beginning Linux programming读书笔记(2)之shell编程
2.Shellprogramming
一点哲学：
UNIX建立在高度重用的基础之上
$ ls -al | more
说明:显示指定工作目录下之内容（列出目前工作目录所含之档案及子目录)。
-a 显示所有档案及目录(ls内定将档案名或目录名称开头为"."的视为隐藏档,不会列出)
-l 除档案名称外,亦将档案型态,权限,拥有者,档案大小等资讯详细列出
你能用很多的小的脚本组成大的复杂的程序
If the utility needs to run faster, it’s quite common to prototype utilities in the shell and reimplement them later in C or C++, Perl, Python, or some other language that executes more swiftly once an idea has proven its worth.
什么是shell：
shell是一个程序，它作为你与操作系统之间的接口，使你能够输入命令去利用操作系统去执行。
管道（pipes）和重定向
输出重定向
$ ls -l >lsoutput.txt 将ls–l的命令的输出内容显示在lsoutput.txt中，如果没有这个txt则会新建它，如果已经存在则会重写这个txt，在这个txt后进行拼接的话，使用>>。
$ kill-HUP 1234 >killout.txt 2>killerr.txt 标准output将会输出在killout里，错误信息则会显示在killerr里面
输入重定向
$ more< killout.txt
管道（Pipes）
你可以用操作符(|)来连接进程
$ ps >psout.txt Ps:显示瞬间进程(process)的状态
$ sort psout.txt > pssort.out
可以简化为 $ ps | sort > pssort.out
把Shell当作 是一种编程语言
Going through this long rigmarole everytime you want to execute a sequence of commands is a bore.You need to store the commands in a file, conventionally referred to as a shell script, so you can execute them whenever you like.
新建一个脚本
#!/bin/sh
# named first
for file in *
do
if grep -q POSIX $file
then
echo $file
fi
done
exit 0
#后是注释的内容
#!告诉操作系统它后面的程序是执行本文件的程序，/bin/shis the default shell program.
执行它
$/bin/sh first
太麻烦？
$ chmod+x first
$ first
Error: command not found?shell环境变量PATH没有指向当前目录，输入PATH=$PATH:即可
Shell语法
变量
不用声明，使用就行，默认情况下，所有的变量都被认为或者是被当作string存储
Within the shell you can access thecontents of a variable by preceding its name with a $.
salutation=”Yes Dear”
echo $salutation
Yes Dear
salutation=7+5
echo$salutation
7+5
注：如果右值有空格那么必须用“”括注。等号的两边不能有等号。
引号
单引号 ’’ 以及反斜杠\将会使得变量removethe special meaning of the $ symbol，双引号则不会
myvar=”Hithere”
echo $myvar
echo “$myvar”
echo ‘$myvar’
echo \$myvar
输出结果
Hi there
Hi there
$myvar
$myvar
环境变量
$HOME The home directory of the current user
$PATH A colon-separated list of directories tosearch for commands
$0 The name of the shell script
$# The number of parameters passed
$$ The process ID of the shell script, often used inside a script for generating unique temporary filenames; for example/tmp/tmpfile_$$
env 命令查看当前系统的环境变量
参数
$1, $2, …The parameters given to the script
$* Alist of all the parameters, 被环境变量中的IFS的第一个字符分隔
$@ A list of all the parameters，与IFS无关
输入参数：命令后直接跟参数 例如
#!/bin/sh
salutation=”Hello”
echo $salutation
echo “The program $0 is nowrunning”
echo “The second parameter was$2”
echo “The first parameter was $1”
echo “The parameter list was $*“
echo “The user’s home directoryis $HOME”
echo “Please enter a newgreeting”
read salutation
echo $salutation
echo “The script is now complete”
exit 0
将上述语句保存为脚本文件filetemp
然后运行
$ filetemp foo bar baz
输出结果：
Hello
Theprogram ./try_var is now running
Thesecond parameter was bar
Thefirst parameter was foo
Theparameter list was foo bar baz
Theuser’s home directory is /home/rick
Pleaseenter a new greeting
Sire
Sire
Thescript is now complete
$
条件
实际上，大部分的脚本大量应用[和test命令，它们是shell的Booleancheck
条件判断的三种类型如下：
String Comparison Result
string1 = string2 True if the strings are equal
string1 != string2 True if the strings are not equal
-n string True if the string is not null
-z string True if the string is null (an empty string)
Arithmetic Comparison Result
expression1 -eq expression2 True if the expressions are equal
expression1 -ne expression2 True if the expressions are not equal
expression1 -gt expression2 True if expression1 is greater thanexpression2
expression1 -ge expression2 True if expression1 is greater than orequal to expression2
expression1 -lt expression2 True if expression1 is less thanexpression2
expression1 -le expression2 True if expression1 is less than orequal to expression2
! expression True if the expression isfalse, and vice versa
File Conditional Result
-d file True if the file is a directory
-e file True if the file exists.
-f file True if the file is a regular file
-g file True if set-group-id is set on file
-r file True if the file is readable
-s file True if the file has nonzero size
-u file Trueif set-user-id is set on file
-w file Trueif the file is writable
-x file True if the file is executable
示例：
If [ -f /bin/bash ]
then
echo "file /bin/bash exists"
fi
注意[之后的空格和各种空格
控制结构
If elif
一个关于变量的问题
if [ $var = 2 ] 与if [ “$var” = 2 ] 的不同当var为空的时候 对应的不同 尽量有括号
另外赋值的=两边不能有空格，但是比较的时候必须是有空格的
for语句
for variable in values
do
statements
done
示例程序：
#!/bin/sh
#注意分号的存在
for file in $(ls f*.sh); do
lpr $file
done
exit 0
while循环
while condition do
statements
done
#!/bin/sh
echo“Enter password”
read trythis
while [“$trythis” != “secret” ]; do
echo“Sorry, try again”
read trythis
done
exit 0
until(直到…)
until condition
do
statements
done
#!/bin/bash
until who | grep “$1” > /dev/null
do
sleep 60
done
# now ring the bell and announce the expected user.
echo -e‘\a’
echo“**** $1 has just logged in ****“
exit 0
Case语句
#!/bin/sh
Echo “Is it morning? Please answer yes or no”
read timeofday
case “$timeofday” in
yes | y| Yes | YES ) echo “Good Morning”;;
n* | N*) echo “Good Afternoon”;;
* ) echo “Sorry, answer not recognized”;;
esac
exit 0
注意：分开符;;匹配的更全面：[yY]| [Yy][Ee][Ss] )
ANDList：&&
例子：
#!/bin/sh
touchfile_one
rm -ffile_two
if [ -ffile_one ] && echo “hello” && [ -f file_two ] && echo “there”
then
echo“in if”
else
echo“in else”
fi
exit 0
执行结果：
hello
in else
注意两点 1. 短路闭合特性2. Echo返回值为true
OR List基本同上 ：||
