OS concept#

System call v.s Function call#

User mode v.s. Kernel mode#

• Hardware always in Kernel Mode

Horizontal#

• Services for Application Programs: User Identification, Process Management, Memory Management, File/Directory Management, Inter-process Communication, Signal, I/O Management (e.g., Terminal, Network, etc), …
• OS Kernel: CPU Scheduling, Virtual Memory, File System, Protection, Security, Synchronization, I/O Control, …

User identification#

User 是一種權限，不一定是真實使用者

在 /etc/passwd（local host or NIS DB）裡面記錄

cat /etc/passwd

可以取得資訊，每個 row 是一個 User，format 是：

{User-name}:{Passwd}:{User-Id}:{Group-id}:{Comment}:{Home-dir}:{Shell}

root:*:0:0:System Administrator:/var/root:/bin/sh
daemon:*:1:1:System Services:/var/root:/usr/bin/false
_uucp:*:4:4:Unix to Unix Copy Protocol:/var/spool/uucp:/usr/sbin/uucico
_taskgated:*:13:13:Task Gate Daemon:/var/empty:/usr/bin/false
_networkd:*:24:24:Network Services:/var/networkd:/usr/bin/false
_installassistant:*:25:25:Install Assistant:/var/empty:/usr/bin/false

• crypt()
  • 是一個 one-way func，用於登入帳號
  • 第一個參數 {passwd} 是真正的密碼
  • 第二個參數 {salt bit} 是為了製造同樣 common word 隨機性，System 會選擇 time-base 的參數

  • crypt("apple", "am") → "amADBMNoVAZpc"
    crypt("water", "pm") → "pmRarHxhhU34U"

    bash
  • 系統使用你的帳號跟你輸入的密碼放入比對
  • man 3 crypt 可以查看

Process Management#

• Program is a executable file
• Process is an executing instance of a program
  • ps is a snapshot of the current process
  • top is display processes

CPU Scheduling#

• Time Sharing: the rate is too fast that user won’t realize that they are sharing

• Event can be DISK
• (IMPROTANT) A process want to inline of ready queue, it has to put every data into the memory.

Context Switch

• Time Slice maybe just a nano second
• We have to store the status of last process before kicking.
• Then we have to restore the status of the next process.
• Memory is too slow, so we have L1, L2 cahce on CPU.
• If one process want to ask DISK, CPU will kick out this process into event, but not time slice.

• Exit have many kind.

UNIX Process#

From “init” prcess use fork to build a process tree.

Pipe

• A way to send the output of one command to the input of another Filter
• A program that takes input and transforms it in some way
• wc- gives a count of words/lines/chars
• grep- searches for lines with a given string
• more
• sort- sorts lines alphabetically or numerically

  ls -la | more

  bash

  cat file | wc

  bash

  man ksh | grep “history”

  bash

  ls -l | grep “bowman” | wc

  bash

  who | sort > current_users

  bash

UNIX philosophy:

• Write programs that do one thing and do it well
• Write programs that work together
• Write programs that handle text streams, because that is the universal interface

Memory Management#

File/Directory Management#

File Structure#

• root /: all unix like system’s root.
• lib: library, the include path
• bin: binary, in the system genuinly
• var: dynamic file

chmod <mode> <file(s)>
chmod 700 file.txt

chmod g+rw file.txt

chmod,chown,touch