Some often used bash scripts techniques. Also an example script I wrote for my network homework which does some auto download testing.

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### to start a script:
# !/bin/bash

### to assign variable: (notice no space in between)
vname1='str'
vname2=1

### use assigned variable:
echo $vname1

### simple loop
while(( $int<=100))
do
  echo $int
  # do something here
  let "int++"
done

Suppose we name out script as myscript.sh, to run it, fisrt we givie it permission:

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$ chmod 755 myscript.sh
$ ./myscript.sh

Some some techniques (more will be added):

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### string quotes
NAME="John"
echo "Hi $NAME"  #=> Hi John
echo 'Hi $NAME'  #=> Hi $NAME

### Conditional execution:
git commit; git push 
git commit && git push 
git commit || echo "Commit failed"

Now an example:

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# !/bin/bash
echo start creating files

### create 25 4kb files
int=1
while(( $int<=25 ))
do
  echo $int
  dd if=/dev/urandom of=test/t$int bs=1000 count=4
  let "int++"
done

### create 25 60kb files
int=1
while(( $int<=25 ))
do
  echo $int
  dd if=/dev/urandom of=test/tt$int bs=1000 count=60
  let "int++"
done

echo done creating files

At the end, a very good reference link: https://devhints.io/bash

Git commands

Often used commands:

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### 创建
git clone <link> <dir>
git init


### 互动
git status
git add <file>
git commit -m <message>
git commit -m -a <message>


### 分支
git branch # 列出分支
git branch <branch name> # 创建
git branch -d <brach name> # 删除

git checkout <branch name> # 转移
git checkout -b <branch name> # 创建并转移

git merge <branch name> # 合并

### 记录
git log

### 同步
git pull # 同步且merge至当前branch
git fetch # 仅同步

Virture machine

1. Software emulation of an abstract machine

– Give programs illusion they own the machine
– Make it look like hardware has features you want

2. Two types of “Virtual Machine”s

– Process VM: supports the execution of a single program; this functionality typically provided by OS
– System VM: supports the execution of an entire OS and its applications (e.g., VMWare Fusion, Virtual box,Parallels Desktop, Xen)

3. Process VM

• Programming simplicity
​ – Each process thinks it has all memory/CPU time
​ – Each process thinks it owns all devices
​ – Different devices appear to have same high levelinterface
​ – Device interfaces more powerful than raw hardware
​ » Bitmapped display  windowing system
​ » Ethernet card  reliable, ordered, networking (TCP/IP)
• Fault Isolation
​ – Processes unable to directly impact other processes
​ – Bugs cannot crash whole machine
• Protection and Portability
​ – Java interface safe and stable across many platforms

What is an operating system?

• Special layer of software that provides application
software access to hardware resources
​ – Convenient abstraction of complex hardware devices
​ – Protected access to shared resources
​ – Security and authentication
​ – Communication amongst logical entities

What is an Operating System,… Really?

• Most Likely:
​ – Memory Management
​ – I/O Management
​ – CPU Scheduling
​ – Communications? (Does Email belong in OS?)
​ – Multitasking/multiprogramming?
• What about?
​ – File System?
​ – Multimedia Support?
​ – User Interface?
​ – Internet Browser?

473. Matchsticks to Square

22. Generate Parentheses

216. Combination Sum III

40. Combination Sum II

39. Combination Sum

90. Subsets II

78. Subsets