Important core attribute knowledge of Linux files

Keywords: Linux ftp network Attribute

1, users

  • Linux is a multi-user, multi task operating system

    • Multiple users: multiple users log in at the same time
    • Multitask: each user can perform multiple tasks

  • User:

    • Management of Linux system
    • Users are generally divided into user name and user ID (UID)
    • The user name is convenient for the administrator to use, and the user ID is the only difference between a user (the computer only recognizes the ID)

  • File and process corresponding user relationship

    • All users and processes in linux must have user correspondence.
    • File existence must correspond to user
    • Process running must correspond to user

  • Linux user classification

    • Super administrator: root

      • UID is 0.
      • If the user ID (UID) has root permission
      [root@oldboyedu ~]# id root   #View user ID
  d=0(root) gid=0(root) groups=0(root)
[root@oldboyedu ~]# id
uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[root@oldboyedu ~]# su - oldboy		#Switching users
[oldboy@oldboyedu ~]$ logout
[root@oldboyedu ~]# vim /etc/passwd    #Modify user ID
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
systemd-network:x:192:192:systemd Network Management:/:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
polkitd:x:999:998:User for polkitd:/:/sbin/nologin
tss:x:59:59:Account used by the trousers package to sandbox the tcsd daemon:/dev/null:/sb
in/nologin
abrt:x:173:173::/etc/abrt:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
postfix:x:89:89::/var/spool/postfix:/sbin/nologin
oldboy:x:0:1000:oldboy:/home/oldboy:/bin/bash
tcpdump:x:72:72::/:/sbin/nologin                 
"/etc/passwd" 22L, 1017C written
[root@oldboyedu ~]# su - oldboy   
Last login: Fri Apr 10 18:52:44 CST 2020 from 10.0.0.1 on pts/0
[root@oldboyedu ~]# cd /root/
[root@oldboyedu /root]# cd ..
[root@oldboyedu /]# ^C
[root@oldboyedu /]# logout
[root@oldboyedu ~]# id
uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[root@oldboyedu ~]# su - oldboy
Last login: Fri Apr 10 18:53:33 CST 2020 on pts/0
[root@oldboyedu ~]# id
uid=0(root) gid=1000(oldboy) groups=1000(oldboy) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[root@oldboyedu ~]# whoami 	#Query current user
root
      • Note: do not change the UID value of users other than root to 0, which is easy to cause operation error due to too large permission

  • Virtual user

    • The actual user, but it is not allowed to log in
    • What is the value?
      • Meet the requirements of file or process operation owner. But it will not bring management risk
    • UID: 1-499

  • Ordinary users

    • The actual user allows him to log in, created as an administrator, and helps the administrator manage the system.

    • UID:1000~60000

      [root@oldboyedu ~]# useradd oldgirl		#Create a normal user
[root@oldboyedu ~]# id oldgirl
uid=1001(oldgirl) gid=1001(oldgirl) groups=1001(oldgirl)

1.1. How to find the lost root password?

  • Restart the system. During the startup process, quickly press the direction keys and on the keyboard. The purpose is to inform the bootstrapper that we need to select different operations on the boot page in order to pause the bootstrapper.
  • Press the e key on the keyboard to enter editing mode and find the line of Linux 16. Move the cursor all the way to the back of LANG=en_US.UTF-8, with a space, and then add init=/bin/sh. In particular, you need to write after UTF-8, keep the same line, and pay attention to the space. Because the screen is too small, it will automatically add \ line feed, which is normal.

  • Mount root

  • Select the user name to change the password, and select root to change it. You can change it to the user you want to change

  • Enter the same new password twice. Note that no characters will appear on the screen when entering the password.

    • If the password you enter is too simple, you will be prompted to warn BAD PASSWORD: the password fails the dictionary check - it is too simple / systematic). You can ignore it and continue to enter the password, but it is recommended to set a more complex password to ensure security

  • Update system information touch /.authorelabel

  • Finally, enter the following command to restart the system

2. User groups

  • To assemble or organize (a person's family, for example)

  • Every user in Linux must have an organization, which is called user group

    • User groups are also shown to people. User groups also have unique identification. group ID is referred to as GID for short

    • By default, a user is created, but no user group is created. He will automatically create a user group, the same user group as the user name, and the same GID

      [root@oldboyedu ~]# id oldgirl
uid=1001(oldgirl) gid=1001(oldgirl) groups=1001(oldgirl)

2.1 correspondence between users and user groups

  • A user corresponds to a user group

  • One user corresponds to multiple user groups

  • Multiple users correspond to one user group

    [root@oldboyedu ~]# useradd oldgirl
[root@oldboyedu ~]# id oldgirl
uid=1001(oldgirl) gid=1001(oldgirl) groups=1001(oldgirl)
[root@oldboyedu ~]# useradd -g root text	#Create user specified user group
[root@oldboyedu ~]# id text
uid=1002(text) gid=0(root) groups=0(root)
[root@oldboyedu ~]# groupadd sa		#Create user group
[root@oldboyedu ~]# useradd -g sa uu
[root@oldboyedu ~]# id uu
uid=1003(uu) gid=1002(sa) groups=1002(sa)
[root@oldboyedu ~]# id oldboy
uid=1000(oldboy) gid=1000(oldboy) groups=1000(oldboy)
[root@oldboyedu ~]# id root
uid=0(root) gid=0(root) groups=0(root)

3. File permissions

  • Linux system has strict requirements on file security permission

    [root@oldboyedu /]# ls -lih /etc/sysconfig/
total 108K
33575490 -rw-r--r--. 1 root root  412 Apr  4 18:09 anaconda
34063111 -rw-r--r--. 1 root root  483 Apr  4 18:08 authconfig
16841044 drwxr-xr-x. 2 root root   43 Apr  4 18:22 cbq
16881480 drwxr-xr-x. 2 root root    6 Aug  9  2019 console
33743063 -rw-r--r--. 1 root root  150 Mar 18 07:59 cpupower
34061028 -rw-------. 1 root root  110 Aug  9  2019 crond
34061967 -rw-------. 1 root root 1.4K Apr 11  2018 ebtables-config
1			2       3  4    5     6    7  8    9   10

  • Rwxrxrxrx: file permissions

    • 9 characters, every three characters in a group
      • The first three characters represent the permissions of the primary user
      • The middle three characters represent the permissions of the user group
      • The next three characters represent the permissions of other users
    • r
      • Read permission
      • Represented by the number 4
    • w
      • Writable permissions
      • Represented by the number 2
    • x
      • Executable rights
      • Represented by the number 1
      • No authority

  • Column 789: last modified time of document

    • There are three file times

      • Modification time: the content of modify file has been modified
      • Access time: the time when the content of the access file has been accessed
      • Change time: the time when the change file property changes

    • stat view files all the time

      [root@oldboyedu ~]# stat /etc/hosts
  File: '/etc/hosts'
  Size: 158       	Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d	Inode: 16777794    Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Context: system_u:object_r:net_conf_t:s0
Access: 2020-04-10 19:27:37.963000075 +0800
Modify: 2013-06-07 22:31:32.000000000 +0800
Change: 2020-04-04 18:05:04.725808731 +0800
 Birth: -

      • -c access part

        • %A display character permission

          [root@oldboyedu ~]# stat -c %A /etc/hosts
-rw-r--r--

#Exercise 1. Modify access time and change time
[root@oldboyedu ~]# echo oldboy > test.txt
[root@oldboyedu ~]# stat test.txt 
  File: 'test.txt'
  Size: 7         	Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d	Inode: 34060899    Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Context: unconfined_u:object_r:admin_home_t:s0
Access: 2020-04-10 20:26:41.347028313 +0800
Modify: 2020-04-10 20:26:41.347028313 +0800
Change: 2020-04-10 20:26:41.347028313 +0800
 Birth: -
[root@oldboyedu ~]# echo oldboy >> test.txt
[root@oldboyedu ~]# stat test.txt 
  File: 'test.txt'
  Size: 14        	Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d	Inode: 34060899    Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Context: unconfined_u:object_r:admin_home_t:s0
Access: 2020-04-10 20:26:41.347028313 +0800
Modify: 2020-04-10 20:26:59.226028455 +0800
Change: 2020-04-10 20:26:59.226028455 +0800
 Birth: -

#Exercise 2. Change time
[root@oldboyedu ~]# stat test.txt 
  File: 'test.txt'
  Size: 14        	Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d	Inode: 34060899    Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Context: unconfined_u:object_r:admin_home_t:s0
Access: 2020-04-10 20:26:41.347028313 +0800
Modify: 2020-04-10 20:26:59.226028455 +0800
Change: 2020-04-10 20:26:59.226028455 +0800
 Birth: -
[root@oldboyedu ~]# chmod +x test.txt 
[root@oldboyedu ~]# stat test.txt 
  File: 'test.txt'
  Size: 14        	Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d	Inode: 34060899    Links: 1
Access: (0755/-rwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Context: unconfined_u:object_r:admin_home_t:s0
Access: 2020-04-10 20:26:41.347028313 +0800
Modify: 2020-04-10 20:26:59.226028455 +0800
Change: 2020-04-10 20:29:18.668029566 +0800

  • First column

    • File node

      • The representation is the number 33574978, which is called the file index node
      • Similar to ID card, index node is the unique identification of file in the system
      • To find a file, you have to go through the index node to find it
      • The concept of index node is derived from ext file system (ext2, ext3, ext4)

    • Index node

      • The inode is a piece of storage space of the hard disk, with the size of 256 bytes or 512 bytes
      • The data stored in the index node is the attribute of the file
      • In the index node, only the file name is not included (the file name is in the block of the parent directory)

    • inode and block

      • When the file system is created by formatting, a large number of inode s and block s will be generated
      • inode stores the attribute information of the file and the pointer to the file entity (similar to soft connection)
      • block is used to store the contents of files (large blocks)
      • Block is called disk block, which is the entity unit used to store the actual data (the maximum size of ext file system is generally 4KB). That is to say, it is used to store the real data, such as ordinary file data such as photos and videos. A single large file needs to occupy multiple block blocks for storage. If a single file is too small to occupy the whole block, the remaining space cannot be used.

    • View inode

      [root@oldboyedu /]# df -i
Filesystem      Inodes IUsed   IFree IUse% Mounted on
devtmpfs        480007   370  479637    1% /dev
tmpfs           482665     1  482664    1% /dev/shm
tmpfs           482665  1227  481438    1% /run
tmpfs           482665    16  482649    1% /sys/fs/cgroup
/dev/sda3      9960960 72530 9888430    1% /
/dev/sda1       131072   334  130738    1% /boot
tmpfs           482665     1  482664    1% /run/user/0

    • View block

      [root@oldboyedu /]# df -h
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs        1.9G     0  1.9G   0% /dev
tmpfs           1.9G     0  1.9G   0% /dev/shm
tmpfs           1.9G   12M  1.9G   1% /run
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
/dev/sda3        19G  2.0G   18G  11% /
/dev/sda1       253M  161M   93M  64% /boot
tmpfs           378M     0  378M   0% /run/user/0

    • Two cases of disk full

      • inode full (file inodes are full)
      • block full

    • Tips:

      • no space left no device reports an error. There is no space on the device (the disk is full.)
      • Inspection steps
        • df -h
        • Df-i see if the inode is full
      #Simulated disk full
[root@oldboyedu ~]# mkdir -p /app/logs
[root@oldboyedu ~]# dd if=/dev/zero of=/dev/sdc bs=8K count=10
10+0 records in
10+0 records out
81920 bytes (82 kB) copied, 0.000135709 s, 604 MB/s
[root@oldboyedu ~]# mkfs.ext4 /dev/sdc
mke2fs 1.42.9 (28-Dec-2013)
/dev/sdc is not a block special device.
Proceed anyway? (y,n) y

Filesystem too small for a journal
Discarding device blocks: done                            
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
Stride=0 blocks, Stripe width=0 blocks
16 inodes, 80 blocks
4 blocks (5.00%) reserved for the super user
First data block=1
1 block group
8192 blocks per group, 8192 fragments per group
16 inodes per group

Allocating group tables: done                            
Writing inode tables: done                            
Writing superblocks and filesystem accounting information: done

[root@oldboyedu ~]# mount -o loop /dev/sdc /app/logs/
[root@oldboyedu ~]# df -h
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs        1.9G   52K  1.9G   1% /dev
tmpfs           1.9G     0  1.9G   0% /dev/shm
tmpfs           1.9G   12M  1.9G   1% /run
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
/dev/sda3        19G  2.0G   18G  11% /
/dev/sda1       253M  161M   93M  64% /boot
tmpfs           378M     0  378M   0% /run/user/0
/dev/loop0       73K   14K   54K  21% /app/logs
[root@oldboyedu ~]# cd /app/logs/
[root@oldboyedu /app/logs]# touch {1..15}.txt
touch: cannot touch '6.txt': No space left on device
touch: cannot touch '7.txt': No space left on device
touch: cannot touch '8.txt': No space left on device
touch: cannot touch '9.txt': No space left on device
touch: cannot touch '10.txt': No space left on device
touch: cannot touch '11.txt': No space left on device
touch: cannot touch '12.txt': No space left on device
touch: cannot touch '13.txt': No space left on device
touch: cannot touch '14.txt': No space left on device
touch: cannot touch '15.txt': No space left on device
[root@oldboyedu /app/logs]# df -h
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs        1.9G   52K  1.9G   1% /dev
tmpfs           1.9G     0  1.9G   0% /dev/shm
tmpfs           1.9G   12M  1.9G   1% /run
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
/dev/sda3        19G  2.0G   18G  11% /
/dev/sda1       253M  161M   93M  64% /boot
tmpfs           378M     0  378M   0% /run/user/0
/dev/loop0       73K   15K   53K  23% /app/logs
[root@oldboyedu /app/logs]# df -i
Filesystem      Inodes IUsed   IFree IUse% Mounted on
devtmpfs        480007   375  479632    1% /dev
tmpfs           482665     1  482664    1% /dev/shm
tmpfs           482665  1233  481432    1% /run
tmpfs           482665    16  482649    1% /sys/fs/cgroup
/dev/sda3      9960960 72669 9888291    1% /
/dev/sda1       131072   334  130738    1% /boot
tmpfs           482665     1  482664    1% /run/user/0
/dev/loop0          16    16       0  100% /app/logs
      • Why is inode full?
        • Too many small files
        • During initialization, the number of block s is far greater than the number of inode s

    • Emulated disk full

      [root@oldboyedu /app/logs]# rm -rf *
[root@oldboyedu /app/logs]# ll
[root@oldboyedu /app/logs]# cp /bin/cat ddd
[root@oldboyedu /app/logs]# cp /bin/cat ddddd
cp: error writing 'ddddd': No space left on device
cp: failed to extend 'ddddd': No space left on device
[root@oldboyedu /app/logs]# df -h
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs        1.9G   80K  1.9G   1% /dev
tmpfs           1.9G     0  1.9G   0% /dev/shm
tmpfs           1.9G   12M  1.9G   1% /run
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
/dev/sda3        19G  2.0G   18G  11% /
/dev/sda1       253M  161M   93M  64% /boot
tmpfs           378M     0  378M   0% /run/user/0
/dev/loop0       73K   72K     0 100% /app/logs

  • The third column

    • Number of file hard connections

      • Number of normal file hard connections is 1

      • Number of directory hard connections is 2

        • Because there is also its own directory (.)

          [root@oldboyedu /]# ls -lihd /data/ /data/.
34060900 drwxr-xr-x. 2 root root 137 Apr 10 20:36 /data/
34060900 drwxr-xr-x. 2 root root 137 Apr 10 20:36 /data/.

Posted by Hybrid Kill3r on Fri, 10 Apr 2020 06:44:04 -0700

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