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本章节关于 The Linux Command Line 读书笔记，语言通俗易懂，适合入门，并且图书是开源性质
官网(包含下载链接)
中文开源翻译
Freedom is the power to decide what your computer does, and the only way to have this freedom is to know what your computer is doing.Freedom is a computer that is without secrets, one where everything can be known if you care enought to find out.
It's been said that "graphical user interfaces make easy tasks easy, while command line interfaces make difficult tasks possible" and this is still very true today.
Linux is not just a piece of software; it's also a small parg of the larger Unix culture, which has its own languuage and history.
This book is divided into four parts, each covering some aspect of the command line experience:
Part 1 - Learning The Shell starts our exploration of the basic language of the command line including such things as the structure of commands, file system naviagtion, command line editing, and finding help and documentation for commands.
Part 2 - Configuration And The Environment covers editing configuration files that control the computers oparation from the command line.
Part 3 - Command Tasks and Essential Tools explores many of the ordinary tasks that are commonly performed form the command line. Unix-like operating systems, such as Linux, contain many "classic" command line programs that are used to perform powerful operations on data.
Part 4 - Writing Shell Scripts introduces shell programming, an admittedly rudimentary, but easy to learn, technique for automating many common computeing tasks. By learning shell programming, you will become familiar with concepts that canbe applied to many other programming languages.
Technically speaking, Linux is the name of the operating systems's kernel, nothing more.
This book is an ongoing project, like many open source software projects.
The shell is a program that takes keyboard commands and passes them to the operating system to carry tou.
date
cal to display a calendar of the curruent month
df to see the current amount of free space on our disk drives
free to display the amount of free memory
介词定位术
on our Linux system
at the prompt
in this chapter
pwd - Print name of current working directory
cd - Change directory
cd Changes the working directory to your home directory
cd - Changes the working directory to the previous working directory
cd ~username
ls - List directory contents
ls get a list of files and subdirectories contained in the current working directory
ls /usr specify the directory to list
ls ~ /usr specify multiple direcotories
ls -l change the format of the output to reveal more detail
This brings us to a very important point about how most commands work. Commands are often followed by one or more options that modify their behavior, and further, by one or more arguments, the items upon which the command acts. So most commands look kind of like this:
the ls command has a large of possible options. The most common are listed below.
less /etc/passwd to examine the file that defines all teh sysmtem's user accounts.
The less program was designed as an improved replacement of an earlier Unix program call more.
命令行界面：双击选中，点击滑轮粘贴
/ Where everything begins.
/bin Contains binaries(programs) that must be present for the system to boot and run.
/boot the Linux kernal, initail RAM disk image, and the boot loader.
/boot/grub/grub.conf
/boot/vmlinuz
/dev device nodes, "Everything is a file"
/etc/passwd
/etc/crontab
/home
/lib similiar to DLLS
/lost+found for recovery
/media` on modern Linux system, for removable media
/mnt on older Linux system, for removable media
/opt to hold commercial software products
/proc a virtual file system, will give us a picture of how the kernel sees the computer
/proc/cpuinfo
/root the home directory for the root account
/sbin vital system tasks for the superuser
/tmp
/usr the directory is likely the largest one
/usr/bin installed by the Linux distribution, hold thousands of programs
/usr/lib
/usr/local
/usr/sbin system administration programs
netplan
/usr/share
/usr/share/doc
/var data that is likely to change is stored, such as databases, spool files, user mail.
/var/log log files
In most Unix-like systems it is possible to have a file referenced by multiple names.
foo -> foo.2.7
libc.so.6 -> libc.2.6.so
cp - Copy files and directories
mv - Move/rename files and directories
mkdir - Create directories
rm - Remove files and directories
ln - Create hard and symblic links
These five commands are among the most frequently user Linux commands.
Since the shell uses filenames so much, it provides special charaters to help us rapidly specify groups of filenames. These special charaters are called wildcards.
Wildcards can be used with any command that accepts filenames as arguments.
mkdir direcotory...
When three periods follow an argument in the description of a command(as above), it means that the argument can be repeated.
rm的options
这节的小练习，多敲几遍
ln -s item symbolic-link
We will attempt to remove mysterious options and arguments and even create our own commands.
an executable program
a command built into the shell itself
a shell function, related with the environment
an alias
type - Display a Command's Type
是 shell builtin, executable program 还是 alias，拉出来看看
which - Display an Executable's Location only works for executable programs
man - Disaplay a Program's Manual Page
whatis - Display One-Line Manual Page Descriptions
help - Get Help for Shell Builtins
Creating Our Own Commands with alias
alias foo='cd /usr; ls; cd -'
unalias foo
alias
In this lesson we are going to unleash what may be the coolest feature of the command line. It's called I/O redirection. I/O redirection allows us to change where output goes and where input comes from. Normally, output goes to the screen and input comes form the keyboard, but with I/O redirection, we can change that.
Many of the programs produce output of some kind.
results
status and error messages
按照一切都是文件的规则，程序会把正常的输出值 to a special file called standard output and 错误状态信息 to another file called standard error. In addition, many programs take input from a facility called standard input, which is, by default, attached to the keyboard.
cat - Concatenate Files
The cat command reads one or more files and copies them to standard output.
We can use it to display files without paging.
Cat is often used to display shourt text files.
Using the pipe operator | (vertical lar), the standard output of one command can be piped into the standard input of another.
Each time we type a command and press the Enter key, bash perform serveral substituions(replacement) upon the text before it carries out our command.
echo - It prints its text arguments on standard output.
bash uses a library (a shared collection of routines that different programs can use) called Readline to implement command line editing.
the hidden file .bash_history in our home directory kept the list of history commands.
Operating systems in the Unix tradition differ from those in the MS-DOS tradition in that they are not only multitasking systems, but also multi-user systems.
多人同时登录必须保证各自操作的安全，不能随意操作别人的文件，不能破坏电脑
Reading, Writing, and Executing
chmod - Change File Mode
文件和文件夹略有不同
示例说明
To authenticating using sudo, requires the user's own password.
passwd - Changing Your Password
Processes are how Linux organizes the different programs waiting for their turn at the CPU.
init --> init scripts --> system services, namely daemon programs, which just sit in the background
The kernel maintains information about each process to help keep things organized.
As we discussed earlier, the shell maintains a body of information during our shell session called the environment.
environment variables
shell variables, and some programmatic data, namely aliases and shell funtions
When we log on to the system, the bash program starts, and reads a serias of configuration scripts called startup files, which define the default environment shared by all users.
a login shell session, a virtual console session, username and password
a non-login shell session, in the GUI
Since we know where the startup files are and what they contain, we can modify them to customize our environment.
The changes we have made to our .bashrc will not take effect until we close our terminal session and start a new one because the .bashrc file is only read at the beginning of a session.
source ~/.bashrc
vi is almost always available.
vi is lightweiht and fast.
We don't want other Linux and Unix users to think we are cowards.
The Prompt is defined by an environment variable named PS1(short for "prompt string 1")
Most terminal emulator programs respond to certain non-printing character sequences to control such things as character attributes(such as color, bold text, and the dreaded blinking text) adn cursor position.
Most distributions fall into one of two camps of packaging technologies: the Debian .deb camp and the Red Hat .rmp camp
If a package requires a shared resource such as a shared library, it is said to have a dependency.
Package management systems usually consist of two types of tools.
Low-level tools which handles tasks such as installing and removing package files
High-level tools that perform metadata searching and dependency resolution
If a package file has been downloaded from a source other than a repository, it can be installed directly(though without dependency resolution) using a low-level tool.
display information about an installed package
dpkg --status emacs or sudo apt-cache show tmux
Most network devices recving this packet will reply to it, allowing the network connection to be verified.
performance statistics
The ip program is a multi-purpose network configuration tool that makes use of the full range networking of features available in mordern Linux kernels.
ip [OPTIONS] OBJECT { COMMAND | help } 如 ip address
The first, called lo, is the loopback interface, a virtual interface that the system uses to "talk to itself" and the second, called eth0, is the Ethernet interface.
updatedb 定时任务统计文件名
Throughout the history of computing, there has been a struggle to get the most data into the smallest available space, whether that space be memory, storage devices, or network bandwidth.
compression algorithoms: lossless and lossy
无压缩空间的的文件，避免再次压缩，文件会变大，因会增加额外信息 gzip picture.zip
.tar for a 'plain' tar archive
.tgz for a gzipped archive
tar --help | less
其中 f 指定文件 archive.tar
c 关联 create
x 关联 extract
a slightly odd way of expressing options, 可不加 leading dash
Why compile software?
Availability. Linux 发布版包括了一些常用软件，但是不是全部. In some case, need to compiling
Timeliness. 尝试新版本
Simply put, compiling is the process of translating source code (the human-readable description of a program written by a programmer) into the native language of the computer's processor.
machine language
assembly language --> assembler --> machine language
high-level programming language --> compiler --> 有些会编译成 汇编语言
linking, libraries, executable program file
A program called linker is used to form the connections between the output of the compiler and the libraries that the compiled program requires.
编译型语言
解释性语言
The file Makefile describes the relationships and dependencies among the components that comprise the finished program.
重要文件 可研究 README INSTALL
In the simplest terms, a shell script is a file containing a series of commands. The shell reads this file and carries the commands as though they have been entered directly on the command line.
Write a script
Make the script executable
Put the script somewhere the shell can find it
编写脚本 vim hello
第一行 The #! charater sequence is, in fact, a special constuct called a shebang. The shebang is used to tell the kernel the name of the interpreter that should be used to execute the script that follows.
第二行 # 注释
第三行 同命令行模式下 echo 'Hello World!'
修改为可执行，并调整执行路径
The if statement has the following syntax:
Commands(including the scripts and shell functions we write) issue a value to the system when they terminate, called an exit status.This value, which is an integer in the range of 0 to 255, indicates the success of failure of the command's execution. By convention, a value of zero indicates success and any other value indicates failure. The shell provides a parameter that we can use to examine the exit status. Here we see it in action:
By far, the command used most frequently with if is test. The test command performs a variety of checks and comparisions. It has two equvalent forms. The first, shown here:
test expression
And the second, more popular form, shown here:
[ expression ]
"$FILE" 注意加引号
exit defaults to the exit status of the last command executed
Modern versions of bash include a compound command that acts an enhanced replacement for test.
It uses the following syntax:
[[ expression ]]
兼容 [ expressoin ]
支持正则 string1 =~ regex
