Linux Classroom Series – 14/Jul/2020

Change your password

• Command for changing the password

passwd [user]

• Lets login as user john and change his passwd
• Login as sudo user and change password of other user
• root user can change password of any user

Change mode

• Syntax

chmod [reference][operator][mode] file ..

• Reference
  • u => owner
  • g => group
  • o => others
  • a => all
• Operators
  • • => Add the modes
  • • => Remove the specific modes
  • = => The modes specified are made exact
• Examples

mkdir -p modtest/{a,b,c}
ls -al modtest/
chmod g=rw modtest/a
ls -al modtest/

umask

• UMask is known as User Mask or it is also called as User file creation Mask.
• This is a base permission or a default permission when a new file or folder is created in linux

touch testfile
mkdir testdir

• The default permissions for file are 644 and for directory are 775
• Lets add following entry into a file /etc/profile and add the entry umask 0027

777
027
750
rwxr-x---

• Now observe directory permissions for the newly created directory

Processes

• A process is basically a program that is executing.

• Executing the program happens in a sequential

• To run anything on OS, a process has to be created

• For any process to run

  • A program is loaded into memory and it becomes process. Process has 4 sections
    • Stack:
      • Stack contains temporary data such as what method/function paramters, return addresses & local
    • Heap: This is dynamically allocated memory to process
    • Data: Global & static variables
    • Text: Program counter, Process registers

• Process LifeCycle

  • Start
  • Ready
  • Running
  • Waiting
  • Terminated or Exited

• Lets understand commands which we would examine w.r.t process

  • ps: Report the snapshot of current process
  • top: Display task
  • jobs: List active jobs
  • bg: Place the job in the background
  • fg: Place the job in the foreground
  • kill: Send a signal to a process
  • Killall: Kill a process by name
  • shutdown: Shutdown or reboot a system

• Daemon Programs:

  • When a linux starts, kernel has to run some activites, so it initiates those activities as process and launches a program called as init
  • init runs a series of shell scripts called as init scripts, which start all the system services.
  • Many of these services are implemented as daemon programs, Programs that jus run in the background and generally they don’t have UI.