Create files, file permissions, ownership, and sticky bits

If you need to create a file of a given size , have access to dd command .

dd Will put the given input , Clone a copy and write it to the output .

stdin、 Both device files and ordinary files can be used as input ,stdout、 Device files and ordinary files can also be used as output .

Log in and copy

      
      

       
       $ dd if=/dev/zero of=out.data bs=1M count=10
      
      
      
      

       
       
1.
      
      

if Represents the input file ,/dev/zero It's a character device , Will keep coming back 0 Value bytes .

of Represents the output file .

bs Represents the block size ,count Number of Representatives , Multiplying the two is the file size .

The unit of measure of block size ：

c  byte  1B

w  word  2B

b  block  512B

k  kilobytes  1024B

M  Megabytes  1024KB

G 1024MB

dd Command can be used to test the operation speed of memory .

Linux Every file in the system is associated with multiple permission types . In these permissions , It usually includes three types of permissions , user 、 User groups and other entities .

user user It's the owner of the document , User group group It's a collection of multiple users , The system allows these users to access files in some form , Other users others Is any user other than these two .

Command ls -l You can list the permissions of the file , The first column of output results is often as follows ：

-rw-r--r--

drwxr-xr-x

The first letter indicates the type of file ：

-： Ordinary documents

d： Catalog

c： Character device

b： Block device

l： A symbolic link

s： Socket

p： The Conduit

The remaining nine characters are divided into three groups , The first group of corresponding users user jurisdiction , The second group corresponds to the user group group jurisdiction , The third group corresponds to other users others jurisdiction .

The three characters of each group of permissions correspond to the file reading 、 Write and executable permissions , If permission is specified , Will write the corresponding characters , If you don't have permission , It will say “-”.

The first character corresponds to the read permission , If it can be read , Just write “r”.

The second character corresponds to write permission , Can rewrite , Just write “w”.

The third character corresponds to the executable permission , Executable , Just write “x”.

user user Another one is called setuid（S） Authority , Appears in executable permissions x Location .setuid Permissions allow users to execute executables with the permissions of their owners , Form the following ：

-rwS------

The directory is also read 、 Write and execute permissions , However, the read permission of the directory is to allow reading the list of files and subdirectories in the directory , Write permission allows you to create or delete files or subdirectories in a directory , Execution permission is to allow access to files and subdirectories in the directory .

The permissions of the second and third groups are the same as those of the user group . However, there is no user group setuid Of , The corresponding is setgid. It allows files to be executed with the same valid group permissions as the group of the directory owner . Form the following ：

----rwS---

No other users setuid and setgid.

The directory has a special permission , It is called viscous position （sticky bit）, You can understand it as write protection , When a directory has the sticky bit set , Only the user who created the directory can delete the files in the directory , Even user groups and other users have write permission . Sticky bits are written in other user permission groups x Location , Use t perhaps T To express . If there is only viscous bit , Namely t, If the execution permission and sticky bit are set at the same time , Just use T. Form the following ：

-------rwt