学习体会：本章内容比较多，我这里按照自己的理解重新组织了一下，希望对读过这本书的同学有帮助：

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shell programming 是linux学习的必经之路，也是必须学好的，经过一段个人体会是：
shell 编程主要是考察你对shell 的一些结构组织，标识，命令的理解，这些都是日后读scrips所必需掌握的东西。
也许实践才是学习shell的最好方式，所以这一章虽然读完，感觉很多东西只是理解并没有达到能应用自如，这一定需要比较长时间的实践。
这章节，作者依旧娓娓的将shell里面的各种复杂的知识做了归纳，由浅入深，非常适合基础差的同学。
感觉，有经验的如果能仔细看一下这个章节，也对自己的是个系统整理的过程。

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What this chapter tell us:
- what the shell is

- basic consideration

-subtleties of syntax:
 variables,conditions,program control
 lists
 functions
 commands and commands execution
 here document 

- debugging

- grep and regular expression
- find

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- what the shell is
Beginning of this chapter:+
1 why we should use the shell?
- shell could easy to run at the very complex program which are not time-critical。
- shell use the interpreted ( 解释性) language so not only your can execure the commands and utilitie on the shell but also you can write them.
[hsy75] what is command and utilities?
e.g.
$ls -al | more
this command use the ls and more utilites.


2 what is shell
is a program that acts as the interface between you and the linux system which support the commands.

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- basic consideration
Pipes and redirection ( 也就是用于控制shell的输入输出)
Redirection:
[hsy75]个人理解，在default之外的输入输出定义都叫做redirection(< > >> ):
default:
-0 input
-1 output
-2 error

>> used to append to the file:
e.g.
$ps >> out.txt

>& used to combine two outputs
[hsy75]2>&1 often used to get the error code to a file
e.g.
$kill -l 1234 >outerr.txt 2>&1

Pipes ( | )
[hsy75] Pipes 就是自动同事处理多个流程到一个你想要的结果
process connected by pipes can run simultaneously and auto rescheduled as data flow between them.
[frankhuang@localhost bin]$ ps |sort|more
11220 pts/1    00:00:00 ps
11221 pts/1    00:00:00 sort
11222 pts/1    00:00:00 more
 6800 pts/1    00:00:00 bash
  PID TTY          TIME CMz

---

 Shell programming = script. creating
 1 wildcard 通配符 expansion
shell programming 你必须了解通配符：
*
?
set
^set
{}
and so on:

2 to make a script. executable
[frank@localhost ch02]$ chmod +x first

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-subtleties of syntax:
变量variables
[hsy75] 一般shell 变量就是指： a variable by preceding its name with a $
常用的检验变量的方法就是用 echo 把他打印出来

变量定义时候要注意下面几种quote 的不同
" " 括号内均为变量 [hsy75] 好的习惯是用这个东西把变量wrap起来，从而避免shell出错 e.g $"test"
'  ' 这表示里面的是字符串 这个也很有用 当你想输出默认字符时候
   eg. 
    y='$'$x   // x=100
   echo y
   $100
   这里extra 符号$被打印
\ 反斜杠又叫去功能化标识，用来是上述quote 或者$失效

位置参数变量符：
IFS 用来定义字段分隔符
$1
$2  【hsy75] 这里 1，2.。。positional parameters 是shell里面经常用到的

$* 去除ifs定义的分割符
$@  不去除ifs定义的分割符

conditionsprogram control
(if for while until case )
[hsy75] 这里和c很像，可以忽略

注意一下：
[ = test
test include 1string comparision /2arithmetic comparison /3file conditionals.


list( AND OR )
e.g
记住这句就ok了
[ -f file1 ] && command for ture || command for false

functions
shell的函数最重要的就是理解位置参数了(positional parameters),用户端的输入或者是函数参数的传递都是通过这个

eg.
用户通过shell输入：
Frank Huang
那位位置变量默认就是：
$* = Frank Huang
$1 = Frank
$2 = Huang

和位置变量非常有关系的是命令set ,set 经常被用来根据变量来转换为位置变量：
经过set 命令后 data 的值就给了位置变量可以被shell利用了
e.g
set $(date)
echo the month is $2

---

 commands
:                    = test
.                    = current shell
echo            这里再一次提到 \的escapte的作用
eval             = $( ... )  [hsy75] extra the content in brace , means give you the value of the value of a vaiable
exec            replace the shell
exit  n
export         just creat an enviroment variable which can be seen by other scripts
expr  (expression evaluate)          = $(( ... ))   [hsy75]注意这个和前面的 $(...)取变量内容不同 是double parenthsis ,
printf
return
set             很有用的是将一组变量的field 和位置变量联系起来
                  set the para variables for the shell
                  eg

                  echo the data is $(date)
                  set $(date)
                  echo the month is $2
                 
shift
trap
unset

two useful commands

find (search the file or directory) [hsy75]diff from grep which find for strings
syntax
find [path] [options] [tests] [actions]
eg.
$find . -mount \(-newer filename -o -name "_*" \) -type f -exec ls -l {}\; -print
[hsy75]上面这个find 看起来很难，我归纳一下书上的解释下面一下就其实简单
$find .[path=curernt directory] -mount [options=do not search the other files system]\[quote the braces using a backslash to escapte the meanings in shell]([combine the tester using the parentheses]-newer[tests = newer ] filename[tests newer's pattern means the filename is older than the finding file] -o[tests operations means OR] -name[tests= name] "_*"[tests'name pattern means name started with an underscore ] \)[escapte the parentheses] -type[tests type] f[tests type =file] -exec[actions exec] ls -l[the command that we use invode by action] {}[used for -exec or -ok means the full path to the current file]\;[means the parameters on the line of action is end so it is a termiator ] -print [another actions that to print the result]

grep  find 的兄弟，经常一起用 find for string ：said by writer it is quite common to have grep as a command passed after a action -exec when using find
grep [options] Pattern [FILES]
[hsy75] 看起来grep 是非常简单的，但是，当你开始接触 regular expression 的时候，sophisticated would occurs

regular expressiom
^      anchor to the beginning of a line
$     anchor to the end of a line
.      any single character
[ ]    range of characters
[:blank:] special match patterns,the blank 可以被其他参数代替

-E  extended matching
[hsy75] 下面这些是给-E扩展用，所以必须都用\来escapte shell 的功能
?
*
+
{n}
{n,}
{n,m}

e.g
[frank@localhost ch02]$ grep Th.[[:space:]] words2.txt
The handle toward my hand? Come, let me clutch thee.
The curtain'd sleep; witchcraft celebrates
Thy very stones prate of my whereabout,
[frank@localhost ch02]$ grep Th[匹配Th].[匹配附加任何一个字母][[:space:]][后面紧跟 空格] words2.txt[被查找的文件名]



[frankhuang@localhost ch02]$ grep -E [a-z]\{9\} words2.txt
Proceeding from the heat-oppressed brain?
And such an instrument I was to use.
Thus to mine eyes. Now o'er the one halfworld
The curtain'd sleep; witchcraft celebrates
Pale Hecate's offerings, and wither'd murder,
With Tarquin's ravishing strides, towards his design
Thy very stones prate of my whereabout,
[frankhuang@localhost ch02]$ grep -E[扩展支持开启] [a-z][range of characters] \[Escape for shell of {] {9[有9个字符匹配也就是单词长度为9] \} words2.txt

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and commands execution
1 $(command) = '\command'\
we often need to capture the result of a command exexution for use in the shell cript ,the above syntax helps put the output of command into a variable .
the ability to put the result of a command into a script. variable is very powerful.
e.g

set $(who)
writer = $(who)
echo $writer

2 $((arithmetic expansion))  = $(expr arithmetic expansion) 注意这里是双括号

Parameter expansion
3 ${variable}  变量拓展和保护
3.1用来保护shell的扩展时候，只扩展对应的变量
e.g.

for i in 1 2
do
    creat_file $test_{i}
    creat_file $test_i   报错

done

3.2 参数扩展
${param:default} 
类似的还有下面变量扩展参数
#       从头删除匹配字符，如果多个匹配，删除最小匹配长度，并返回
##    从头删除匹配字符，如果多个匹配，删除最长匹配长度，并返回
%    从尾巴删除匹配字符，如果多个匹配，删除最小匹配长度，并返回
%% 从尾巴删除匹配字符，如果多个匹配，删除最长匹配长度，并返回
参数扩展，在redirection output的时候非常有用，
since linux are havily round the idea of filters, the result of one operation must oftern be redirected manually.

e.g.
for image in *.gif
do
cjpeg $image > ${image%%gif}jpg
【hsy75】解释扩展如下
cjpeg[a program to convert from gif to jpg] $image[get the file name from the *.gif] >[redirection] ${[圈定需要替换的变量] image%%[从尾巴删除匹配字符gif]gif } jpg
done

比如 draw_0001.gif  将变成 draw_0001.jpg

eg.
#!/bin/sh

unset foo
echo ${foo:-bar}

foo=fud
echo ${foo:-bar}

foo=/usr/bin/X11/startx
echo ${foo#*/}
echo ${foo##*/}

bar=/usr/local/etc/local/networks
echo ${bar%local*}
echo ${bar%%local*}

exit 0


bar
fud
usr/bin/X11/startx
startx
/usr/local/etc/
/usr/

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 here document
其实就是像自动命令的输入，可以和编辑器绑定起来，从而实现通过文件读写来直接运行编辑器里面的scripts
e.g
#!/bin/sh

ed[编译器ed] textfile <<!FunkyStuff![here document的makers]
3 [ed 编译器命令 move to line 3]
d [ed 编译器命令 删除一行]
.,\[这里避免shell的作用，从而shell执行的时候可以将$后面的东西看成是非变量，从而保证ed的command 正确执行]$s/is/was/ [替换 is 为was]
w
q
!FunkyStuff!

exit 0

This is line 1
This is line 2
This is line 3  被删除
This is line 4  被替换

将变成
This is line 1
This is line 2
This was line 4

---

debugging
-n check syntax error only
-v Echoes commands before running
-x after running

set -o xtrace
set +o xtrace

其他的调试选项
-u
-o nounset
-o verbose


我们用上面提到的例子再来说明一下如何调试：
1 没有调试：
[frankhuang@localhost ch02]$ ./param
bar
fud
usr/bin/X11/startx
startx
/usr/local/etc/
/usr/

2 打开set trace调试
[frankhuang@localhost ch02]$ set -o xtrace
++ echo -ne '\033]0;frankhuang@localhost:~/beginning_linux/beginning/ch02'
[frankhuang@localhost ch02]$ ./param
+ ./param
bar
fud
usr/bin/X11/startx
startx
/usr/local/etc/
/usr/
++ echo -ne '\033]0;frankhuang@localhost:~/beginning_linux/beginning/ch02'

3关闭set trace 调试
[frankhuang@localhost ch02]$ set +o xtrace
+ set +o xtrace
[frankhuang@localhost ch02]$ ./param
bar
fud
usr/bin/X11/startx
startx
/usr/local/etc/
/usr/

4
[frankhuang@localhost ch02]$ sh -x ./param
+ unset foo
+ echo bar
bar
+ foo=fud
+ echo fud
fud
+ foo=/usr/bin/X11/startx
+ echo usr/bin/X11/startx
usr/bin/X11/startx
+ echo startx
startx
+ bar=/usr/local/etc/local/networks
+ echo /usr/local/etc/
/usr/local/etc/
+ echo /usr/
/usr/
+ exit 0

5
[frankhuang@localhost ch02]$ sh -v ./param
#!/bin/sh

unset foo
echo ${foo:-bar}
bar

foo=fud
echo ${foo:-bar}
fud

foo=/usr/bin/X11/startx
echo ${foo#*/}
usr/bin/X11/startx
echo ${foo##*/}
startx

bar=/usr/local/etc/local/networks
echo ${bar%local*}
/usr/local/etc/
echo ${bar%%local*}
/usr/

exit 0

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going graphical - dialog utility
图形输入工具，这个再编写大型的shell配置文件时候经常用到
e.g

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the doc in under development