Run-Level is just a fancy world of “mode” or “condition”. When a computer runs a Linux-based operating system, it usually has seven (7) ways how it behaves. But, it is indeed cooler to say “there are 7 different run-levels in Linux.”

All the run-levels are:

  1. Run-level 0: Shut down mode: all operation is halted and the computer will be totally shut or turned off.
  2. Run-level 1: Single user mode:
  3. Run-level 2: Multi-user mode without networking
  4. Run-level 3: Multi-user mode with networking: normal mode.
  5. Run-level 4: Custom user mode. Special mode for a particular condition as defined by the user.
  6. Run-level 5: Run-level 3 plus Graphical User Interface/”Windows”.
  7. Run-level 6: Reboot mode: the computer just restarted and re-initialized everything in preparation to go to different run level.

So, there are special folders /directories for each run-level, usually located in /etc/rc.d. And you can expect to find rc0.d, rc1.d, rc2.d, rc3.d, rc4.d, rc5.d and rc6.d – each for its corresponding run-level.

And, at the start of every run-level, the operating system will execute all scripts/program inside that special folder alphabetically.

chkconfig is usually the program that used to manage which program to run for each run-level