linux-commands

Linux Prompts:
    $ -> Normal User
    # -> Root User (Administrator)

Linux Command Standard Syntax:
Command-name {options} {inputs} 

Dot (.) denotes present working directory 
DotDot(..) denotes parent directory 

Options:
    - <Single-Character>
    -- <Single-Word>

    Ex: uname --help 

Files/Directories stating with dot(.) are hidden files and directories in Linux

List out existing files and directories:
$ ls 
$ ls -a 
$ ls -l 
$ ls -lt 
$ ls -ltr


Files:
 -> Create Files 
    $ touch file-name
 -> Remove Files 
    $ rm file-name 
 -> Rename a file 
    $ mv source destination 

    Case 1: If destination doesn't exist it will re-name the file. 

    Case 2: If destination exists it will overwrite the file

 -> Copy a file 
    $ cp source  destination 

Directories:

/ -> Root Directory

Types of files:
    d - Directory   
    - - Regular file 
    l - Link
    b - block devices
    c - character files 
    S - socket files 
    p - Named pipe file 

To check in which directory you are in 
    $ pwd

Change from one directory to another
    $ cd <directory>

    $ cd -> This will take you to home directory 
    $ cd - -> This will take you to previous dir 
    $ cd .. -> Takes you to parent directory 

 -> Create Directory 
    $ mkdir demo 

 -> Remove a directory 
    rm / rmdir (Empty Dir)
    $ rm -r dirname

 -> Copy a directory 
    $ cp -r dir1 dir2

 -> Renaming / Moving a directory
    $ mv source destination

    ** If destination does not exists: Rename the directory
    ** If destination exists:
         -> Destination is a file:
                Invalid operation
         -> Destination is a Directory:
                File/Directory will move inside that directory

Filters:
    -> See the complete content inside a file..
       $ cat passwd
       $ cat -n passwd 
       $ tac passwd 

    -> By line numbers.
        --> Print from starting of the file 
            $ head passwd 
                It print top 10 lines 
            $ head -5 passwd 
                It print top 5 lines 

        --> Print from ending of the file 
            $ tail passwd 
                It prints last 10 lines

            $ tail -3 passwd 
                It prints last 3 lines 

    -> Search a word and print those lines 
        Syntax: grep word file 
        $ grep ec2-user passwd 

    -> Column based filter 
        Syntax: cut -d delimeter -f number file 
        $ cut -d : -f 1 passwd 
        $ cut -d : -f 1,5 passwd
        $ cut -d : -f 1-5 passwd

EDITORS:
    ** vi / vim (so widely used)
    ** gedit
    ** nano  
    

Finding Files:
    -> Finding the files can be done with "find" command.
        Syntax: find <location> <criteria> 
        Ex: find / -name sample.conf
            find /boot -type d 
            find / -name xyz -type d
            find / -name xyz -type f
            find / -name "*.conf"
            
    -> Finding the files can be done also with "locate" command (Only in Redhat family)
       By default that package (software) is not available by default.
       In order to install it just follow the following commands.
        $ sudo yum install mlocate -y 
        $ sudo updatedb 
        
       Now you can find the files using locate command.
        $ locate "*.conf"
        
Utilties:
       -> Downloading the files can be done with "wget" command.
          By default wget command is not installed in your system.
          
          $ sudo yum install wget -y 
          
          Now you can download the file using wget.
          Syntax: wget <URL>
          
          
       -> Browsing over command line
          $ curl <URL>
          
          Also, You can download the files with curl command.
          $ curl URL -o filename
          
       -> Compression Utilities.
           -> Extract a tar file.
              $ tar -xf file.tar.gz / file.tar
              
            tar does not apply compression it is only a packing tool to create a file from multiple files and directories.
            But the compression is applied with 
                gunzip  -> .gz
                bunzip2 -> .bz2
           
           -> TRY THIS AS TASK 
              Create a tar file with .gz and .bz2 extension
              By default bunzip2 is not installed 
              $ sudo yum install bzip2 -y
             
Pipes: (|)
    Pipes will convert STDOUT to STDIN

Restart /Shutdown:
  Restart:
    $ sudo init 6
    $ sudo reboot
    $ sudo shutdown -r now
    
  Shutdown:
    $ sudo init 0
    $ sudo halt
    $ sudo shutdown -h now
    
  -> To verify whether the system is rebooted or not. 
    $ uptime
    
Process Management:
   -> Every command you execute will create a PID (Process ID , which is unique)
    To list out all the process we are going to use "ps" command.
    Every process is going to have a parent process ID.
    

Admin topics:

All admins activities cannot be performed by a normal users. You need to be a root user to perform the tasks.
Root logins are not allowed in companies and we alternatively use sudo to gain access and perform activities.

All the commands we are going to perform we will add sudo to starting of the command.

$ sudo <command>
  The above command will be executed as a root user and also it will be logged in the system.
  
1. User management
   In RedHat Linux, to add a user it is mandatory to have a group.
   
   Groups:
     -> To add a group , Use the following
        $ sudo groupadd demo
        
     -> To check whether the group is added or not , we can use the following command.
        $ cat /etc/group | grep demo
        
        Del Group: groupdel
        Modify Group: groupmod
       
   Users:
     -> TO create a user, use the following.
        $ sudo useradd -g demo raju
        
     -> To check whether the user is added or not 
        $ id raju
        $ cat /etc/passwd | grep raju
        
     -> Add user to other groups. 
        $ sudo usermod -a -G adm raju
        
     -> To set a password to raju user
        $ sudo passwd raju
        
     -> To swtich from one user to another user then use "su"
        $ su -  raju

  Sudoers:
     -> To add raju user to execute commands as root user 
        $ sudo visudo
        
2. Package management
     -> Redhat used RPM (RPM Package manager formerly called as Redhat Package Manager), it is like .exe 
        $ sudo yum list installed
        $ sudo yum list available
        $ sudo yum list (or) $ sudo yum list all
        $ sudo yum install zip -y
        $ sudo yum remove zip -y
        $ sudo yum update zip -y
        
3. Service management.
     -> To manage the services we have "systemctl" in centos7 OS, where as we use to have "service" command earlier on centos 6.
     -> To list all services which are active.
        $ sudo systemctl list-units -t service 
     -> To list all services which includes inactive.
        $ sudo systemctl list-units -t service --all
        
     -> TO start/stop/restart a service.
        $ sudo systemctl start httpd
        $ sudo systemctl stop httpd
        $ sudo systemctl restart httpd
        
     -> To check the status of httpd.
        $ sudo systemctl status httpd
        
     -> To start the service at the time of reboot automatically then 
        $ sudo systemctl enable httpd
        $ sudo systemctl disable httpd

        
4. Network management
    -> Info about IP address.
      $ ip a
      
    -> Server name information.
      $ hostname
      $ hostname -f 
      
    -> Port Info
      Every IP is going to have ports from 0-65535 
      $ sudo netstat -lntp 
      
5. Disk management (Just intro)
    -> To check the disks on the server
      $ sudo fdisk -l
      
    -> To check the mounted file systems
      $ mount
      $ df -h
     In order to access the disk we need to mount it.
     
6. File permissions
  -> Ownerships
    Owner:
       $ sudo chown username filename
    Group Owner:
       $ sudo chgrp username filename
       
       $ sudo chown username:groupname file
       $ sudo chown username:groupname directory -R
    
  -> Permissions
     $ chmod u+x file 
     $ chmod o-r file