linux-under-the-hood.txt

1.1
  - user interacts with shell/gui -> create Processes
    processes using libraries via system-calls and signals interact with kernal.    kernal using pluggable drivers can access hardware directly.
    networking happen in kernel level.
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1.2
  - cpu access Disk, RAM and Network.
  - to perform task effectively, the kernal manage the hardware via modules.
     - memory management -> to manage ram.
     - Scheduling -> to slice the time of cpu on different proc
     - drivers -> to access disk, networking and other hardware device.
  - application can communicate with kernal through system-calls.
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1.3 
  - lsmod -> command to print all loaded driver
  - modinfo driver-name -> to get more info about a driver
  - linux store driver of device at /dev/** , 
    when listing ls -l
     - first character in permissions [b|c]rwxr-xr-x:
        kernel distinguish in 2 types of devices
        - b ->  block devices is used to store block of data.
        - c -> used to stream data, like a serial port, which is a char device.
     - two number major, minor: internal representation in the kernel for this file in /dev/
     - /dev/** interfaces for user, kernel deals with the major and minor number
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1.4
  - glibc gnu version of libc, c standard library, all linux distribuation use it.
    a generic interface that allow application to access kernel using system-calls.
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1.5
  - shell; used by user and application. application can feed it with variables[can be stored
     in config file]. linux store config in /usr/lib/systemd/system which contain where to get 
     the required env vars.  which can be used to give application the needed information.
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1.6
  - Linux is a file oriented os
  - everything is happening as a file: device access, I/O handling, inter process communication..
  - Every process keep a table of file descriptors that shwos files that are currently in use.
  - Common fd are:
    - 0 -> STDIN
    - 1 -> STDOUT
    - 2 -> STDERR
  - to list all pid-> ls /proc. to list fd used by specific proc -> ls /proc/pid/fd 

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SysAdmin
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vagrant is a virtual machine that use virtual box to execute and run vm.
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 we can interupt the booting process start in grub mode press e 
  and we now can edit how the kernel start [rbgb (start all svc by default)
     confirm(useful in troubleshooting problem), single(can start other runlevel-> 
       init 3[usermode and networking support])]
  
  ps auxw | egrep '\[' <- KERNEL PROCESSES.

  kernel modules are located in /lib/modules