插入排序

概述

  • 将新元素插入到有序序列中。

    步骤

  • 假设序列中第一个元素为有序序列，将其余元素都当做待插入元素处理
  • 每次当前待插入元素与有序序列最后一个元素开始依次进行比较，如果小于被比较元素，则进行交换，交换后继续进行比较直至有序序列第一个元素
  • 将列表分为有序区，待插入的无序区

代码实现

Python 插入排序
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def sort_cr(v_list):
    for i in range(len(v_list)):
        for j in range(i,0,-1):
            if v_list[j] < v_list[j-1]:
                v_list[j],v_list[j-1] = v_list[j-1],v_list[j]
    return v_list
C 插入排序
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void svoid insertion_sort(int array[],int first,int last)
{
    int i,j;
    int temp;
    for(i=first+1;i<last;i++)
    {
        temp=array[i];
        j=i-1;
        //与已排序的数逐一比较，大于temp时，该数移后
        while((j>=0)&&(array[j]>temp))
        {
            array[j+1]=array[j];
            j--;
        }
        //存在大于temp的数
        if(j!=i-1)
            array[j+1]=temp;
    }
}

性能分析

时间复杂度

时间复杂度为O(n^2)

算法稳定性
  • 比较是从有序序列的末尾开始，也就是想要插入的元素和已经有序的最大者开始比起，如果比它大则直接插入在其后面，否则一直往前找直到找到它该插入的位置。
  • 如果碰见一个和插入元素相等的，那么插入元素把想插入的元素放在相等元素的后面。
  • 相等元素的前后顺序没有改变，从原无序序列出去的顺序就是排好序后的顺序，排序是稳定的。

选择排序

概述

  • 走访列表长度次，每次选出最小（或最大）的元素，重新排顺序

步骤

  • 第一次选出最小（或最大）的元素，放在列表的起始位置，即与开头元素交换
  • 接下来每次选出最小（或最大）的元素，与索引等于当前的循环次数的元素进行交换
  • 将列表分为已选择出来的有序区，等待选择的无序区

代码实现

Python 选择排序
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def sort_xz_1(v_list):
    for i in range(len(v_list)-1):
        min = i
        for j in range(i+1,len(v_list)):
            if v_list[min] > v_list[j]:
                min = j
        v_list[min],v_list[i] = v_list[i],v_list[min]
    return v_list
C选择排序
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void swap(int*a,int*b)
{
    int temp;
    temp=*a;
    *a=*b;
    *b=temp;
}
void select_sort(int A[],int n)
{
    register int i,j,min,m;
    for(i=0;i<n-1;i++)
    {
        min=i;//查找最小值
        for(j=i+1;j<n;j++)
        {
            if(A[min]>A[j])
            {
                min=j;
            }
        }
        if(min!=i)
        {
            swap(&A[min],&A[i]);
            printf("第%d趟排序结果为:\n",i+1);
            for(m=0;m<n;m++)
            {
                if(m>0)
                {
                    printf("");
                }
                printf("%d",A[m]);
            }
            printf("\n");
        }
    }
}

性能分析

时间复杂度

比较时间复杂度为O(n^2)
交换时间复杂度未O(n)

算法稳定性
  • 在一趟选择中，如果一个元素比当前元素小，而该小的元素又出现在一个和当前元素相等的元素后面，那么交换后稳定性就被破坏了。
  • 举个例子，序列5 8 5 2 9，我们知道第一遍选择第1个元素5会和2交换，那么原序列中两个5的相对前后顺序就被破坏了，所以选择排序是一个不稳定的排序算法。

冒泡排序

概述

  • 重复地走访过要排序的元素列，依次比较两个相邻的元素，如果他们的顺序错误就把他们交换过来。走访元素的工作是重复地进行直到没有相邻元素需要交换，也就是说该元素列已经排序完成。
  • 这个算法的名字由来是因为越大的元素会经由交换慢慢“浮”到数列的顶端（升序或降序排列），就如同碳酸饮料中二氧化碳的气泡最终会上浮到顶端一样，故名“冒泡排序”。

步骤

  • 走n-1次，两两相比，当次当前的比较元素的索引最大值为未被比较元素（未冒泡）的索引-1
  • 每次从第一个元素开始进行两两比较。如果当前元素比下一个元素大，就交换他们两个，接下来有下一个元素与自己的下一个元素进行两两比较。
  • 当次最后的元素是本轮比较结果中最大的数，已冒好泡，下次无需理会。
  • 重复对未冒泡好的左区进行冒泡处理
  • 每轮比较结束后 ，已冒泡元素新增一个

代码实现

Python 冒泡排序

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def sort_mp_py(v_list):
    for i in range(len(v_list)-1):
        for j in range(len(v_list)-i-1):
            if v_list[j] > v_list[j+1]:
                v_list[j],v_list[j+1] = v_list[j+1],v_list[j]
    return v_list

C冒泡排序

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void sort_mp_c (elemType arr[], int len) {
    elemType temp;
    int i, j;
    for (i=0; i<len-1; i++) 
        for (j=0; j<len-1-i; j++) { 
            if (arr[j] > arr[j+1]) {  
                temp = arr[j];
                arr[j] = arr[j+1];
                arr[j+1] = temp;
            }
        }
}

性能分析

时间复杂度

平均时间复杂度为O(n^2)

算法稳定性

  • 冒泡排序就是把小的元素往前调或者把大的元素往后调。比较是相邻的两个元素比较，交换也发生在这两个元素之间。
  • 如果两个元素相等，是不会再交换的；如果两个相等的元素没有相邻，那么即使通过前面的两两交换把两个相邻起来，这时候也不会交换，所以相同元素的前后顺序并没有改变，所以冒泡排序是一种稳定排序算法。

Java数据类型与运算符细节点

注释

默认

javadoc默认处理以public或protected修饰的类、接口、方法、成员变量、构造器、内部类之前的文档注释

包注释

包注释不是直接放在java源文件中，必须另外通过一个标准的HTML文件（包描述文件）指定

类型

强类型

所有变量必须先声明后使用，指定类型的变量只能接受类型与之匹配的值

转换

  • 强制类型转换，截取左边保留右边
  • 基本类型包装类都提供了parseXxx(String str)静态方法用于将字符串转换为基本类型 Boolean\Byte\Short\Integer\Long\Character\Float\Double
  • 表达式类型自动提升

最小单元

  • 1byte，2short，2char，4int，4float，8long，8double，boolean 1位即可归属计算机分配内存时最小单元8字节
  • (byte>short)/char>int>long>float>double

进制

  • 2进制0b，8进制0，十六进制0x（1015：af）
  • 最高位符号位
  • 计算机以补码的形式保存所有证书，补码=反码+1，符号位保持不变

无穷大

  • 正无穷大: Double或Float类的POSITIVE_INFINITY,正浮点数数除以0得到，表示正溢出
  • 负无穷大: Double或Float类的NEGATIVE_INFINITY,负浮点数数除以0得到，表示负溢出

运算处理

  • 移位运算时考虑有效位数，低于int类型先自动转换为int类型再移位
  • 左移n位就相当于乘以2的n次方，右移n位就相当于除以2的n次方
  • 移位运算不会改变操作数本身，只是得到了一个新的运算结果
  • ！=如操作数都是引用类型，则两者之间具有父子关系才可以进行比较，指向的不是同一个对象就返回true
  • 短路逻辑运算&&||这两个当第一操作数位相应值时不对第二个操作数进行计算，非短路则都会执行计算
  • 单目运算符、复制运算符、三目运算符 从右向左结合运算，其余的从左向右
  • 分隔符>单目>强制类型转换>乘除余>加减>移位>关系>等价>按位与>按位异或>按位或>条件与>条件或>三目运算符>赋值

异常错误

  • 非数：Double或Float类的NaN,0.0除以0.0或对一个负数开方得到，表示出错
  • ArithmeticException：/by zero 一个整数值除以0，抛出如左异常
  • 整数类型进行求余运算，第二个不能是0 否则将引发零异常。如是浮点数则结果是非数

补充

  • java7中引入 数值中可以使用下划线进行分割
  • 当程序第一次使用某个字符串直接量时，java会使用常量池（constant pool）来缓存

Java流程控制与数组细节点

1、使用if。。。else时，一定要先处理包含范围更小的情况

2、switch控制表达式的数据类型：byte、short、char、int、枚举类型、java.lang.String​

3、do while 循环条件后必须有一个分号，表明循环结束

4、for循环中，continue结束本次循环，循环跌代体一样会得到执行​

5、break结束循环,return 结束整个方法 两者都会结束循环

6、java中的标签只有放在循环语句之前才有作用，放在break所在的循环的外层循环之前才有意义

7、break 标签：结束标签指定的循环 而不是break所在的循环

8、continue加标签如break一样

9、java的数组要求所有的数组元素具有相同的数据类型

10、定义数组时不能指定数组的长度，经过初始化才可以使用，实际开发过程中定义和初始化同时完成

11、不要同时使用静态初始化（分配初始值）和动态初始化（指定数组的长度）

12、​数组索引越界异常（索引值小于0或大于等于长度）：

13、foreach循环自动遍历数组和集合的每个元素，通常用于遍历输出，通常不要对循环变量进行赋值，没有太大意义且极容易引起错误，如果希望改变数组元素的值不要用foreach循环

14、数组元素（堆内存）和数组变量（栈内存）再内存里是分开​存放的

15、方法结束，存放着 方法中定义的局部变量的内存栈都会被自然销毁

16、数组元素（对象）没有任何引用变量引用它时，系统垃圾回收机制会在合适时回收它

17、让机制回收数组所占内存空间：将数组变量赋值为null，切断变量和元素的引用关系​

18、a数组变量赋值给​b数组变量，并没有改变b的数组长度（直到消失），而是改变了b的指引，b原先指向的数组元素失去了引用，成为垃圾，等待回收

19、定义一个Object[]类型的数组可以拓展到多维数组

20、从数组底层的运行机制上看，java​没有多维数组，只是提供了支持多维数组的语法，多维数组本质上都是以为数组。（三维数组每个数组元素是二维数组）

21、char类型转换成int类型数字，直接减去48，他们的ASCII码值恰好相差48

虚拟主机配置多个系统多个域名

在网站根目录htdocs下创建多个子文件夹，一个系统对应一个子文件夹

在网站根目录htdocs下创建”.htaccess”文件。

一个域名对应一段代码​
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       RewriteEngine On

       RewriteCond %{HTTP_HOST} ^(www.)?【此处填域名】$

       RewriteCond %{REQUEST_URI} !^/【此处填htdocs下子文件夹名称】/

       RewriteCond %{REQUEST_FILENAME} !-f

       RewriteCond %{REQUEST_FILENAME} !-d

       RewriteRule ^(.*)$ /【此处填htdocs下子文件夹名称】/$1

       RewriteCond %{HTTP_HOST} ^(www.)?【此处填域名】$

       RewriteRule ^(/)?$ 【此处填包含二级目录下的首页完整路径】 [L]

       ##### 示例(www.fzg5.com绑定htdocs下的fzg5子文件夹）

​       RewriteEngine On

       RewriteCond %{HTTP_HOST} ^(www.)?fzg5.com$

       RewriteCond %{REQUEST_URI} !^/fzg5/

       RewriteCond %{REQUEST_FILENAME} !-f

       RewriteCond %{REQUEST_FILENAME} !-d

       RewriteRule ^(.*)$ /fzg5/$1

       RewriteCond %{HTTP_HOST} ^(www.)?fzg5.com$

       RewriteRule ^(/)?$ fzg5/index.html [L]

Git Manual（已更新） 

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## Git Manual                                                                                

### NAME
       git - the stupid content tracker

### SYNOPSIS
       git [--version] [--help] [-C <path>] [-c <name>=<value>]
           [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
           [-p|--paginate|-P|--no-pager] [--no-replace-objects] [--bare]
           [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
           [--super-prefix=<path>]
           <command> [<args>]

### DESCRIPTION
       Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.

       See gittutorial(7) to get started, then see giteveryday(7) for a useful minimum set of commands. The Git User's Manual[1] has a more in-depth introduction.

       After you mastered the basic concepts, you can come back to this page to learn what commands Git offers. You can learn more about individual Git commands with "git help
       command". gitcli(7) manual page gives you an overview of the command-line command syntax.

       A formatted and hyperlinked copy of the latest Git documentation can be viewed at https://git.github.io/htmldocs/git.html.

### OPTIONS
       --version
           Prints the Git suite version that the git program came from.

       --help
           Prints the synopsis and a list of the most commonly used commands. If the option --all or -a is given then all available commands are printed. If a Git command is named
           this option will bring up the manual page for that command.

           Other options are available to control how the manual page is displayed. See git-help(1) for more information, because git --help ...  is converted internally into git help
           ....

       -C <path>
           Run as if git was started in <path> instead of the current working directory. When multiple -C options are given, each subsequent non-absolute -C <path> is interpreted
           relative to the preceding -C <path>.

           This option affects options that expect path name like --git-dir and --work-tree in that their interpretations of the path names would be made relative to the working
           directory caused by the -C option. For example the following invocations are equivalent:

               git --git-dir=a.git --work-tree=b -C c status
               git --git-dir=c/a.git --work-tree=c/b status

       -c <name>=<value>
           Pass a configuration parameter to the command. The value given will override values from configuration files. The <name> is expected in the same format as listed by git
           config (subkeys separated by dots).

           Note that omitting the = in git -c foo.bar ...  is allowed and sets foo.bar to the boolean true value (just like [foo]bar would in a config file). Including the equals but
           with an empty value (like git -c foo.bar= ...) sets foo.bar to the empty string which git config --type=bool will convert to false.

       --exec-path[=<path>]
           Path to wherever your core Git programs are installed. This can also be controlled by setting the GIT_EXEC_PATH environment variable. If no path is given, git will print
           the current setting and then exit.

       --html-path
           Print the path, without trailing slash, where Git's HTML documentation is installed and exit.

       --man-path
           Print the manpath (see man(1)) for the man pages for this version of Git and exit.

       --info-path
           Print the path where the Info files documenting this version of Git are installed and exit.

       -p, --paginate
           Pipe all output into less (or if set, $PAGER) if standard output is a terminal. This overrides the pager.<cmd> configuration options (see the "Configuration Mechanism"
           section below).

       -P, --no-pager
           Do not pipe Git output into a pager.

       --git-dir=<path>
           Set the path to the repository. This can also be controlled by setting the GIT_DIR environment variable. It can be an absolute path or relative path to current working
           directory.

       --work-tree=<path>
           Set the path to the working tree. It can be an absolute path or a path relative to the current working directory. This can also be controlled by setting the GIT_WORK_TREE
           environment variable and the core.worktree configuration variable (see core.worktree in git-config(1) for a more detailed discussion).

       --namespace=<path>
           Set the Git namespace. See gitnamespaces(7) for more details. Equivalent to setting the GIT_NAMESPACE environment variable.

       --super-prefix=<path>
           Currently for internal use only. Set a prefix which gives a path from above a repository down to its root. One use is to give submodules context about the superproject that
           invoked it.

       --bare
           Treat the repository as a bare repository. If GIT_DIR environment is not set, it is set to the current working directory.

       --no-replace-objects
           Do not use replacement refs to replace Git objects. See git-replace(1) for more information.

       --literal-pathspecs
           Treat pathspecs literally (i.e. no globbing, no pathspec magic). This is equivalent to setting the GIT_LITERAL_PATHSPECS environment variable to 1.

       --glob-pathspecs
           Add "glob" magic to all pathspec. This is equivalent to setting the GIT_GLOB_PATHSPECS environment variable to 1. Disabling globbing on individual pathspecs can be done
           using pathspec magic ":(literal)"

       --noglob-pathspecs
           Add "literal" magic to all pathspec. This is equivalent to setting the GIT_NOGLOB_PATHSPECS environment variable to 1. Enabling globbing on individual pathspecs can be done
           using pathspec magic ":(glob)"

       --icase-pathspecs
           Add "icase" magic to all pathspec. This is equivalent to setting the GIT_ICASE_PATHSPECS environment variable to 1.

       --no-optional-locks
           Do not perform optional operations that require locks. This is equivalent to setting the GIT_OPTIONAL_LOCKS to 0.

       --list-cmds=group[,group...]
           List commands by group. This is an internal/experimental option and may change or be removed in the future. Supported groups are: builtins, parseopt (builtin commands that
           use parse-options), main (all commands in libexec directory), others (all other commands in $PATH that have git- prefix), list-<category> (see categories in command-list.txt), nohelpers (exclude helper commands), alias and config (retrieve command list from config variable completion.commands)

### GIT COMMANDS
       We divide Git into high level ("porcelain") commands and low level ("plumbing") commands.

### HIGH-LEVEL COMMANDS (PORCELAIN)
       We separate the porcelain commands into the main commands and some ancillary user utilities.

#### Main porcelain commands
       git-add(1)
           Add file contents to the index.

       git-am(1)
           Apply a series of patches from a mailbox.

       git-archive(1)
           Create an archive of files from a named tree.

       git-bisect(1)
           Use binary search to find the commit that introduced a bug.

       git-branch(1)
           List, create, or delete branches.

       git-bundle(1)
           Move objects and refs by archive.

       git-checkout(1)
           Switch branches or restore working tree files.

       git-cherry-pick(1)
           Apply the changes introduced by some existing commits.

       git-citool(1)
           Graphical alternative to git-commit.

       git-clean(1)
           Remove untracked files from the working tree.

       git-clone(1)
           Clone a repository into a new directory.

       git-commit(1)
           Record changes to the repository.

       git-describe(1)
           Give an object a human readable name based on an available ref.

       git-diff(1)
           Show changes between commits, commit and working tree, etc.

       git-fetch(1)
           Download objects and refs from another repository.

       git-format-patch(1)
           Prepare patches for e-mail submission.

       git-gc(1)
           Cleanup unnecessary files and optimize the local repository.

       git-grep(1)
           Print lines matching a pattern.

       git-gui(1)
           A portable graphical interface to Git.

       git-init(1)
           Create an empty Git repository or reinitialize an existing one.

       git-log(1)
           Show commit logs.

       git-merge(1)
           Join two or more development histories together.

       git-mv(1)
           Move or rename a file, a directory, or a symlink.

       git-notes(1)
           Add or inspect object notes.

       git-pull(1)
           Fetch from and integrate with another repository or a local branch.

       git-push(1)
           Update remote refs along with associated objects.

       git-range-diff(1)
           Compare two commit ranges (e.g. two versions of a branch).

       git-rebase(1)
           Reapply commits on top of another base tip.

       git-reset(1)
           Reset current HEAD to the specified state.

       git-revert(1)
           Revert some existing commits.

       git-rm(1)
           Remove files from the working tree and from the index.

       git-shortlog(1)
           Summarize git log output.

       git-show(1)
           Show various types of objects.

       git-stash(1)
           Stash the changes in a dirty working directory away.

       git-status(1)
		   Show the working tree status.

       git-submodule(1)
           Initialize, update or inspect submodules.

       git-tag(1)
           Create, list, delete or verify a tag object signed with GPG.

       git-worktree(1)
           Manage multiple working trees.

       gitk(1)
           The Git repository browser.

#### Ancillary Commands
       Manipulators:

       git-config(1)
           Get and set repository or global options.

       git-fast-export(1)
           Git data exporter.

       git-fast-import(1)
           Backend for fast Git data importers.

       git-filter-branch(1)
           Rewrite branches.

       git-mergetool(1)
           Run merge conflict resolution tools to resolve merge conflicts.

       git-pack-refs(1)
           Pack heads and tags for efficient repository access.

       git-prune(1)
           Prune all unreachable objects from the object database.

       git-reflog(1)
           Manage reflog information.

       git-remote(1)
           Manage set of tracked repositories.

       git-repack(1)
           Pack unpacked objects in a repository.

       git-replace(1)
           Create, list, delete refs to replace objects.

       Interrogators:

       git-annotate(1)
           Annotate file lines with commit information.

       git-blame(1)
		   Show what revision and author last modified each line of a file.

       git-count-objects(1)
           Count unpacked number of objects and their disk consumption.

       git-difftool(1)
           Show changes using common diff tools.

       git-fsck(1)
           Verifies the connectivity and validity of the objects in the database.

       git-help(1)
           Display help information about Git.

       git-instaweb(1)
           Instantly browse your working repository in gitweb.

       git-merge-tree(1)
           Show three-way merge without touching index.

       git-rerere(1)
           Reuse recorded resolution of conflicted merges.

       git-show-branch(1)
           Show branches and their commits.

       git-verify-commit(1)
           Check the GPG signature of commits.

       git-verify-tag(1)
           Check the GPG signature of tags.

       git-whatchanged(1)
           Show logs with difference each commit introduces.

       gitweb(1)
           Git web interface (web frontend to Git repositories).

#### Interacting with Others
       These commands are to interact with foreign SCM and with other people via patch over e-mail.

       git-archimport(1)
           Import a GNU Arch repository into Git.

       git-cvsexportcommit(1)
           Export a single commit to a CVS checkout.

       git-cvsimport(1)
           Salvage your data out of another SCM people love to hate.

       git-cvsserver(1)
           A CVS server emulator for Git.

       git-imap-send(1)
           Send a collection of patches from stdin to an IMAP folder.

	   git-p4(1)
           Import from and submit to Perforce repositories.

       git-quiltimport(1)
           Applies a quilt patchset onto the current branch.

       git-request-pull(1)
           Generates a summary of pending changes.

       git-send-email(1)
           Send a collection of patches as emails.

       git-svn(1)
           Bidirectional operation between a Subversion repository and Git.

### LOW-LEVEL COMMANDS (PLUMBING)
       Although Git includes its own porcelain layer, its low-level commands are sufficient to support development of alternative porcelains. Developers of such porcelains might start
       by reading about git-update-index(1) and git-read-tree(1).

       The interface (input, output, set of options and the semantics) to these low-level commands are meant to be a lot more stable than Porcelain level commands, because these
       commands are primarily for scripted use. The interface to Porcelain commands on the other hand are subject to change in order to improve the end user experience.

       The following description divides the low-level commands into commands that manipulate objects (in the repository, index, and working tree), commands that interrogate and
       compare objects, and commands that move objects and references between repositories.

#### Manipulation commands
       git-apply(1)
           Apply a patch to files and/or to the index.

       git-checkout-index(1)
           Copy files from the index to the working tree.

       git-commit-graph(1)
           Write and verify Git commit-graph files.

       git-commit-tree(1)
           Create a new commit object.

       git-hash-object(1)
           Compute object ID and optionally creates a blob from a file.

       git-index-pack(1)
           Build pack index file for an existing packed archive.

       git-merge-file(1)
           Run a three-way file merge.

       git-merge-index(1)
           Run a merge for files needing merging.

       git-mktag(1)
           Creates a tag object.

       git-mktree(1)
           Build a tree-object from ls-tree formatted text.

	   git-multi-pack-index(1)
           Write and verify multi-pack-indexes.

       git-pack-objects(1)
           Create a packed archive of objects.

       git-prune-packed(1)
           Remove extra objects that are already in pack files.

       git-read-tree(1)
           Reads tree information into the index.

       git-symbolic-ref(1)
           Read, modify and delete symbolic refs.

       git-unpack-objects(1)
           Unpack objects from a packed archive.

       git-update-index(1)
           Register file contents in the working tree to the index.

       git-update-ref(1)
           Update the object name stored in a ref safely.

       git-write-tree(1)
           Create a tree object from the current index.

#### Interrogation commands
       git-cat-file(1)
           Provide content or type and size information for repository objects.

       git-cherry(1)
           Find commits yet to be applied to upstream.

       git-diff-files(1)
           Compares files in the working tree and the index.

       git-diff-index(1)
           Compare a tree to the working tree or index.

       git-diff-tree(1)
           Compares the content and mode of blobs found via two tree objects.

       git-for-each-ref(1)
           Output information on each ref.

       git-get-tar-commit-id(1)
           Extract commit ID from an archive created using git-archive.

       git-ls-files(1)
           Show information about files in the index and the working tree.

       git-ls-remote(1)
           List references in a remote repository.

       git-ls-tree(1)
		   List the contents of a tree object.

       git-merge-base(1)
           Find as good common ancestors as possible for a merge.

       git-name-rev(1)
           Find symbolic names for given revs.

       git-pack-redundant(1)
           Find redundant pack files.

       git-rev-list(1)
           Lists commit objects in reverse chronological order.

       git-rev-parse(1)
           Pick out and massage parameters.

       git-show-index(1)
           Show packed archive index.

       git-show-ref(1)
           List references in a local repository.

       git-unpack-file(1)
           Creates a temporary file with a blob's contents.

       git-var(1)
           Show a Git logical variable.

       git-verify-pack(1)
           Validate packed Git archive files.

       In general, the interrogate commands do not touch the files in the working tree.

#### Synching repositories
       git-daemon(1)
           A really simple server for Git repositories.

       git-fetch-pack(1)
           Receive missing objects from another repository.

       git-http-backend(1)
           Server side implementation of Git over HTTP.

       git-send-pack(1)
           Push objects over Git protocol to another repository.

       git-update-server-info(1)
           Update auxiliary info file to help dumb servers.

       The following are helper commands used by the above; end users typically do not use them directly.

       git-http-fetch(1)
           Download from a remote Git repository via HTTP.

       git-http-push(1)
 		   Push objects over HTTP/DAV to another repository.

       git-parse-remote(1)
           Routines to help parsing remote repository access parameters.

       git-receive-pack(1)
           Receive what is pushed into the repository.

       git-shell(1)
           Restricted login shell for Git-only SSH access.

       git-upload-archive(1)
           Send archive back to git-archive.

       git-upload-pack(1)
           Send objects packed back to git-fetch-pack.

#### Internal helper commands
       These are internal helper commands used by other commands; end users typically do not use them directly.

       git-check-attr(1)
           Display gitattributes information.

       git-check-ignore(1)
           Debug gitignore / exclude files.

       git-check-mailmap(1)
           Show canonical names and email addresses of contacts.

       git-check-ref-format(1)
           Ensures that a reference name is well formed.

       git-column(1)
           Display data in columns.

       git-credential(1)
           Retrieve and store user credentials.

       git-credential-cache(1)
           Helper to temporarily store passwords in memory.

       git-credential-store(1)
           Helper to store credentials on disk.

       git-fmt-merge-msg(1)
           Produce a merge commit message.

       git-interpret-trailers(1)
           add or parse structured information in commit messages.

       git-mailinfo(1)
           Extracts patch and authorship from a single e-mail message.

       git-mailsplit(1)
           Simple UNIX mbox splitter program.

	   git-merge-one-file(1)
           The standard helper program to use with git-merge-index.

       git-patch-id(1)
           Compute unique ID for a patch.

       git-sh-i18n(1)
           Git's i18n setup code for shell scripts.

       git-sh-setup(1)
           Common Git shell script setup code.

       git-stripspace(1)
           Remove unnecessary whitespace.

### CONFIGURATION MECHANISM
       Git uses a simple text format to store customizations that are per repository and are per user. Such a configuration file may look like this:

           #
           # A '#' or ';' character indicates a comment.
           #

           ; core variables
           [core]
                   ; Don't trust file modes
                   filemode = false

           ; user identity
           [user]
                   name = "Junio C Hamano"
                   email = "gitster@pobox.com"

       Various commands read from the configuration file and adjust their operation accordingly. See git-config(1) for a list and more details about the configuration mechanism.

### IDENTIFIER TERMINOLOGY
       <object>
           Indicates the object name for any type of object.

       <blob>
           Indicates a blob object name.

       <tree>
           Indicates a tree object name.

       <commit>
           Indicates a commit object name.

       <tree-ish>
           Indicates a tree, commit or tag object name. A command that takes a <tree-ish> argument ultimately wants to operate on a <tree> object but automatically dereferences
           <commit> and <tag> objects that point at a <tree>.

       <commit-ish>
           Indicates a commit or tag object name. A command that takes a <commit-ish> argument ultimately wants to operate on a <commit> object but automatically dereferences <tag>
           objects that point at a <commit>.

       <type>
		   Indicates that an object type is required. Currently one of: blob, tree, commit, or tag.

       <file>
           Indicates a filename - almost always relative to the root of the tree structure GIT_INDEX_FILE describes.

### SYMBOLIC IDENTIFIERS
       Any Git command accepting any <object> can also use the following symbolic notation:

       HEAD
           indicates the head of the current branch.

       <tag>
           a valid tag name (i.e. a refs/tags/<tag> reference).

       <head>
           a valid head name (i.e. a refs/heads/<head> reference).

       For a more complete list of ways to spell object names, see "SPECIFYING REVISIONS" section in gitrevisions(7).

### FILE/DIRECTORY STRUCTURE
       Please see the gitrepository-layout(5) document.

       Read githooks(5) for more details about each hook.

       Higher level SCMs may provide and manage additional information in the $GIT_DIR.

### TERMINOLOGY
       Please see gitglossary(7).

### ENVIRONMENT VARIABLES
       Various Git commands use the following environment variables:

#### The Git Repository
       These environment variables apply to all core Git commands. Nb: it is worth noting that they may be used/overridden by SCMS sitting above Git so take care if using a foreign
       front-end.

       GIT_INDEX_FILE
           This environment allows the specification of an alternate index file. If not specified, the default of $GIT_DIR/index is used.

       GIT_INDEX_VERSION
           This environment variable allows the specification of an index version for new repositories. It won't affect existing index files. By default index file version 2 or 3 is
           used. See git-update-index(1) for more information.

       GIT_OBJECT_DIRECTORY
           If the object storage directory is specified via this environment variable then the sha1 directories are created underneath - otherwise the default $GIT_DIR/objects
           directory is used.

       GIT_ALTERNATE_OBJECT_DIRECTORIES
           Due to the immutable nature of Git objects, old objects can be archived into shared, read-only directories. This variable specifies a ":" separated (on Windows ";"
           separated) list of Git object directories which can be used to search for Git objects. New objects will not be written to these directories.

           Entries that begin with " (double-quote) will be interpreted as C-style quoted paths, removing leading and trailing double-quotes and respecting backslash escapes. E.g.,
           the value "path-with-\"-and-:-in-it":vanilla-path has two paths: path-with-"-and-:-in-it and vanilla-path.

       GIT_DIR
           If the GIT_DIR environment variable is set then it specifies a path to use instead of the default .git for the base of the repository. The --git-dir command-line option also sets this value.

       GIT_WORK_TREE
           Set the path to the root of the working tree. This can also be controlled by the --work-tree command-line option and the core.worktree configuration variable.

       GIT_NAMESPACE
           Set the Git namespace; see gitnamespaces(7) for details. The --namespace command-line option also sets this value.

       GIT_CEILING_DIRECTORIES
           This should be a colon-separated list of absolute paths. If set, it is a list of directories that Git should not chdir up into while looking for a repository directory
           (useful for excluding slow-loading network directories). It will not exclude the current working directory or a GIT_DIR set on the command line or in the environment.
           Normally, Git has to read the entries in this list and resolve any symlink that might be present in order to compare them with the current directory. However, if even this
           access is slow, you can add an empty entry to the list to tell Git that the subsequent entries are not symlinks and needn't be resolved; e.g.,
           GIT_CEILING_DIRECTORIES=/maybe/symlink::/very/slow/non/symlink.

       GIT_DISCOVERY_ACROSS_FILESYSTEM
           When run in a directory that does not have ".git" repository directory, Git tries to find such a directory in the parent directories to find the top of the working tree,
           but by default it does not cross filesystem boundaries. This environment variable can be set to true to tell Git not to stop at filesystem boundaries. Like
           GIT_CEILING_DIRECTORIES, this will not affect an explicit repository directory set via GIT_DIR or on the command line.

       GIT_COMMON_DIR
           If this variable is set to a path, non-worktree files that are normally in $GIT_DIR will be taken from this path instead. Worktree-specific files such as HEAD or index are
           taken from $GIT_DIR. See gitrepository-layout(5) and git-worktree(1) for details. This variable has lower precedence than other path variables such as GIT_INDEX_FILE,
           GIT_OBJECT_DIRECTORY...

#### Git Commits
       GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE, GIT_COMMITTER_NAME, GIT_COMMITTER_EMAIL, GIT_COMMITTER_DATE, EMAIL
           see git-commit-tree(1)

#### Git Diffs
       GIT_DIFF_OPTS
           Only valid setting is "--unified=??" or "-u??" to set the number of context lines shown when a unified diff is created. This takes precedence over any "-U" or "--unified"
           option value passed on the Git diff command line.

       GIT_EXTERNAL_DIFF
           When the environment variable GIT_EXTERNAL_DIFF is set, the program named by it is called, instead of the diff invocation described above. For a path that is added,
           removed, or modified, GIT_EXTERNAL_DIFF is called with 7 parameters:

               path old-file old-hex old-mode new-file new-hex new-mode

           where:

       <old|new>-file
           are files GIT_EXTERNAL_DIFF can use to read the contents of <old|new>,

       <old|new>-hex
           are the 40-hexdigit SHA-1 hashes,

       <old|new>-mode
           are the octal representation of the file modes.

           The file parameters can point at the user's working file (e.g.  new-file in "git-diff-files"), /dev/null (e.g.  old-file when a new file is added), or a temporary file
           (e.g.  old-file in the index).  GIT_EXTERNAL_DIFF should not worry about unlinking the temporary file --- it is removed when GIT_EXTERNAL_DIFF exits.

           For a path that is unmerged, GIT_EXTERNAL_DIFF is called with 1 parameter, <path>.

 		   For each path GIT_EXTERNAL_DIFF is called, two environment variables, GIT_DIFF_PATH_COUNTER and GIT_DIFF_PATH_TOTAL are set.

       GIT_DIFF_PATH_COUNTER
           A 1-based counter incremented by one for every path.

       GIT_DIFF_PATH_TOTAL
           The total number of paths.

#### other
       GIT_MERGE_VERBOSITY
           A number controlling the amount of output shown by the recursive merge strategy. Overrides merge.verbosity. See git-merge(1)

       GIT_PAGER
           This environment variable overrides $PAGER. If it is set to an empty string or to the value "cat", Git will not launch a pager. See also the core.pager option in git-
           config(1).

       GIT_EDITOR
           This environment variable overrides $EDITOR and $VISUAL. It is used by several Git commands when, on interactive mode, an editor is to be launched. See also git-var(1) and
           the core.editor option in git-config(1).

       GIT_SSH, GIT_SSH_COMMAND
           If either of these environment variables is set then git fetch and git push will use the specified command instead of ssh when they need to connect to a remote system. The
           command-line parameters passed to the configured command are determined by the ssh variant. See ssh.variant option in git-config(1) for details.

       + $GIT_SSH_COMMAND takes precedence over $GIT_SSH, and is interpreted by the shell, which allows additional arguments to be included. $GIT_SSH on the other hand must be just
       the path to a program (which can be a wrapper shell script, if additional arguments are needed).

       + Usually it is easier to configure any desired options through your personal .ssh/config file. Please consult your ssh documentation for further details.

       GIT_SSH_VARIANT
           If this environment variable is set, it overrides Git's autodetection whether GIT_SSH/GIT_SSH_COMMAND/core.sshCommand refer to OpenSSH, plink or tortoiseplink. This
           variable overrides the config setting ssh.variant that serves the same purpose.

       GIT_ASKPASS
           If this environment variable is set, then Git commands which need to acquire passwords or passphrases (e.g. for HTTP or IMAP authentication) will call this program with a
           suitable prompt as command-line argument and read the password from its STDOUT. See also the core.askPass option in git-config(1).

       GIT_TERMINAL_PROMPT
           If this environment variable is set to 0, git will not prompt on the terminal (e.g., when asking for HTTP authentication).

       GIT_CONFIG_NOSYSTEM
           Whether to skip reading settings from the system-wide $(prefix)/etc/gitconfig file. This environment variable can be used along with $HOME and $XDG_CONFIG_HOME to create a
           predictable environment for a picky script, or you can set it temporarily to avoid using a buggy /etc/gitconfig file while waiting for someone with sufficient permissions
           to fix it.

       GIT_FLUSH
           If this environment variable is set to "1", then commands such as git blame (in incremental mode), git rev-list, git log, git check-attr and git check-ignore will force a
           flush of the output stream after each record have been flushed. If this variable is set to "0", the output of these commands will be done using completely buffered I/O. If
           this environment variable is not set, Git will choose buffered or record-oriented flushing based on whether stdout appears to be redirected to a file or not.

       GIT_TRACE
           Enables general trace messages, e.g. alias expansion, built-in command execution and external command execution.

           If this variable is set to "1", "2" or "true" (comparison is case insensitive), trace messages will be printed to stderr.

           If the variable is set to an integer value greater than 2 and lower than 10 (strictly) then Git will interpret this value as an open file descriptor and will try to write the trace messages into this file descriptor.

           Alternatively, if the variable is set to an absolute path (starting with a / character), Git will interpret this as a file path and will try to append the trace messages to
           it.

           Unsetting the variable, or setting it to empty, "0" or "false" (case insensitive) disables trace messages.

       GIT_TRACE_FSMONITOR
           Enables trace messages for the filesystem monitor extension. See GIT_TRACE for available trace output options.

       GIT_TRACE_PACK_ACCESS
           Enables trace messages for all accesses to any packs. For each access, the pack file name and an offset in the pack is recorded. This may be helpful for troubleshooting
           some pack-related performance problems. See GIT_TRACE for available trace output options.

       GIT_TRACE_PACKET
           Enables trace messages for all packets coming in or out of a given program. This can help with debugging object negotiation or other protocol issues. Tracing is turned off
           at a packet starting with "PACK" (but see GIT_TRACE_PACKFILE below). See GIT_TRACE for available trace output options.

       GIT_TRACE_PACKFILE
           Enables tracing of packfiles sent or received by a given program. Unlike other trace output, this trace is verbatim: no headers, and no quoting of binary data. You almost
           certainly want to direct into a file (e.g., GIT_TRACE_PACKFILE=/tmp/my.pack) rather than displaying it on the terminal or mixing it with other trace output.

           Note that this is currently only implemented for the client side of clones and fetches.

       GIT_TRACE_PERFORMANCE
           Enables performance related trace messages, e.g. total execution time of each Git command. See GIT_TRACE for available trace output options.

       GIT_TRACE_SETUP
           Enables trace messages printing the .git, working tree and current working directory after Git has completed its setup phase. See GIT_TRACE for available trace output
           options.

       GIT_TRACE_SHALLOW
           Enables trace messages that can help debugging fetching / cloning of shallow repositories. See GIT_TRACE for available trace output options.

       GIT_TRACE_CURL
           Enables a curl full trace dump of all incoming and outgoing data, including descriptive information, of the git transport protocol. This is similar to doing curl
           --trace-ascii on the command line. This option overrides setting the GIT_CURL_VERBOSE environment variable. See GIT_TRACE for available trace output options.

       GIT_TRACE_CURL_NO_DATA
           When a curl trace is enabled (see GIT_TRACE_CURL above), do not dump data (that is, only dump info lines and headers).

       GIT_REDACT_COOKIES
           This can be set to a comma-separated list of strings. When a curl trace is enabled (see GIT_TRACE_CURL above), whenever a "Cookies:" header sent by the client is dumped,
           values of cookies whose key is in that list (case-sensitive) are redacted.

       GIT_LITERAL_PATHSPECS
           Setting this variable to 1 will cause Git to treat all pathspecs literally, rather than as glob patterns. For example, running GIT_LITERAL_PATHSPECS=1 git log -- '*.c' will
           search for commits that touch the path *.c, not any paths that the glob *.c matches. You might want this if you are feeding literal paths to Git (e.g., paths previously
           given to you by git ls-tree, --raw diff output, etc).

       GIT_GLOB_PATHSPECS
           Setting this variable to 1 will cause Git to treat all pathspecs as glob patterns (aka "glob" magic).

       GIT_NOGLOB_PATHSPECS
           Setting this variable to 1 will cause Git to treat all pathspecs as literal (aka "literal" magic).

	   GIT_ICASE_PATHSPECS
           Setting this variable to 1 will cause Git to treat all pathspecs as case-insensitive.

       GIT_REFLOG_ACTION
           When a ref is updated, reflog entries are created to keep track of the reason why the ref was updated (which is typically the name of the high-level command that updated
           the ref), in addition to the old and new values of the ref. A scripted Porcelain command can use set_reflog_action helper function in git-sh-setup to set its name to this
           variable when it is invoked as the top level command by the end user, to be recorded in the body of the reflog.

       GIT_REF_PARANOIA
           If set to 1, include broken or badly named refs when iterating over lists of refs. In a normal, non-corrupted repository, this does nothing. However, enabling it may help
           git to detect and abort some operations in the presence of broken refs. Git sets this variable automatically when performing destructive operations like git-prune(1). You
           should not need to set it yourself unless you want to be paranoid about making sure an operation has touched every ref (e.g., because you are cloning a repository to make a
           backup).

       GIT_ALLOW_PROTOCOL
           If set to a colon-separated list of protocols, behave as if protocol.allow is set to never, and each of the listed protocols has protocol.<name>.allow set to always
           (overriding any existing configuration). In other words, any protocol not mentioned will be disallowed (i.e., this is a whitelist, not a blacklist). See the description of
           protocol.allow in git-config(1) for more details.

       GIT_PROTOCOL_FROM_USER
           Set to 0 to prevent protocols used by fetch/push/clone which are configured to the user state. This is useful to restrict recursive submodule initialization from an
           untrusted repository or for programs which feed potentially-untrusted URLS to git commands. See git-config(1) for more details.

       GIT_PROTOCOL
           For internal use only. Used in handshaking the wire protocol. Contains a colon : separated list of keys with optional values key[=value]. Presence of unknown keys and
           values must be ignored.

       GIT_OPTIONAL_LOCKS
           If set to 0, Git will complete any requested operation without performing any optional sub-operations that require taking a lock. For example, this will prevent git status
           from refreshing the index as a side effect. This is useful for processes running in the background which do not want to cause lock contention with other operations on the
           repository. Defaults to 1.

       GIT_REDIRECT_STDIN, GIT_REDIRECT_STDOUT, GIT_REDIRECT_STDERR
           Windows-only: allow redirecting the standard input/output/error handles to paths specified by the environment variables. This is particularly useful in multi-threaded
           applications where the canonical way to pass standard handles via CreateProcess() is not an option because it would require the handles to be marked inheritable (and
           consequently every spawned process would inherit them, possibly blocking regular Git operations). The primary intended use case is to use named pipes for communication
           (e.g.  \\.\pipe\my-git-stdin-123).

           Two special values are supported: off will simply close the corresponding standard handle, and if GIT_REDIRECT_STDERR is 2>&1, standard error will be redirected to the same
           handle as standard output.

       GIT_PRINT_SHA1_ELLIPSIS (deprecated)
           If set to yes, print an ellipsis following an (abbreviated) SHA-1 value. This affects indications of detached HEADs (git-checkout(1)) and the raw diff output (git-diff(1)).
           Printing an ellipsis in the cases mentioned is no longer considered adequate and support for it is likely to be removed in the foreseeable future (along with the variable).

### DISCUSSION
       More detail on the following is available from the Git concepts chapter of the user-manual[2] and gitcore-tutorial(7).

       A Git project normally consists of a working directory with a ".git" subdirectory at the top level. The .git directory contains, among other things, a compressed object
       database representing the complete history of the project, an "index" file which links that history to the current contents of the working tree, and named pointers into that
       history such as tags and branch heads.

       The object database contains objects of three main types: blobs, which hold file data; trees, which point to blobs and other trees to build up directory hierarchies; and
       commits, which each reference a single tree and some number of parent commits.

       The commit, equivalent to what other systems call a "changeset" or "version", represents a step in the project's history, and each parent represents an immediately preceding step. Commits with more than one parent represent merges of independent lines of development.

       All objects are named by the SHA-1 hash of their contents, normally written as a string of 40 hex digits. Such names are globally unique. The entire history leading up to a
       commit can be vouched for by signing just that commit. A fourth object type, the tag, is provided for this purpose.

       When first created, objects are stored in individual files, but for efficiency may later be compressed together into "pack files".

       Named pointers called refs mark interesting points in history. A ref may contain the SHA-1 name of an object or the name of another ref. Refs with names beginning ref/head/
       contain the SHA-1 name of the most recent commit (or "head") of a branch under development. SHA-1 names of tags of interest are stored under ref/tags/. A special ref named HEAD
       contains the name of the currently checked-out branch.

       The index file is initialized with a list of all paths and, for each path, a blob object and a set of attributes. The blob object represents the contents of the file as of the
       head of the current branch. The attributes (last modified time, size, etc.) are taken from the corresponding file in the working tree. Subsequent changes to the working tree
       can be found by comparing these attributes. The index may be updated with new content, and new commits may be created from the content stored in the index.

       The index is also capable of storing multiple entries (called "stages") for a given pathname. These stages are used to hold the various unmerged version of a file when a merge
       is in progress.

### FURTHER DOCUMENTATION
       See the references in the "description" section to get started using Git. The following is probably more detail than necessary for a first-time user.

       The Git concepts chapter of the user-manual[2] and gitcore-tutorial(7) both provide introductions to the underlying Git architecture.

       See gitworkflows(7) for an overview of recommended workflows.

       See also the howto[3] documents for some useful examples.

       The internals are documented in the Git API documentation[4].

       Users migrating from CVS may also want to read gitcvs-migration(7).

### AUTHORS
       Git was started by Linus Torvalds, and is currently maintained by Junio C Hamano. Numerous contributions have come from the Git mailing list <git@vger.kernel.org[5]>.
       http://www.openhub.net/p/git/contributors/summary gives you a more complete list of contributors.

       If you have a clone of git.git itself, the output of git-shortlog(1) and git-blame(1) can show you the authors for specific parts of the project.

### REPORTING BUGS
       Report bugs to the Git mailing list <git@vger.kernel.org[5]> where the development and maintenance is primarily done. You do not have to be subscribed to the list to send a
       message there. See the list archive at https://public-inbox.org/git for previous bug reports and other discussions.

       Issues which are security relevant should be disclosed privately to the Git Security mailing list <git-security@googlegroups.com[6]>.

### SEE ALSO
       gittutorial(7), gittutorial-2(7), giteveryday(7), gitcvs-migration(7), gitglossary(7), gitcore-tutorial(7), gitcli(7), The Git User's Manual[1], gitworkflows(7)

### GIT
       Part of the git(1) suite

### NOTES
        1. Git User's Manual
           git-htmldocs/user-manual.html

        2. Git concepts chapter of the user-manual
           git-htmldocs/user-manual.html#git-concepts

        3. howto
           git-htmldocs/howto-index.html

        4. Git API documentation
           git-htmldocs/technical/api-index.html

        5. git@vger.kernel.org
           mailto:git@vger.kernel.org

        6. git-security@googlegroups.com
           mailto:git-security@googlegroups.com

POP vs OOP

[TOC]

POP

Procedure-Oriented Programming

以过程为中心，试图使问题满足语言的过程性方法，强调编程的算法

OOP

Object-Oriented Programming

以对象为中心，试图让语言来满足问题的要求，强调数据

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