Chap4:探究操作系统[The Linux Command Line]

1 learn some more commands:

　　(1) ls-List directory contents

　　(2) file -Determine file type

　　(3) less-View file contents

2 ls

　　(1)Type ls to see a list of files and subdirectories contained in the current working directory.

　　(2)Besides the current working directory,we can specify the directory to list,like   ls /user

　　(3)Or even specify multiple directories,like   ls ~ / user (home directory)

　　(4)change the format of the ouput to reveal more detail,like  ls - l,we changed the output to the long format.

3 options and arguments

　　The ls command has a large number of possible options.

　　(1)-a　　--all　　List all filee,even those with names that begin with a period,which are normally not listed(i.e.hidden)

　　(2)-d　　--directory　　Ordinaryly,if a directory is specified, ls will list the contents of the directory, not the directory itself. Use this option in conjunction with the -l option to see details about the directory rather than its contents.

　　(3)-F　　--classify　　This option will append an indicator character to the end of each listed name.

　　(4)-h　　--human-readable　　In long format listings ,diaplay file sizes in human readable format rather than in bytes.

　　(5)-l　　　　　　Display results in long format.

　　(6)-r　　--reverse　　Display the results in reverse order.Normally,ls display its results in ascending alphabetical order.

　　(7)-S　　　　　Sort results by file size.

　　(8)-t　　　　　 Sort by modification time.

4 long format output

　　This format contains a great deal of useful imformation.

　　format-　　-rw-r--r--　　file's number of hard links　the user name of the file's owner　　the name of the group which owns the file　　size of the file in bytes　　date and time of the file's last modification　　name of the file　

　　here is the examples directory from an Ubuntu system:

　　-rw-r--r-- 1 root root 3576296 2007-04-03 11:05 Experience ubuntu.ogg

5 determine  a file's type

　　The file command will print a brief description of the file's contents.

6 view text files　

　　The  less command is a program to view text files. The table below lists the most common keyboard commands used by less.

　　(1) Page Up or b-Scroll back one page

　　(2) Page Down or space-Scroll forward one page

　　(3) UP Arrow-Scroll Up one line

　　(4) Down Arrow-Scrow Down one line

　　(5) G -Move to the end of the text file

　　(6) 1G or g-Move to the beginning of the text file

　　(7) /characters-Search forward for the next occurrence of characters

　　(8) n-Search forward for the next occurrence of the previous search

　　(9) h-Display help screen

　　(10) q-Quit less

7 less is more

　　The less program was designed as an improved replacement of an earlier Unix program called more.less falls into the class of programs called "pagers", programs that allow the easy viewing of long text documents in a page by page manner.Whereas the more program could only page forward, the less program allows paging both forward and backward and has many other features as well.

8 wander around the file system

　　list just a few of the directories we can explore.

/	The root directory.Where everything begins.
/bin	Contains binaries (programs) that must be present for the system to boot and run.
/boot	Contains the linux kernel, intial RAM disk image (for drivers needed at boot time), and the boot loader. Interesting files: /boot/grub/grub.conf or menu.lst, which are used to configure the boot loader. /boot/vmlinuz,the linux kernel.
/dev	This is a special directory which contains device nodes. "Everything is a file" also applies to devices. Here is where the kernel maintains a list of all the devices it understands.
/etc	The /etc directory contains all of the system-wide configuration files. It also contains a collection of shell scripts which start each of the system services at boot time. Everything in this directory should be readable text. Interesting files:While everything in /etc is interesting, here are some of my all-time favorites: /etc/crontab, a file that defines when automated jobs will run. /etc/fstab, a table of storage devices and their associated mount points. /etc/passwd, a list of the user accounts.
/home	In normal configurations, each user is given a directory in /home. Ordinary users can only write files in their home directories. This limitation protects the system from errant user activity.
/lib	Contains shared library files used by the core system programs. These are similar to DLLs in Windows.
/lost+found	Each formatted partition or device using a Linux file system, such as ext3, will have this directory. It is used in the case of a partial recovery from a file system corruption event. Unless something really bad has happened to your system, this directory will remain empty.
/media	On modern Linux systems the /media directory will contain the mount points for removable media such USB drives, CD-ROMs, etc. that are mounted automatically at insertion.
/mnt	On older Linux systems, the /mnt directory contains mount points for removable devices that have been mounted manually.
/opt	The /opt directory is used to install “optional” software. This is mainly used to hold commercial software products that may be installed on your system.
/proc	The /proc directory is special. It's not a real file system in the sense of files stored on your hard drive. Rather, it is a virtual file system maintained by the Linux kernel. The “files” it contains are peepholes into the kernel itself. The files are readable and will give you a picture of how the kernel sees your computer.
/root	This is the home directory for the root account.
/sbin	This directory contains “system” binaries. These are programs that perform vital system tasks that are generally reserved for the superuser.
/tmp	The /tmp directory is intended for storage of temporary, transient files created by various programs. Some configurations cause this directory to be emptied each time the system is rebooted.
/usr	The /usr directory tree is likely the largest one on a Linux system. It contains all the programs and support files used by regular users.
/usr/bin	/usr/bin contains the executable programs installed by your Linux distribution. It is not uncommon for this directory to hold thousands of programs.
/usr/lib	The shared libraries for the programs in /usr/bin.
/usr/local	The /usr/local tree is where programs that are not included with your distribution but are intended for system- wide use are installed. Programs compiled from source code are normally installed in /usr/local/bin. On a newly installed Linux system, this tree exists, but it will be empty until the system administrator puts something in it.
/usr/sbin	Contains more system administration programs.
/usr/share	/usr/share contains all the shared data used by programs in /usr/bin. This includes things like default configuration files, icons, screen backgrounds, sound files, etc.
/usr/share/doc	Most packages installed on the system will include some kind of documentation. In /usr/share/doc, we will find documentation files organized by package.
/var	With the exception of /tmp and /home, the directories we have looked at so far remain relatively static, that is, their contents don't change. The /var directory tree is where data that is likely to change is stored. Various databases, spool files, user mail, etc. are located here.
/var/log	/var/log contains log files, records of various system activity. These are very important and should be monitored from time to time. The most useful one is /var/log/messages. Note that for security reasons on some systems, you must be the superuser to view log files.

9 symbolic link(soft link or symlink)

　　In most Unix-like systems it is possible to have a file referenced by multiple names.

10 hard link

　　Hard links also allow files to have multiple names , but they do it in a different way.