1.Package Management on Red hat Linux
1.1 Yellowdog Updater Modified (Yum)
• Options Available with Yum
option description
search Allows you to search for packages available for download
list Lists all the packages available for download
install Installs a package or packages
check-update Lists packages available for update
update Updates the package specified or downloads and installs any available updates
remove Removes a package or packages from your host
info Provides package information about a package
provides Tells you what package a particular file or feature belongs to
list updates Lists all the packages with updates available only
list available Lists all the available packages only
localinstall Installs a locally downloaded RPM package
clean all Cleans up downloaded package files that are no longer needed
1.2 Configuring Yum
Yum has its configuration files stored under /etc/yum.conf and /etc/yum.repos.d/, and it stores state files in the directories /var/lib/yum and /var/cache/yum.
You will not normally need to make changes to the default /etc/yum.conf. An example of when you might want to change configuration is to add a proxy server that Yum should use when downloading packages. The defaults will generally suit most environments. A full list of the available configuration settings for Yum is available through the yum.conf man page.
$man yum.conf
The files defining what repositories are available to Yum are contained in the /etc/yum.repo.d directory. Let’s look at a generic repository file:
$cat/etc/yum.repo.d/myrepo.repo
[myrepo]
name=myrepo
baseurl=http://myrepo.mydomain.com/pub/linux/releases/$releasever/$basearch/os/
enabled=1
gpgcheck=1
gpgkey=http://myrepo.mydomain.com/linux/RPM-GPG-KEY-linux
option description
[repo-id] The repo_id is the unique name for the repository.
name The name is a description of the repository.
baseurl The baseurl is the URL to the repository. This is generally an HTTP URL, much like
you would use in a web browser.
enabled You can enable or disable the package by specifying 0 for disabled and 1 for enabled.
gpgcheck This option tells Yum to check the GPG keys used to “sign” packages so that you can
be sure the package hasn’t been tampered with. Specifying 0 turns off checking and
1 indicates checking is on.
gpgkey This is the URL where Yum should find the repository’s GPG key.
1.3 Red Hat Package Management (RPM)
• Options Available with rpm
options and Flags description
-q|--query Allows you to query the RPM database and find information about installed
packages on your host
-i|--install Installs a local package on your host
-e|--erase Removes a package from your host
-U|--upgrade Upgrades an existing package or installs a package on your host if it is not
already installed
-F|--freshen Upgrades a package only if an earlier version is installed
-V|--verify Verifies an installed package
#rpm --query --all
This command, which uses the --query flag together with the --all flag, indicating that you are querying all packages, lists all the packages installed.
1.4 Compiling from Source
• Configure
$ ./configure
• Compile and Make
# make
• Install
$ sudo make install
• Uninstall
$sudo make uninstall
2012年3月2日星期五
Users and Groups
1.Introducing sudo
On Red Hat, the sudo command is not enabled by default, and you’ll need to enable it. To do this, you need to use a command called visudo to edit the sudo command’s configu-ration file, /etc/sudoers. To do this, you need to log on as the root user and run the visudo command.
# visudo
把下面这行注释去掉，This enables any member of a group called wheel to use the sudo command.
# %wheel ALL=(ALL) ALL
用下面的命令，把ataylo用户添加到wheel组里，这样ataylor就可以使用sudo命令了。
# usermod –G wheel ataylor
2.Creating Users
$ sudo useradd –m –c 'John Smith' jsmith
Option Description
-c Add a description of the user
-d homedir The user’s home directory
-m Create the user’s home directory
-M Do not create the user’s home directory (Red Hat only)
-s shell Specify the shell the user will use
2.1 User Default Settings
On both Red Hat and Ubuntu distributions, the defaults are contained in the /etc/default/useradd file, and you can display the current defaults using the following command:
$ sudo /usr/sbin/useradd -D 或者 $ sudo cat /etc/default/useradd
内容如下
# useradd defaults file
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
Option Description
SHELL The path to the default shell
HOME The path to the user’s home directory
SKEL The directory to use to provide the default contents of a user’s new home directory
GROUP The default group ID
INACTIVE The maximum number of days after password expiration that a password can be changed
EXPIRE The default expiration date of user accounts
The useradd -D Defaults（例:$ sudo useradd -D -s /bin/bash）
Option Description
-b path/to/default/home Specifies the path prefix of a new user’s home directory
-e date Specifies the default expiration date
-f days Specifies the number of days after a password has expired before the account will be disabled
-g group Specifies the default group
-s shell Specifies the default shell
2.2 Creating Groups
So how do we tell what groups our new user belongs to? To check the details of a particular user, we can use the id command.
$ id jsmith
uid=1003(jsmith) gid=1003(jsmith) groups=1003(jsmith)
• create new groups
$ sudo groupadd printing
$ sudo groupadd finance
• create users to some groups
$ sudo useradd -m -c 'Anne Taylor' -G printing,finance ataylor
2.3 Deleting Users and Groups
• delete a user
$ sudo userdel ataylor
You can force Linux to delete the user’s home directory using the –r option of the userdel command. This will delete the /home/username directory and all files in it, but it won’t delete any files outside of this directory that might also belong to the user.
But if you do decide to delete a user, you can run the command find / -user UID –o –group GID to find all the files associated with the user you have just deleted.
• delete a group
$ sudo groupdel finance
2.4 Passwords
$ passwd
$ sudo passwd jsmith
2.5 Password Aging
$ sudo chage -M 30 ataylor
2.6 Disabling Users
As the root user, you can also use the passwd command to disable and enable user accounts using the –l, or lock, option. For example, consider the following:
$ sudo passwd –l ataylor
You can then unlock the user using the –u, or unlock, option.
$ sudo passwd –u ataylor
This doesn’t lock a user out but disables the user’s getting shell access.
$ sudo usermod –s /sbin/nologin
2.7 Storing User and Group Data
The first file, /etc/passwd, contains a list of all users and their details. Listing 4-12 shows examples of some passwd entries.
Listing 4‑12. /etc/passwd Entries
root:x:0:0:root:/root:/bin/bash
daemon:x:2:2:daemon:/sbin:/sbin/nologin
Each entry can be broken into its component pieces, separated by a colon.
username:password:UID:GID:GECOS:Home Directory:Shell
↑备注，说明
On Linux hosts, information about groups is stored in the /etc/groups file. Listing 4-13 shows a sample from this file.
Listing 4‑13. Sample of the /etc/groups File
root:x:0:root
ataylor:x:501:finance,printing
The /etc/group file is structured much like the /etc/passwd file, with the data separated by a colon. The file is broken into a group name, a password, the GID, and a comma- separated list of the members of that group.
groupname:password:GID:member,member
↑组下面的用户
2.8 Configuring Your Shell and Environment
• Environment Variables
Name Used For
HOME The user’s home directory
LANG Defines which language files applications should use
LS_COLORS Defines colors used by the ls command
MAIL The location of the user’s mailbox
PATH A colon-separated list of directories where shells look for executable files
PS1 Defines the normal prompt
SHELL The current shell
_ Contains the last command executed in this session
• Command aliases
$ alias rm='rm -i'
$ unalias rm
$ alias
On Red Hat, the sudo command is not enabled by default, and you’ll need to enable it. To do this, you need to use a command called visudo to edit the sudo command’s configu-ration file, /etc/sudoers. To do this, you need to log on as the root user and run the visudo command.
# visudo
把下面这行注释去掉，This enables any member of a group called wheel to use the sudo command.
# %wheel ALL=(ALL) ALL
用下面的命令，把ataylo用户添加到wheel组里，这样ataylor就可以使用sudo命令了。
# usermod –G wheel ataylor
2.Creating Users
$ sudo useradd –m –c 'John Smith' jsmith
Option Description
-c Add a description of the user
-d homedir The user’s home directory
-m Create the user’s home directory
-M Do not create the user’s home directory (Red Hat only)
-s shell Specify the shell the user will use
2.1 User Default Settings
On both Red Hat and Ubuntu distributions, the defaults are contained in the /etc/default/useradd file, and you can display the current defaults using the following command:
$ sudo /usr/sbin/useradd -D 或者 $ sudo cat /etc/default/useradd
内容如下
# useradd defaults file
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
Option Description
SHELL The path to the default shell
HOME The path to the user’s home directory
SKEL The directory to use to provide the default contents of a user’s new home directory
GROUP The default group ID
INACTIVE The maximum number of days after password expiration that a password can be changed
EXPIRE The default expiration date of user accounts
The useradd -D Defaults（例:$ sudo useradd -D -s /bin/bash）
Option Description
-b path/to/default/home Specifies the path prefix of a new user’s home directory
-e date Specifies the default expiration date
-f days Specifies the number of days after a password has expired before the account will be disabled
-g group Specifies the default group
-s shell Specifies the default shell
2.2 Creating Groups
So how do we tell what groups our new user belongs to? To check the details of a particular user, we can use the id command.
$ id jsmith
uid=1003(jsmith) gid=1003(jsmith) groups=1003(jsmith)
• create new groups
$ sudo groupadd printing
$ sudo groupadd finance
• create users to some groups
$ sudo useradd -m -c 'Anne Taylor' -G printing,finance ataylor
2.3 Deleting Users and Groups
• delete a user
$ sudo userdel ataylor
You can force Linux to delete the user’s home directory using the –r option of the userdel command. This will delete the /home/username directory and all files in it, but it won’t delete any files outside of this directory that might also belong to the user.
But if you do decide to delete a user, you can run the command find / -user UID –o –group GID to find all the files associated with the user you have just deleted.
• delete a group
$ sudo groupdel finance
2.4 Passwords
$ passwd
$ sudo passwd jsmith
2.5 Password Aging
$ sudo chage -M 30 ataylor
2.6 Disabling Users
As the root user, you can also use the passwd command to disable and enable user accounts using the –l, or lock, option. For example, consider the following:
$ sudo passwd –l ataylor
You can then unlock the user using the –u, or unlock, option.
$ sudo passwd –u ataylor
This doesn’t lock a user out but disables the user’s getting shell access.
$ sudo usermod –s /sbin/nologin
2.7 Storing User and Group Data
The first file, /etc/passwd, contains a list of all users and their details. Listing 4-12 shows examples of some passwd entries.
Listing 4‑12. /etc/passwd Entries
root:x:0:0:root:/root:/bin/bash
daemon:x:2:2:daemon:/sbin:/sbin/nologin
Each entry can be broken into its component pieces, separated by a colon.
username:password:UID:GID:GECOS:Home Directory:Shell
↑备注，说明
On Linux hosts, information about groups is stored in the /etc/groups file. Listing 4-13 shows a sample from this file.
Listing 4‑13. Sample of the /etc/groups File
root:x:0:root
ataylor:x:501:finance,printing
The /etc/group file is structured much like the /etc/passwd file, with the data separated by a colon. The file is broken into a group name, a password, the GID, and a comma- separated list of the members of that group.
groupname:password:GID:member,member
↑组下面的用户
2.8 Configuring Your Shell and Environment
• Environment Variables
Name Used For
HOME The user’s home directory
LANG Defines which language files applications should use
LS_COLORS Defines colors used by the ls command
MAIL The location of the user’s mailbox
PATH A colon-separated list of directories where shells look for executable files
PS1 Defines the normal prompt
SHELL The current shell
_ Contains the last command executed in this session
• Command aliases
$ alias rm='rm -i'
$ unalias rm
$ alias
Linux Basics
0.Others
• 启动到text模式
修改/etc/inittab文件，将id:5:initdefault:一行中的5改为3
• 启动到xwindow模式
init 5
1.Getting Help
• man ls
• ls --help
• man -K user
This would search all man pages for the keyword user and return a list of all man pages con-taining
the term. You will then be prompted to view each page that is returned, skip a page and go to the next
page, or quit the search.
• whatis useradd
• apropos whoam
The apropos command also searches the whatis database but searches for strings rather than complete
words.
2.Users and Groups
• Users
/etc/passwd
• Groups
/etc/group
3.Services and processes
• ps -A
ps command with the -A flag (for all) to list all the processes currently running on our host.
• top
The top command starts an interactive monitoring tool that updates every few seconds with the top
running processes on your host.
4.Files and File Systems
• pwd
• linux directory structure
Directory Description
/bin/ User commands and binaries.
/boot/ Files used by the boot loader. (We talk about boot loaders in Chapter 5.)
/dev/ Device files.
/etc/ System configuration files.
/home/ User’s home directories.
/lib/ Shared libraries and kernel modules.
/media/ Removable media is usually mounted here (see Chapter 8).
/mnt/ Temporary mounted file systems are usually mounted here (see Chapter 8).
/opt/ Add-on application software packages.
/proc/ Kernel and process status data is stored in here in text- file format.
/root/ The root user’s home directory.
/sbin/ System binaries.
/srv/ Data for services provided by this host.
/tmp/ Directory for temporary files.
/usr/ User utilities, libraries, and applications.
/var/ Variable or transient files and data, for example logs, mail queues, and print jobs.
• ls(ls-la)
the l and a switches have been added to the ls command. The l switch, which is an abbreviation of
long, uses a long listing format, which as you can see shows a lot more information. The a switch tells
ls to list all files and directories, even hidden ones.
-rw-r--r--1rootroot02008-07-1520:47.autofsck
drwxr-xr-x2rootroot40962008-05-1804:11bin
Each line of the listing contains seven pieces of information about each object:
- Unix file type
- Permissions
- Number of hard links
- User and group ownership
- Size
- Time and date
- Name
4.1 File Types and Permissions
• File types
Type Description
- File
d Directory
l Link
c Character devices
b Block devices
s Socket
p Named pipe
• Permissions
Each file on your host has three classes of permissions:
- User
- Group
- Other (everyone else)
- 表示删除权限
= 表示使之成为唯一的权限
• df -h
This command displays all of your disks and storage devices and the free space present on them.
We’ve executed the command and added the -h switch, which returns human- readable sizes.
4.4 Date and Time
• ls-lu
display the last accessed time for a file by listing it with the -u switch
• ls-lc
list creation dates by using the -c switch
• date
Using date on the command line without any options will return the current time and date like so:
Tue Aug 1913:01:20 EST 2008
5.Working with Files
5.1 Reading Files
• cat /etc/hosts
The cat command is so named because it “concatenates and prints files
• less /etc/services
From inside the less interface, you can scroll through the file.
- spacebar : 下一页
- Enter key: 下一行To scroll backward,
- B key : 上一行（也可以用上下键）
- Q key : 退出
• grep localhost /etc/hosts
you can see a very simple grep search for the string localhost in the file /etc/hosts.
$greplocalhost/etc/hosts
127.0.0.1 localhost.localdomain localhost localhost
You can also recursively search down into lower directories by adding the -r switch like so:
$grep -r localhost /etc
5.2 Searching for Files
• find /home/ -type f -iname myfile*
The -iname option searches for a case- insensitive pattern, in this case, all files starting with myfile.
• find /home/ -type f -iname myfile* -exec grep mail {} \;
• find / -nouser -o -nogroup
This command, run as root, will search the whole directory tree for any files that don’t belong to a valid
user or group.
5.3 Copying Files
• cp -i /home/jsmith/*.c ./
We’ve also added the -i switch to make sure we’re prompted if a file already exists.
• cp -r /home/jsmith /backup
The previous line copies the /home/jsmith directory and all files and directories beneath it to the /backup
directory.
• cp -p /home/jsmith /backup
Lastly, when copying files using the cp command, some items about the file, such as dates, times, and
permissions, can be changed or updated. If you want to preserve the original values on the copy, you can
use the -p switch.
• cat /home/jsmith/myfile > /home/jsmith/yourfile
The use of the > symbol sends the output from one command to the command or action on the other side
of the > symbol. In this case, the output of the cat command is redirected into a file called yourfile.
If this file doesn’t exist, it will be created. If it does exist, its content will be overwritten.
• cat /home/jsmith/myfile >> /home/jsmith/yourfile
Using the >> syntax will append the output from the cat of myfile to the end of yourfile.
If yourfile does not exist, it will be created.
5.4 Moving and Renaming Files
• mv -i ~/myfile ~/mynewfile
The -i option again ensures we get prompted if the target file already exists. You can also rename files in
place with the mv command:
5.5 Deleting Files
• rm -fr /home/jsmith/backup
5.6 Linking Files
• ln -s /home/jsmith/myfile
The previous line would create a symbolic link called myfile to the /home/jsmith/myfile file.
5.7 Linking Files
• vi file
• 启动到text模式
修改/etc/inittab文件，将id:5:initdefault:一行中的5改为3
• 启动到xwindow模式
init 5
1.Getting Help
• man ls
• ls --help
• man -K user
This would search all man pages for the keyword user and return a list of all man pages con-taining
the term. You will then be prompted to view each page that is returned, skip a page and go to the next
page, or quit the search.
• whatis useradd
• apropos whoam
The apropos command also searches the whatis database but searches for strings rather than complete
words.
2.Users and Groups
• Users
/etc/passwd
• Groups
/etc/group
3.Services and processes
• ps -A
ps command with the -A flag (for all) to list all the processes currently running on our host.
• top
The top command starts an interactive monitoring tool that updates every few seconds with the top
running processes on your host.
4.Files and File Systems
• pwd
• linux directory structure
Directory Description
/bin/ User commands and binaries.
/boot/ Files used by the boot loader. (We talk about boot loaders in Chapter 5.)
/dev/ Device files.
/etc/ System configuration files.
/home/ User’s home directories.
/lib/ Shared libraries and kernel modules.
/media/ Removable media is usually mounted here (see Chapter 8).
/mnt/ Temporary mounted file systems are usually mounted here (see Chapter 8).
/opt/ Add-on application software packages.
/proc/ Kernel and process status data is stored in here in text- file format.
/root/ The root user’s home directory.
/sbin/ System binaries.
/srv/ Data for services provided by this host.
/tmp/ Directory for temporary files.
/usr/ User utilities, libraries, and applications.
/var/ Variable or transient files and data, for example logs, mail queues, and print jobs.
• ls(ls-la)
the l and a switches have been added to the ls command. The l switch, which is an abbreviation of
long, uses a long listing format, which as you can see shows a lot more information. The a switch tells
ls to list all files and directories, even hidden ones.
-rw-r--r--1rootroot02008-07-1520:47.autofsck
drwxr-xr-x2rootroot40962008-05-1804:11bin
Each line of the listing contains seven pieces of information about each object:
- Unix file type
- Permissions
- Number of hard links
- User and group ownership
- Size
- Time and date
- Name
4.1 File Types and Permissions
• File types
Type Description
- File
d Directory
l Link
c Character devices
b Block devices
s Socket
p Named pipe
• Permissions
Each file on your host has three classes of permissions:
- User
- Group
- Other (everyone else)
With chmod, each class is abbreviated to a single letter:
u: User
g: Group
o: Other or everyone
a: All
chmod u+x myfile
chmod u-x,og+w myfile
chmod 654 myfile
行动：
+ 表示添加权限 - 表示删除权限
= 表示使之成为唯一的权限
We can also apply the permissions of one class to another class by using the = symbol:
chmod u=g myfile --用户和组的权限一样
You can also set permissions for multiple files by listing each file separated by space like so:
chmod u+r file1 file2 file3
You can also use the asterisk symbol to specify all files and add the -R switch to recurse into lower
directories like so:
chmod -R u+x /usr/local/bin/*
其中：rwx也可以用数字来代替
r ------------4
w -----------2
x ------------1
- ------------0
当大家都明白了上面的东西之后，那么我们常见的以下的一些权限就很容易都明白了：
-rw------- (600) 只有所有者才有读和写的权限
-rw-r--r-- (644) 只有所有者才有读和写的权限，组群和其他人只有读的权限
-rwx------ (700) 只有所有者才有读，写，执行的权限
-rwxr-xr-x (755) 只有所有者才有读，写，执行的权限，组群和其他人只有读和执行的权限
-rwx--x--x (711) 只有所有者才有读，写，执行的权限，组群和其他人只有执行的权限
-rw-rw-rw- (666) 每个人都有读写的权限
-rwxrwxrwx (777) 每个人都有读写和执行的权限
r ------------4
w -----------2
x ------------1
- ------------0
当大家都明白了上面的东西之后，那么我们常见的以下的一些权限就很容易都明白了：
-rw------- (600) 只有所有者才有读和写的权限
-rw-r--r-- (644) 只有所有者才有读和写的权限，组群和其他人只有读的权限
-rwx------ (700) 只有所有者才有读，写，执行的权限
-rwxr-xr-x (755) 只有所有者才有读，写，执行的权限，组群和其他人只有读和执行的权限
-rwx--x--x (711) 只有所有者才有读，写，执行的权限，组群和其他人只有执行的权限
-rw-rw-rw- (666) 每个人都有读写的权限
-rwxrwxrwx (777) 每个人都有读写和执行的权限
4.2 Users, Groups, and Ownership
use the chown command to change user and group ownership.
chown jsmith myfile
chown jsmith:admin myfile
chown-R jsmith:admin /home/jsmith/*
4.3 Size and Space
• ls-lh
-rw-rw-r--1jsmithjsmith51K2008-08-1723:47myfile
On the previous line, you can see that the myfile file is 51 kilobytes in size.
• du-sh /usr/local/bin
4.7M /usr/local/bin
The -s switch summarizes the total, and the -h switch displays the size in a human- readable form.
This command displays all of your disks and storage devices and the free space present on them.
We’ve executed the command and added the -h switch, which returns human- readable sizes.
4.4 Date and Time
• ls-lu
display the last accessed time for a file by listing it with the -u switch
• ls-lc
list creation dates by using the -c switch
• date
Using date on the command line without any options will return the current time and date like so:
Tue Aug 1913:01:20 EST 2008
5.Working with Files
5.1 Reading Files
• cat /etc/hosts
The cat command is so named because it “concatenates and prints files
• less /etc/services
From inside the less interface, you can scroll through the file.
- spacebar : 下一页
- Enter key: 下一行To scroll backward,
- B key : 上一行（也可以用上下键）
- Q key : 退出
• grep localhost /etc/hosts
you can see a very simple grep search for the string localhost in the file /etc/hosts.
$greplocalhost/etc/hosts
127.0.0.1 localhost.localdomain localhost localhost
You can also recursively search down into lower directories by adding the -r switch like so:
$grep -r localhost /etc
5.2 Searching for Files
• find /home/ -type f -iname myfile*
The -iname option searches for a case- insensitive pattern, in this case, all files starting with myfile.
• find /home/ -type f -iname myfile* -exec grep mail {} \;
• find / -nouser -o -nogroup
This command, run as root, will search the whole directory tree for any files that don’t belong to a valid
user or group.
5.3 Copying Files
• cp -i /home/jsmith/*.c ./
We’ve also added the -i switch to make sure we’re prompted if a file already exists.
• cp -r /home/jsmith /backup
The previous line copies the /home/jsmith directory and all files and directories beneath it to the /backup
directory.
• cp -p /home/jsmith /backup
Lastly, when copying files using the cp command, some items about the file, such as dates, times, and
permissions, can be changed or updated. If you want to preserve the original values on the copy, you can
use the -p switch.
• cat /home/jsmith/myfile > /home/jsmith/yourfile
The use of the > symbol sends the output from one command to the command or action on the other side
of the > symbol. In this case, the output of the cat command is redirected into a file called yourfile.
If this file doesn’t exist, it will be created. If it does exist, its content will be overwritten.
• cat /home/jsmith/myfile >> /home/jsmith/yourfile
Using the >> syntax will append the output from the cat of myfile to the end of yourfile.
If yourfile does not exist, it will be created.
5.4 Moving and Renaming Files
• mv -i ~/myfile ~/mynewfile
The -i option again ensures we get prompted if the target file already exists. You can also rename files in
place with the mv command:
5.5 Deleting Files
• rm -fr /home/jsmith/backup
5.6 Linking Files
• ln -s /home/jsmith/myfile
The previous line would create a symbolic link called myfile to the /home/jsmith/myfile file.
5.7 Linking Files
• vi file
vim 替换^M（转）
【原因】
回车表示：dos中CRLF 回车(CR, ASCII 13, r) 换行(LF, ASCII 10, n)，两个符号。linux中只有一个。
在Linux下使用vi来查看一些在Windows下创建的文本文件，有时会发现在行尾有一些“^M”。
【処理方法】
1.使用dos2unix命令。一般的分发版本中都带有这个小工具，使用起来很方便:
$ dos2unix myfile.txt
$ dos2unix myfile.txt
2.使用vi的替换功能。启动vi，进入命令模式，输入以下命令:
:%s/^M$//g # 去掉行尾的^M。
:%s/^M$//g # 去掉行尾的^M。
:%s/^M//g # 去掉所有的^M。
:%s/^M/[ctrl-v]+[enter]/g # 将^M替换成回车。
:%s/^M/r/g # 将^M替换成回车。
注意：这里的“^M”要使用“CTRL-V CTRL-M”生成，而不是直接键入“^M”。
2012年2月28日星期二
2012年2月19日星期日
Mounting cdrom in cent os 5 linux
CDRROM, type as root
For DVD
Code:
mount /dev/cdrom /mnt
Code:
mount /dev/dvd /mnt
2011年11月28日星期一
使用SELECT * 语句的缺点（转）
使用SELECT * 语句的缺点
1、SELECT * 语句取出表中的所有字段，不论该字段的数据对调用的应用程序是否有用，这会对服务器资源造成浪费，甚至会对服务器的性能产生一定的影响。
2、如果表的结构在以后发生了改变，那么SELECT * 语句可能会取到不正确的数据甚至是出错。使用字段名可以快速定位影响范围。
3、执行SELECT * 语句时，SQL Server首先要查找出表中有哪些列，然后才能开始执行SELECT * 语句，这在某些情况会产生性能问题。
4、使用SELECT * 语句将不会使用到覆盖索引，不利于查询的性能优化。
5、在文档角度来看，SELECT * 语句没有列明将要取出哪些字段进行操作，所以也是不推荐的。
2、如果表的结构在以后发生了改变，那么SELECT * 语句可能会取到不正确的数据甚至是出错。使用字段名可以快速定位影响范围。
3、执行SELECT * 语句时，SQL Server首先要查找出表中有哪些列，然后才能开始执行SELECT * 语句，这在某些情况会产生性能问题。
4、使用SELECT * 语句将不会使用到覆盖索引，不利于查询的性能优化。
5、在文档角度来看，SELECT * 语句没有列明将要取出哪些字段进行操作，所以也是不推荐的。
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