Bash编程学习笔记（二） - Linux - Tech - JavaEye论坛

读写Shell变量

命令替代操作符及其描述

$variable 得到一个变量的值,如果没有被初始化则为Null 没有被初始化返回NULL
${variable} 得到一个变量的值,如果没有被初始化则为Null,通常在其他的东西要添加到变量值的时候使用 同上
${variable:-string} 得到一个变量的值,如果这个变量没有被定义则返回一个确定的值        variable存在而且不是空值的时候返回变量的值,否在那返回null
${variable:=string} 得到一个变量没有被定义则把一个已知的值赋给它并返回这个值       variable存在不是空值的时候返回值,否则string赋给variable,返回string
${variable:?string} 如果变量没有被定义则显示一条消息 variable存在而且不是空值的时候,返回变量的值,否则显示字符串variable,并在其后显示message
${variable:+string} 测试一个变量存在与否 variable存在而且不是空值的时候返回string,否则返回null

Java代码

1. yaoyuan-desktop$echo $name  
2.   
3. yaoyuan-desktop$name=yaoyuan  
4. yaoyuan-desktop$echo $name  
5. yaoyuan  
6. yaoyuan-desktop$echo $place  
7.   
8. yaoyuan-desktop$echo ${name:-John} ${place:-Portland}  
9. yaoyuan Portland  
10. yaoyuan-desktop$echo ${place?"Not defined"}  
11. bash: place: Not defined  
12. yaoyuan-desktop$echo ${name:+"Not defined"}  
13. Not defined  
14. yaoyuan-desktop$echo ${place:+"Not defined"}  
15.   
16. yaoyuan-desktop$echo ${place:="San Francisro"}  
17. San Francisro  
18. yaoyuan-desktop$echo ${name:-John} ${place:-Portland}  
19. yaoyuan San Francisro  

yaoyuan-desktop$echo $nameyaoyuan-desktop$name=yaoyuanyaoyuan-desktop$echo $nameyaoyuanyaoyuan-desktop$echo $placeyaoyuan-desktop$echo ${name:-John} ${place:-Portland}yaoyuan Portlandyaoyuan-desktop$echo ${place?"Not defined"}bash: place: Not definedyaoyuan-desktop$echo ${name:+"Not defined"}Not definedyaoyuan-desktop$echo ${place:+"Not defined"}yaoyuan-desktop$echo ${place:="San Francisro"}San Francisroyaoyuan-desktop$echo ${name:-John} ${place:-Portland}yaoyuan San Francisro

'',"",*,\的使用

Java代码

1. yaoyuan-desktop$name=yaoyuan  
2. yaoyuan-desktop$echo $name  
3. yaoyuan  
4. yaoyuan-desktop$name=yao yuan  
5. bash: yuan：找不到命令  
6. yaoyuan-desktop$echo $name  
7. yaoyuan  
8. yaoyuan-desktop$name=yaoyuan*  
9. yaoyuan-desktop$echo $name  
10. yaoyuan  
11. yaoyuan-desktop$name=yaoyuan  
12. yaoyuan-desktop$echo $name  
13. yaoyuan  
14. yaoyuan-desktop$echo "$name"  
15. yaoyuan*  
16. yaoyuan-desktop$echo "Is Not Hello World! "  
17. Is Not Hello World!   
18. yaoyuan-desktop$echo '$name'  
19. $name  
20. yaoyuan-desktop$echo '$name'  
21. $name  

yaoyuan-desktop$name=yaoyuanyaoyuan-desktop$echo $nameyaoyuanyaoyuan-desktop$name=yao yuanbash: yuan：找不到命令yaoyuan-desktop$echo $nameyaoyuanyaoyuan-desktop$name=yaoyuan*yaoyuan-desktop$echo $nameyaoyuanyaoyuan-desktop$name=yaoyuanyaoyuan-desktop$echo $nameyaoyuanyaoyuan-desktop$echo "$name"yaoyuan*yaoyuan-desktop$echo "Is Not Hello World! "Is Not Hello World!yaoyuan-desktop$echo '$name'$nameyaoyuan-desktop$echo '$name'$name

Shell命令替换

$(command)

Java代码

1. yaoyuan-desktop$command=pwd  
2. yaoyuan-desktop$echo "The value of command is: $command."  
3. The value of command is: pwd.  
4. yaoyuan-desktop$command=$(pwd)  
5. yaoyuan-desktop$echo "The value of command is: $command."  
6. The value of command is: /home/yaoyuan.  

yaoyuan-desktop$command=pwdyaoyuan-desktop$echo "The value of command is: $command."The value of command is: pwd.yaoyuan-desktop$command=$(pwd)yaoyuan-desktop$echo "The value of command is: $command."The value of command is: /home/yaoyuan.

命令替换适用于任何命令

Java代码

1. yaoyuan-desktop$echo "The date and time is $(date)."  
2. The date and time is 2008年 10月 12日 星期日 01:41:28 CST.  

yaoyuan-desktop$echo "The date and time is $(date)."The date and time is 2008年 10月 12日 星期日 01:41:28 CST.

创建只读的用户自定义的变量

declare -r [name-list]
typeset -r [name-list]
readonly [name-list]

用于阻止对name-list中列出的变量赋新值

Java代码

1. yaoyuan@yaoyuan-desktop:~/ScriptFile$ declare -r name=Yao place=Yuan  
2. yaoyuan@yaoyuan-desktop:~/ScriptFile$ echo $name $place  
3. Yao Yuan  
4.   
5. yaoyuan@yaoyuan-desktop:~/ScriptFile$ name=Yuan  
6. bash: name: 只读变量  
7. yaoyuan@yaoyuan-desktop:~/ScriptFile$ place="YaoYuan"  
8. bash: place: 只读变量  

yaoyuan@yaoyuan-desktop:~/ScriptFile$ declare -r name=Yao place=Yuanyaoyuan@yaoyuan-desktop:~/ScriptFile$ echo $name $placeYao Yuanyaoyuan@yaoyuan-desktop:~/ScriptFile$ name=Yuanbash: name: 只读变量yaoyuan@yaoyuan-desktop:~/ScriptFile$ place="YaoYuan"bash: place: 只读变量

只读变量的值不能重设

Java代码

1. yaoyuan@yaoyuan-desktop:~/ScriptFile$ unset name  
2. bash: unset: name：无法重置：只读的 variable  
3. yaoyuan@yaoyuan-desktop:~/ScriptFile$ unset place  
4. bash: unset: place：无法重置：只读的 variable  

yaoyuan@yaoyuan-desktop:~/ScriptFile$ unset namebash: unset: name：无法重置：只读的 variableyaoyuan@yaoyuan-desktop:~/ScriptFile$ unset placebash: unset: place：无法重置：只读的 variable

从标准输入设备读入

read [options][variable-list]

用于从标准输入设备读入一行，把读入的词赋给在variable-list中的变量

Java代码

1. yaoyuan@yaoyuan-desktop:~/ScriptFile$ cat read_demo  
2. #! /bin/bash  
3. echo -n "Enter input: "  
4. read line  
5. echo "You entered: $line"  
6. echo -n "Enter another line: "  
7. read word1 word2 word3  
8. echo "The first word is: $word1"  
9. echo "The second word is : $word2"  
10. echo "The rest of the line is: $word3"  
11. exit 0  
12. yaoyuan@yaoyuan-desktop:~/ScriptFile$ ./read_demo  
13. Enter input: yaoyuan  
14. You entered: yaoyuan  
15. Enter another line: hello shell, I love you  
16. The first word is: hello  
17. The second word is : shell,  
18. The rest of the line is: I love you  

yaoyuan@yaoyuan-desktop:~/ScriptFile$ cat read_demo#! /bin/bashecho -n "Enter input: "read lineecho "You entered: $line"echo -n "Enter another line: "read word1 word2 word3echo "The first word is: $word1"echo "The second word is : $word2"echo "The rest of the line is: $word3"exit 0yaoyuan@yaoyuan-desktop:~/ScriptFile$ ./read_demoEnter input: yaoyuanYou entered: yaoyuanEnter another line: hello shell, I love youThe first word is: helloThe second word is : shell,The rest of the line is: I love you