whois google.com

6. Ping: to check connectivity status (ping google.com)

System and Hardware information

1. Print all information: Used to print system information.

$ uname -a

2. Print kernel name:

$ uname -s

3. Print kernel release:

$ uname -r

4. Print Architecture:

$ uname -m

5. Print Operating System:

$ uname -o

Shutdown

1. shutdown -h now: immediately power off & shutdown the system.

2. shutdown -h +5 : shutdown the system after 5 minutes.

3. shutdown -r now: reboot the system.

4. shutdown -Fr now: orce file system check during reboot.

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