th2zz

CS577 Algorithm Notesheet

2018-05-05

Linear algebra

2017-12-11

summary and notes upon compeletion of the course & self study notes.
https://github.com/th2zz/th2zz.github.io/blob/master/notes/linear_algebra.pdf

Textbook Reference
Introduction to Linear Algebra 4th Edition by Gilbert Strang
Applied Linear Algebra (Undergraduate Texts in Mathematics) 2nd ed. 2018 Edition by Peter J. Olver, Chehrzad Shakiban

chmod

2017-09-29

修改文件或目录的访问权限。

chmod --help

Usage: chmod [OPTION]... MODE[,MODE]... FILE...
  or:  chmod [OPTION]... OCTAL-MODE FILE...
  or:  chmod [OPTION]... --reference=RFILE FILE...
Change the mode of each FILE to MODE.
With --reference, change the mode of each FILE to that of RFILE.

  -c, --changes          like verbose but report only when a change is made
  -f, --silent, --quiet  suppress most error messages
  -v, --verbose          output a diagnostic for every file processed
      --no-preserve-root  do not treat '/' specially (the default)
      --preserve-root    fail to operate recursively on '/'
      --reference=RFILE  use RFILE's mode instead of MODE values
  -R, --recursive        change files and directories recursively
      --help     display this help and exit
      --version  output version information and exit

Each MODE is of the form '[ugoa]*([-+=]([rwxXst]*|[ugo]))+|[-+=][0-7]+'.

文件系统权限是文件系统用来管理用户和用户组对文件或目录的访问权限的规则。

###用户

文件和目录归属于用户(owner)，
分配给用户组(group)，owner 是 group 的成员；
非 owner 和非 group 成员的用户属于其他用户(others)；
还有所有用户(all)。

###访问权限

read 读取文件（当作用与目录时，仅能获取目录下所有文件的文件名）；
write 修改文件（当作用与目录时，可以对文件执行创建、删除和重命名操作）；
execute 执行文件（shell 脚本或可执行文件。当作用与目录时，可以获得文件内容和元信息，但不能获得文件列表）

###符号模式

r read;
w write;
x execute.

1
2
3

ls -l

-rw-r--r-- 1 hu hu  0  3月 31 15:19 test.txt

-rw-r--r-- 代表了文件类型和文件权限：

第一位 - 代表了文件为普通文件（参考文件类型）：

- Regular file
d Directory
l Symbolic link
p Named pipe
s Socket
c Device file(character device)
b Device file(block device)
D Door

后面九位分成三段，分别代表了 owner/group/others 三类用户的访问权限，- 代表没有对应权限：
owner 拥有 read/write 权限；group 和 others 只拥有 read 权限。

###数字模式

| 符号 | 代码 | 权限 |
| | | |
| r | 4/100 | read |
| w | 2/10 | write |
| x | 1/1 | execute |
| - | 0/0 | - |

-rw-r--r-- 可以用数字表示为 0644。

###MODE 规则

[ugoa]*([-+=]([rwxXst]*|[ugo]))+|[-+=][0-7]+ 被用来校验 chmod 命令中的 MODE 参数。

u user 所有者
g group 所有者所在分组
o others 所有其他用户
a all 所有用户（等同于 ugo）

+ 添加权限
- 删除权限
= 设置权限

可能的 MODE 参数：

chmod a+w test.txt 给所有用户添加 test.txt 的写权限；
chmod u=rwx app.js app.js 的所有者拥有对该文件的读写和执行权限；
chmod 624 app.js 分别给三组用户不同的权限（u: rw, g: w, o: r）；
chmod 755 ~/works 分别给三组用户不同的权限（u: rwx, g: rx, o: rx）。

###参考

wiki: Chmod

link

2017-09-21

用于创建符号链接或硬链接。

ln --help

用法：ln [选项]... [-T] 目标 链接名	(第一种格式)
　或：ln [选项]... 目标		(第二种格式)
　或：ln [选项]... 目标... 目录	(第三种格式)
　或：ln [选项]... -t 目录 目标...	(第四种格式)
In the 1st form, create a link to TARGET with the name LINK_NAME.
In the 2nd form, create a link to TARGET in the current directory.
In the 3rd and 4th forms, create links to each TARGET in DIRECTORY.
Create hard links by default, symbolic links with --symbolic.
By default, each destination (name of new link) should not already exist.
When creating hard links, each TARGET must exist.  Symbolic links
can hold arbitrary text; if later resolved, a relative link is
interpreted in relation to its parent directory.

Mandatory arguments to long options are mandatory for short options too.
      --backup[=CONTROL]	为每个已存在的目标文件创建备份文件
  -b				类似--backup，但不接受任何参数
  -d, -F, --directory		创建指向目录的硬链接(只适用于超级用户)
  -f, --force			强行删除任何已存在的目标文件
  -i, --interactive           prompt whether to remove destinations
  -L, --logical               dereference TARGETs that are symbolic links
  -n, --no-dereference        treat LINK_NAME as a normal file if
                                it is a symbolic link to a directory
  -P, --physical              make hard links directly to symbolic links
  -r, --relative              create symbolic links relative to link location
  -s, --symbolic              make symbolic links instead of hard links
  -S, --suffix=SUFFIX         override the usual backup suffix
  -t, --target-directory=DIRECTORY  specify the DIRECTORY in which to create
                                the links
  -T, --no-target-directory   treat LINK_NAME as a normal file always
  -v, --verbose               print name of each linked file
      --help		显示此帮助信息并退出
      --version		显示版本信息并退出

The backup suffix is '~', unless set with --suffix or SIMPLE_BACKUP_SUFFIX.
The version control method may be selected via the --backup option or through
the VERSION_CONTROL environment variable.  Here are the values:

  none, off       不进行备份(即使使用了--backup 选项)
  numbered, t     备份文件加上数字进行排序
  existing, nil   若有数字的备份文件已经存在则使用数字，否则使用普通方式备份
  simple, never   永远使用普通方式备份

Using -s ignores -L and -P.  Otherwise, the last option specified controls
behavior when a TARGET is a symbolic link, defaulting to -P.

###查看

ls -l 可以查看文件是否为链接文件：

1
2
3

ls -l /bin

lrwxrwxrwx 1 root root      24  3月 18 14:25 netcat -> /etc/alternatives/netcat

l 代表文件是链接文件；
netcat 是指向 /etc/alternatives/netcat 的一个文件链接。

###创建

ln -s target link 创建一个名为 link 的文件，链接到 target 文件；
ln -s target 在当前目录创建一个链接到 target 的文件，与 target 同名；
ln -s targets link_dir 在 link_dir 目录创建所有 targets 的链接。

###参考

wiki: 符号链接
 理解 Linux 的硬链接与软链接

chown

2017-08-29

修改文件或目录的所有者或（和）用户组。

chown --help

Usage: chown [OPTION]... [OWNER][:[GROUP]] FILE...
  or:  chown [OPTION]... --reference=RFILE FILE...
Change the owner and/or group of each FILE to OWNER and/or GROUP.
With --reference, change the owner and group of each FILE to those of RFILE.

  -c, --changes          like verbose but report only when a change is made
  -f, --silent, --quiet  suppress most error messages
  -v, --verbose          output a diagnostic for every file processed
      --dereference      affect the referent of each symbolic link (this is
                         the default), rather than the symbolic link itself
  -h, --no-dereference   affect symbolic links instead of any referenced file
                         (useful only on systems that can change the
                         ownership of a symlink)
      --from=CURRENT_OWNER:CURRENT_GROUP
                         change the owner and/or group of each file only if
                         its current owner and/or group match those specified
                         here.  Either may be omitted, in which case a match
                         is not required for the omitted attribute
      --no-preserve-root  do not treat '/' specially (the default)
      --preserve-root    fail to operate recursively on '/'
      --reference=RFILE  use RFILE's owner and group rather than
                         specifying OWNER:GROUP values
  -R, --recursive        operate on files and directories recursively

The following options modify how a hierarchy is traversed when the -R
option is also specified.  If more than one is specified, only the final
one takes effect.

  -H                     if a command line argument is a symbolic link
                         to a directory, traverse it
  -L                     traverse every symbolic link to a directory
                         encountered
  -P                     do not traverse any symbolic links (default)

      --help     display this help and exit
      --version  output version information and exit

Owner is unchanged if missing.  Group is unchanged if missing, but changed
to login group if implied by a ':' following a symbolic OWNER.
OWNER and GROUP may be numeric as well as symbolic.

Examples:
  chown root /u        Change the owner of /u to "root".
  chown root:staff /u  Likewise, but also change its group to "staff".
  chown -hR root /u    Change the owner of /u and subfiles to "root".

###查看归属信息

ls -l 命令可以看到文件的拥有者和用户组：

1
2
3

ls -l

-rw-rw-r-- 1 root hu 27  3月 18 16:50 hello.js

root owner;
hu group.

###修改归属

sudo chown hu hello.js 将 hello.js 的 owner 改为 hu；
sudo chown root:root hello.js 将 hello.js 的 owner 改为 root，group 改为 root。

注意：chown 命令只能被 root 用户执行。