随机数、熵池
bash可以生成随机数，范围是0-32768【65535/2】，它是一个系统变量$RANDOM
[jiakang@jiakang tmp]$ echo $RANDOM
15163
[jiakang@jiakang tmp]$ echo $RANDOM
23259
[jiakang@jiakang tmp]$ echo $RANDOM
25631随机数生成器：熵池
熵池是存放随机数的池子，我们上面bash生成的随机数安全性很低，甚至是通过算法可以破解，所以出现了熵池；
熵池是让/dev/random使用的，它产生的随机数是硬件不规律形成的，比如我们间隔按鼠标的时间，安全性非常高，熵池中的值是剪切过来的，所以可能会被用完出现阻塞用户的进程。
/dev/urandom:也是取熵池中的随机数，但是取完后会用bash自动模拟，不会出现阻塞。
/dev/random
/dev/urandom
mknod创建设备
mknod mydev c 66 0【c代表字符设备,ls – l每行的最前面那个字符代表文件类型,66代表主设备号，0代表次设备号】
[root@jiakang a]# mknod mydev c 66 0
[root@jiakang a]# ls -il
总用量 28
412296 drwxr-xr-x. 3 root root 4096 3月 28 16:34 b
412310 -rwxr-xr-x. 1 root root 133 4月 13 14:34 case.sh
412301 crw-r--r--. 1 root root 66, 0 4月 15 18:42 mydev还可以在创建设备的时候指定使用权限
[root@jiakang a]# mknod -m 640 mydev1 c 66 1
[root@jiakang a]# ls -l
总用量 28
drwxr-xr-x. 3 root root 4096 3月 28 16:34 b
-rwxr-xr-x. 1 root root 133 4月 13 14:34 case.sh
crw-r--r--. 1 root root 66, 0 4月 15 18:42 mydev
crw-r-----. 1 root root 66, 1 4月 15 18:47 mydev1当我们有个硬件连接到linux上面，这个设备会有个自己的设备号，然后我们的linux也有这个设备号，所以就对接上了。linux向这个设备号发送东西，就会发送到对应的硬件上。
下面是两个设备【终端设备】：
[root@jiakang a]# tty
/dev/pts/0
[jiakang@jiakang ~]$ tty
/dev/pts/1
[root@jiakang a]# echo "hello jiakang" >> /dev/pts/1
[jiakang@jiakang ~]$ hello jiakang根目录的重要性
根目录是文件系统的访问路口，如：/a/b，现在把b单独分区了，我要想访问b分区，必须先找到根目录的/a/b，b文件存的是b分区的访问入口；在/a/b/c中，现在再把c单独分区，那访问的时候先找根分区然后找到b文件相应的找到了b分区，再找c文件找到c分区。所以如果根很重要，根分区崩了，其他分区也没了
fdisk查看/创建分区
查看
查看磁盘、分区的使用情况 fdisk -l [/dev/PATH]
[root@jiakang ~]# fdisk -l盘【查看磁盘的使用情况】
Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000478b0
Device Boot Start End Blocks Id System
/dev/sda1 * 1 26 204800 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2 26 287 2097152 82 Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3 287 1306 8182784 83 Linux
[root@jiakang ~]# fdisk -l /dev/sda1【查看某个分区的使用情况】
Disk /dev/sda1: 209 MB, 209715200 bytes
255 heads, 63 sectors/track, 25 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000
创建、创建
fdisk /dev/sda
p：显示当前硬件的分区，包括没保存的改动
n：创建新分区【e：扩展分区 p：主分区】
d：删除一个分区
w：保存退出
q：不保存退出
t：修改分区类型【L：显示支持的分区类型】
l：显示所支持的所有类型
下面的代码我要创建一个新的主分区，但是没有多余的柱面【空间】了
[root@jiakang ~]# fdisk /dev/sda【管理磁盘分区】
WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
switch off the mode (command 'c') and change display units to
sectors (command 'u').
Command (m for help): m【m获取帮助】
Command action
a toggle a bootable flag
b edit bsd disklabel
c toggle the dos compatibility flag
d delete a partition
l list known partition types
m print this menu
n add a new partition
o create a new empty DOS partition table
p print the partition table
q quit without saving changes
s create a new empty Sun disklabel
t change a partition's system id
u change display/entry units
v verify the partition table
w write table to disk and exit
x extra functionality (experts only)
Command (m for help): p【打印显示当前分区使用状况】
Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders【共1305个柱面】
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000478b0
Device Boot Start End Blocks Id System
/dev/sda1 * 1 26 204800 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2 26 287 2097152 82 Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3 287 1306 8182784 83 Linux
Command (m for help): n【创建新分区】
Command action
e extended
p primary partition (1-4)
p【创建主分区】
Selected partition 4
No free sectors available
Command (m for help): q【退出】
...删除第3个主分区，然后再重新划分柱面
...
...
Command (m for help): d【删除分区】
Partition number (1-4): 3【删除第三个主分区】
Command (m for help): p
Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000478b0
Device Boot Start End Blocks Id System
/dev/sda1 * 1 26 204800 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2 26 287 2097152 82 Linux swap / Solaris
Partition 2 does not end on cylinder boundary.创建主分区3，扩展分区4，逻辑分区5
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 3
First cylinder (287-1305, default 287): 【起始柱面，这直接回车就是默认】
Using default value 287
Last cylinder, +cylinders or +size{K,M,G} (287-1305, default 1305): +2G【结束柱面，指定大小】
Command (m for help): p
Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000478b0
Device Boot Start End Blocks Id System
/dev/sda1 * 1 26 204800 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2 26 287 2097152 82 Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3 287 548 2098834 83 Linux【这是新增的主分区3柱面287-548】
Command (m for help): n
Command action
e extended
p primary partition (1-4)
e【创建扩展分区】
Selected partition 4
First cylinder (549-1305, default 549): 【回车】
Using default value 549
Last cylinder, +cylinders or +size{K,M,G} (549-1305, default 1305): 【回车】
Using default value 1305
Command (m for help): n
First cylinder (549-1305, default 549):
Using default value 549
Last cylinder, +cylinders or +size{K,M,G} (549-1305, default 1305): +1G
Command (m for help): p
Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000478b0
Device Boot Start End Blocks Id System
/dev/sda1 * 1 26 204800 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2 26 287 2097152 82 Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3 287 548 2098834 83 Linux
/dev/sda4 549 1305 6080602+ 5 Extended
/dev/sda5 549 680 1060258+ 83 Linux【类型编号83，类型Linux】上面的sda5类型编号是83，类型为Linux，我们可以调到其他类型
Command (m for help): t【修改分区类型】
Partition number (1-5): 5【修改第5个分区】
Hex code (type L to list codes): L【查看有哪些分区类型，83就是刚才的Linux】
0 Empty 24 NEC DOS 81 Minix / old Lin bf Solaris
1 FAT12 39 Plan 9 82 Linux swap / So c1 DRDOS/sec (FAT-
2 XENIX root 3c PartitionMagic 83 Linux c4 DRDOS/sec (FAT-
3 XENIX usr 40 Venix 80286 84 OS/2 hidden C: c6 DRDOS/sec (FAT-
4 FAT16 <32M 41 PPC PReP Boot 85 Linux extended c7 Syrinx
5 Extended 42 SFS 86 NTFS volume set da
...
...
Hex code (type L to list codes): 82【调整为82交换分区】
Changed system type of partition 5 to 82 (Linux swap / Solaris)
Command (m for help): p
Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000478b0
Device Boot Start End Blocks Id System
/dev/sda1 * 1 26 204800 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2 26 287 2097152 82 Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3 287 548 2098834 83 Linux
/dev/sda4 549 1305 6080602+ 5 Extended
/dev/sda5 549 680 1060258+ 82 Linux swap / Solaris【改为了82】w 保存退出修改
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
WARNING: Re-reading the partition table failed with error 16: 设备或资源忙.
The kernel still uses the old table. The new table will be used at
the next reboot or after you run partprobe(8) or kpartx(8)
Syncing disks.
[root@jiakang ~]# fdisk -l
Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000478b0
Device Boot Start End Blocks Id System
/dev/sda1 * 1 26 204800 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2 26 287 2097152 82 Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3 287 548 2098834 83 Linux
/dev/sda4 549 1305 6080602+ 5 Extended
/dev/sda5 549 680 1060258+ 82 Linux swap / Solarispartprobe让内核重新识别分区表
上面的分区表成功后需要内核去识别，/proc/partitions目录是当前内核识别的分区表，可以通过partprobe去让内核重新识别，但是在这可能遇到下面的错误
[root@jiakang ~]# cat /proc/partitions
major minor #blocks name
8 0 10485760 sda
8 1 204800 sda1
8 2 2097152 sda2
8 3 8182784 sda3
[root@jiakang ~]# partprobe
Warning: WARNING: the kernel failed to re-read the partition table on /dev/sda (设备或资源忙). As a result, it may not reflect all of your changes until after reboot.
Warning: 无法以读写方式打开 /dev/sr0 (只读文件系统)。/dev/sr0 已按照只读方式打开。
Warning: 无法以读写方式打开 /dev/sr0 (只读文件系统)。/dev/sr0 已按照只读方式打开。
Error: 无效的分区表 - /dev/sr0 出现递归分区。
[root@jiakang ~]# cat /proc/partitions
major minor #blocks name
8 0 10485760 sda
8 1 204800 sda1
8 2 2097152 sda2
8 3 8182784 sda3
[root@jiakang ~]# ls -l /dev/sr0
brw-rw----+ 1 root cdrom 11, 0 4月 16 09:55 /dev/sr0从上面的结果看，sr0是光驱设备，百度了下/dev/sr0这个设备，也所是光驱。于是管它重载分区表失败是不是由于“Error: 无效的分区表 – /dev/sr0 出现递归分区。”引起的，先把光驱移除了再说，反正是虚拟机上。
把虚拟机上的光驱设备移除后，重新启动系统，再重新新建分区，并重新执行partprobe命令重载分区表，错误信息“Error: 无效的分区表 – /dev/sr0 出现递归分区。”没有了，只是警告提示“设备或资源忙”还在。这时直接格式化新建的逻辑分区/dev/sda5还是提示“没有那个文件或目录”。于是重启系统，再格式化/dev/sda5，这回成功了。
也就是说，之前新建分区格式化失败，是由于虚拟机的光驱/dev/sr0设备导致了分区表递归。另外由于是在同一块硬盘上操作，所以必须要重启系统才能使新建的分区写入分区表生效。
卸载光驱后：
[root@jiakang ~]# partprobe
Warning: WARNING: the kernel failed to re-read the partition table on /dev/sda (设备或资源忙). As a result, it may not reflect all of your changes until after reboot.
[root@jiakang ~]# cat /proc/partitions
major minor #blocks name
8 0 10485760 sda
8 1 204800 sda1
8 2 2097152 sda2
8 3 8182784 sda3重启系统
MDZZ！！！不知道有没有发现上面的刚开始柱面不足,我删除了第3个主分区，我的根目录及所有的文件都在里面，相当于删除了根目录，导致系统崩溃：“客户机操作系统已禁用 CPU。请关闭或重置虚拟机 ”
重新安装了一下linux，这次我加载了两块磁盘，新建了sdb1分区：
[root@localhost ~]# fdisk -l
Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0002b305
Device Boot Start End Blocks Id System
/dev/sda1 * 1 26 204800 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2 26 548 4194304 83 Linux
Partition 2 does not end on cylinder boundary.
/dev/sda3 548 809 2097152 82 Linux swap / Solaris
Partition 3 does not end on cylinder boundary.
/dev/sda4 809 1306 3988480 5 Extended
/dev/sda5 810 1306 3987456 83 Linux
Disk /dev/sdb: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xd25c91c2
Device Boot Start End Blocks Id System
/dev/sdb1 1 200 1606468+ 83 Linux然而这次发现不用内核去识别了，直接就可以自动去识别，而且还可以格式化【我sdb1是新建的刚刚】
[root@localhost ~]# cat /proc/partitions
major minor #blocks name
8 0 10485760 sda
8 1 204800 sda1
8 2 4194304 sda2
8 3 2097152 sda3
8 4 1 sda4
8 5 3987456 sda5
8 16 10485760 sdb
8 17 1606468 sdb1
[root@localhost ~]# mkfs -t ext4 /dev/sdb1
mke2fs 1.41.12 (17-May-2010)
文件系统标签=
操作系统:Linux
...
...查阅资料获取的信息是：不是当前使用的磁盘不需要让内核重新去获取了，教训就是没事就快照，别不小心删了根目录！！
partx
redhat6的partprobe失效可以用partx
[root@localhost ~]# partx -a /dev/sdb1 /dev/sdb