Linux Shell脚本编写——使用结构化命令
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结构化命令中，最基本的类型就是if-then语句
if command
then
command
fi
bash shell的if语句会执行if行定义的那个命令，如果命令的退出状态码是0，则代表成功执行，位于then部分的命令就会执行。如果if行定义的命令的退出状态码是其他，则then部分的命令将不会执行，且if-then语句不能测试跟命令的退出状态码无关的条件
代码2-1
[email protected]:/data# cat demo1 #!/bin/bash if date then echo "hello world" fi [email protected]:/data# ./demo1 Fri Dec 2 05:26:27 CST 2016 hello world
如果故意放置一个不能执行的命令，则不会进入then
代码2-2
[email protected]:/data# cat demo2 #!/bin/bash if aaa then echo "hello world" fi [email protected]:/data# ./demo2 ./demo2: line 2: aaa: command not found
if-then-else
代码2-3
[email protected]:/data# cat demo3 #!/bin/bash if aaa then echo "hello world" else echo "hello java" fi [email protected]:/data# ./demo3 ./demo3: line 2: aaa: command not found hello java
嵌套if，else可有可无，如果所有条件都不符合，则进入else
if command1
then
command set 1
elif command2
then
command set 2
elif command3
then
command set 3
else
command set 4
fi
代码
2-4
[email protected]:/data# cat demo4 #!/bin/bash if aaa then echo "hello world" elif who then echo "hello java" else echo "hello linux" fi [email protected]:/data# ./demo4 ./demo4: line 2: aaa: command not found root pts/0 2016-12-02 04:39 (122.91.222.126) hello java
test命令可以判断3类条件（注：[ condition ]condition两边必须加空格）：
-
数值比较
-
字符串比较
-
文件比较
if test condition
then
commands
fi
或
if[ condition ]
then
commands
fi
| 比较 | 描述 |
| n1 -eq n2 | 检查n1是否与n2相等 |
| n1 -ge n2 | 检查n1是否大于等于n2 |
| n1 -gt n2 | 检查n1是否大于n2 |
| n1 -le n2 | 检查n1是否小于等于n2 |
| n1 -lt n2 | 检查n1是否小于n2 |
| n1 -ne n2 | 检查n1是否不等于n2 |
代码2-5
[email protected]:/data# cat demo3 #!/bin/bash val1=10 val2=20 if [ $val1 -gt $val2 ] then echo "The test value $val1 is greater than $val2 " elif [ $val1 -lt $val2 ] then echo "The test value $val1 is less than $val2" else echo "The test value $val1 is equal $val2 " fi [email protected]:/data# ./demo3 The test value 10 is less than 20
| 比较 | 描述 |
| str1 = str2 | 检查str1是否和str2相同 |
| str1 != str2 | 检查str1是否和str2不同 |
| str1 > str2 | str1是否比str2大 |
| str1 < str2 | str1是否比str2小 |
| -n str1 | 检查str1的长度是否非0 |
| -z str2 | 检查str1的长度是否为0 |
代码2-6
[email protected]:/data# cat demo4 #!/bin/bash testUser=$USER badUser=Tom if [ $USER = $testUser ] then echo "hello $USER" fi if [ $USER = $badUser ] then echo "hello $USER" else echo "User is $USER" fi [email protected]:/data# ./demo4 hello root User is root
字符串顺序
要测试一个字符串是否比另一个字符串大就变得繁琐了：
-
大于小于符号必须转义，否则shell会把它们当做重定向符号而把字符串值当做文件名
-
大于小于顺序和sort命令所采用的不同
代码2-7
[email protected]:/data# cat demo5 #!/bin/bash val1="hello" val2="Hello" if [ $val1 \> $val2 ] then echo "The result is $val1 greater than $val2" elif [ $val1 \< $val2 ] then echo "The result is $val1 less than $val2" fi [email protected]:/data# ./demo5 The result is hello greater than Hello [email protected]:/data# cat demo6 #!/bin/bash val1="hello" val2="world" if [ $val1 \> $val2 ] then echo "The result is $val1 greater than $val2" elif [ $val1 \< $val2 ] then echo "The result is $val1 less than $val2" fi [email protected]:/data# ./demo6 The result is hello less than world
sort和test处理字符串相反，sort默认升序，sort中大写字母大于小写字母，而test中则是小写字母大于大写字母
代码2-8
[email protected]:/data# cat demo7 Hello hello [email protected]:/data# sort demo7 hello Hello
字符串大小
[email protected]:/data# cat demo8 #!/bin/bash val1="testing" val2="" if [ -n $val1 ] then echo "The string ‘$val1‘ is not empty" else echo "The string ‘$val1‘ is empty " fi if [ -n $val2 ] then echo "The string ‘$val2‘ is not empty" else echo "The string ‘$val2‘ is empty " fi if [ -n $val3 ] then echo "The string ‘$val3‘ is not empty" else echo "The string ‘$val3‘ is empty " fi [email protected]ian:/data# ./demo8 The string ‘testing‘ is not empty The string ‘‘ is not empty The string ‘‘ is not empty
代码2-8中：
if [ -n $val1 ]判断val1变量长度是否为0，而它长度正好非0，所以then部分被执行了
if [ -z $val2 ]判断val2变量长度是否为0，而它正好长度为0，所以then部分被执行了
if [ -z $val3 ]判断val3变量长度是否为0，这个变量并未在shell脚本中定义过，所以说明字符串长度仍未0，所以then部分执行
| 比较 | 描述 |
| -d file | 检查file是否存在并是个目录 |
| -e file | 检查file是否存在 |
| -f file | 检查file是否存在并是个文件 |
| -r file | 检查file是否存在并可读 |
| -s file | 检查file是否存在并非空 |
| -w file | 检查file是否存在并可写 |
| -x file | 检查file是否存在并可执行 |
| -O file | 检查file是否存在并属当前用户所拥有 |
| -G file | 检查file是否存在并且默认组与当前用户相同 |
| file1 -nt file2 | 检查file1是否比file2新 |
| file1 -ot file2 | 检查file1是否比file2旧 |
检查目录
代码2-8
[email protected]:/data# cat demo9 #/bin/bash if [ -d $HOME ] then echo "Your home directory exists" cd $HOME ls -a else echo "There is a problem with your HOME directory" fi [email protected]:/data# ./demo9 Your home directory exists . .. .bash_history .bashrc .cache .hivehistory .pip .profile .pydistutils.cfg .rediscli_history .scala_history .selected_editor .spark_history .ssh .vim .viminfo
检查文件是否为空
代码2-9
[email protected]:/data# cat demo1 #!/bin/bash file="text" touch $file if [ -s $file ] then echo "The $file file exists and has data in it" else echo "The $file exists and is empty" fi date > $file if [ -s $file ] then echo "The $file file exists and has data in it" else echo "The $file exists and is empty" fi [email protected]:/data# ./demo1 The text exists and is empty The text file exists and has data in it
使用布尔逻辑来组合测试：
-
[ condition1 ] && [ condition2 ]
-
[ condition1 ] || [ condition2 ]
代码2-10
[email protected]:/data# cat demo2 #!/bin/bash if [ -d $HOME ] && [ -w $HOME/testing ] then echo "The file exists and you can write it" else echo "I cannot write to the file" fi [email protected]:/data# ./demo2 I cannot write to the file
| 符号 | 描述 |
| val++ | 后增 |
| val-- | 后减 |
| ++val | 先增 |
| --val | 先减 |
| ! | 逻辑求反 |
| - | 位求反 |
| ** | 幂运算 |
| << | 位左移 |
| >> | 位右移 |
| & | 位布尔和 |
| | | 位布尔或 |
| && | 逻辑和 |
| || | 逻辑或 |
代码2-11
[email protected]:/data# cat demo3 #!/bin/bash val1=10 if(($val1 ** 2 > 90)) then ((val2=$val1 ** 2)) echo "The square of $val1 is $val2" fi [email protected]:/data# ./demo3 The square of 10 is 100
使用双方括号
[[expression]]提供了test命令未提供的另一特性——模式匹配
代码2-12
[email protected]:/data# cat demo4 #!/bin/bash if [[ $USER=r* ]] then echo "hello $USER" else echo "Sorry. I do not know you" fi [email protected]:/data# ./demo4 hello root
case命令会检查单个变量列表格式的多个值，符号“;;”类似C语言中的break，但是又与C语言有点不太一样，C语言允许没有break，一直到匹配结束，而shell脚本中的case匹配如果没有";;"这个符号则会报错
case variable in
pattern1 | pattern2) command1;;
pattern3) command2;;
*) default commands;;
代码2-13
[email protected]:/data# cat demo5 #!/bin/bash case $USER in root|Tom) echo "Welcome $USER" echo "Please enjoy your visit";; testing) echo "Special testing account";; jessica) echo "Do not forget to log off when you‘re done";; *) echo "Sorry. you are not allowed here";; esac [email protected]:/data# ./demo5 Welcome root Please enjoy your visit
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