Category: Linux

The ln utility is used to create hard links and (with the -s
option) soft links, also known as symbolic links or symlinks. These two
kinds of links are very useful in UNIX-based operating systems.

Suppose that file1 already exists. A hard link, called file2, is created with the command:

$ ln file1 file2

Soft (or Symbolic) links are created with the -s option, as in:

$ ln -s file1 file3

Linux distributions can start and stop the graphical desktop in
various ways. The exact method differs from distribution and among
distribution versions. For the newer systemd-based distributions, the
display manager is run as a service, you can stop the GUI desktop with
the systemctl utility and most distributions will also work with the telinit command, as in:

$ sudo systemctl stop gdm (or sudo telinit 3)

and restart it (after logging into the console) with:

$ sudo systemctl start gdm (or sudo telinit 5)

On Ubuntu versions before 18.04 LTS, substitute lightdm for gdm.

 

 

 

1. At the command line prompt, type su and press Enter. You will then be prompted for the root password, so enter it and press Enter.
   You should end up with a different looking
   prompt, often ending with ‘#’. For example:
   $ su
   Password:
   #
   

   
   

2. Now, you need to create a configuration file to enable your user account to use sudo. Typically, this file is created in the /etc/sudoers.d/ directory with the name of the file the same as your username. For example, for this demo, let’s say your username is student. After doing step 1, you would then create the configuration file for student by doing this:
   
   
   # echo “student ALL=(ALL) ALL” > /etc/sudoers.d/student
   

   
   

3. Finally, some Linux distributions will complain if you do not also change permissions on the file by doing:
   # chmod 440 /etc/sudoers.d/student

4. To enable sudo without password
   # echo “student ALL=(ALL) NOPASSWD:ALL” > /etc/sudoers.d/student
    

init is the first program that is executed when Linux system boots up. All other deamons are child process of this process. 

• init program resides at root filesystem (/sbin/init). Most other processes on the system trace their origin ultimately to init. 
• Besides starting the system, init is responsible for keeping the system running and for shutting it down cleanly.
• One of its responsibilities is to act when necessary as a manager for all non-kernel processes

 Drawbacks of init

• Unix SysV init viewed things as a serial process, divided into a
  series of sequential stages. Each stage required completion before the
  next could proceed. Thus, startup did not easily take advantage of the
  parallel processing that could be done on multiple processors or cores. 
• Furthermore, shutdown and reboot was seen as a relatively rare event;
  exactly how long it took was not considered important. This is no longer
  true, especially with mobile devices and embedded Linux systems.
• Its process ID (PID) is always 1.
• Latest Linux distributions use systemd instead of init

systemd was adopted as alternative to the init. 

• Systems with systemd start up faster than those with earlier init
  methods. This is largely because it replaces a serialized set of steps
  with aggressive parallelization techniques, which permits multiple
  services to be initiated simultaneously.
• /sbin/init now just points to /lib/systemd/systemd; i.e. systemd takes over the init process.