@author winstark

Linux's login logs

• utmp and wtmp is binary file contains logs for logins and logouts in Linux system.
• /var/run/utmp This file contains information about the users who are currently logged onto the system.
• who command users utmp file to display logged in users
• /var/log/wtmp This file is like history for utmp file, i.e. it maintains the logs of all logged in and logged out users (in the past).
• The last command users wtmp file to display listing of last logged in users.

Crontab Linux

• cron is a utility that allows tasks to automatically run in the background of the system at regular intervals by use of the cron deamon.
• crond deamon is the background service that enables cron funtionality.
• Crontab(Cron table) is a file which contains the schedule of cron entries to be run and at what times they are to be run.
• Cron checks these files and directories:
  • /etc/crontab: system crontab.Originally it was usually used to run daily, weekly, monthly jobs.
  • /etc/cron.d: directory that contains systems cronjobs stored for different users.
  • /var/spool/cron: directory that contains user corntables created by the crontab command.
    more

/proc filesystem

The /proc filesystem contains a illusionary filesystem. It does not exist on a disk. Instead, the kernerl creates it in memory. It is used to provide information about the system(originally about processes, hence the name).

systemd and systemctl

A quick practical using systemd service.

• systemd is a Linux service manager. It provides the ability to manage and control services.
• systemctl is a port of systemd that provides the ability to interact with systemd.

Unit & Unit files

• In systemd, a unit refers to any resource that the system knows how to operate on and manage. This is the primary object that systemd tools know how to deal with. These resources are defined using configurations files called unit files.

• Where systemd Unit files found ?
  • /lib/systemd/system : where software install unit files on the system
  • /run/systemd/system
  • /etc/systemd/system

Service Management

sudo systemctl start|stop|restart|reload|enable|disable  application.service  //  control a service 
sudo systemctl status|is-active|is-enabled|is-failed  application.service // check status a service  

System State Overview

systemctl list-units  // list all fo the active units  

systemctl list-units --all // --all to see loaded and inactive units  
systemctl list-units --type=service  // filter  

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