Linux命令行与shell脚本（8）--结构化命令

使用if-then语句

bash shell的if语句会运行if行定义的那个命令。如果该命令的退出状态码是0，位于then部分的命令就会被执行。如果该命令的退出状态码是其他值，then部分的命令就不会执行。

if date
then
echo "it worked"
fi

if abc
then
echo "it worked again"
fi
echo "we are outside of if"

if-then-else 语句

#!/bin/bash
testUser=nouser
if grep $testUser /etc/passwd
then
echo $testUser are system User;
else
echo $testUser are not sysatUser;
fi

嵌套if

可以使用 if-then-elif-then 的嵌套形式

#!/bin/bash
testUser=nouser
if grep $testUser /etc/passwd
then
echo $testUser are system User;
elif grep mysql /etc/passwd
then
echo $testUser are not system User,but mysql are system User;
else
echo $testUser and mysql are not system User
fi

test命令

test命令可以使if语句测试跟命令的退出状态无关的条件，比如数值比较、字符串比较、文件比较。注意test命令无法处理浮点值,test命令用方括号表示

test命令使用标准的数学比较符号来表示字符串比较，用文本代码来表示数值比较。如果你为数值使用了数学运算符号，shell会将它们当成字符串值，可能无法产生正常的结果

test对数值和字符串的比较：

#!/bin/bash
val1=10;
val2=11;
testuser=chenhong;
#number test
if [ $val1 -gt 5 ]
then
echo "The test value $val1 is greater than 5";
fi

if [ $val1 -eq $val2 ]
then
echo "The values are equal"
else
echo "The values are different"
fi
#string test
if [ $USER = $testuser ]
then
echo "Welcom $testuser";
else
echo "This is not $USER";
fi

val1=baseball
val2=hockey

if [ $val1 \> $val2 ]  #shell 也用 < 和 > 操作符进行重定向，所以必须用 \< 或 \> 加以转义
then
echo $val1 is greater than $val2;
else
echo $val1 is less than $val2;
fi

val1=testing;
val2="";

if [ -n "$val1" ] #val1长度是否不为0
then
echo "The string '$val1' is not empty";
else
echo "The string '$val1' is empty";
fi

if [ -z "$val2" ] #$val2长度是否为0
then
echo "The string '$val2' is empty ";
else
echo "The string '$val2' is not empty";
fi
if [ -z "$val3" ] #$val3长度是否为0
then
echo "The string '$val3' is empty ";
else
echo "The string '$val3' is not empty";
fi

test对文件的比较:

#!/bin/bash
if [ -d $HOME ] #测试目录是否存在
then
echo "Your HOME directory is exist";
cd $HOME;
ls -al;
else
echo "Your HOME has some probolem";
fi

if [ -e $HOME ] #测试文件是否存在
then
echo "now check file"
if [ -e $HOME/testing ]
then
echo "Appending date to existing file";
date >> $HOME/testing;
else
echo "Create new file testing";
date > $HOME/testing;
fi
else
echo "Your HOME has some problems";
fi

if [ -e $HOME ] #测试文件是否存在
then
echo "The object is existed,it is a file?";
if [ -f $HOME ] #测试是否为文件
then
echo "Yes,it is a file";
else
echo "No, it is not a file";
fi
else
echo "The object is not existed";
fi

pwfile="/etc/passwd";
if [ -e $pwfile ]
then
echo "The $file is existed";
if [ -r $pwfile ]  #测试文件是否可读
then
echo "I can read the file";
cat $pwfile;
else
echo "I can not read the file";
fi
else
echo "The $file is now existed";
fi

if [ -s $HOME ]  #检查文件是否存在且非空
then
echo $HOME is existed and not empty
fi

logfile="$dir/log";
touch $logfile;
chmod a-w $logfile;
now=`date`;
echo $logfile
if [ -w $logfile ]   #判断文件是否可写
then
echo "First attemp succeeded";
echo "The program ran at : $now " >> $logfile;
else
echo "First attemp failed";
fi
chmod a+w $logfile
if [ -w $logfile ]   #判断文件是否可写
then
echo "Second attemp succeeded";
echo "The program ran at : $now " >> $logfile;
else
echo "Second attemp failed";
fi

touch $HOME/test1;
if [ -x $HOME/test1 ] #测试文件是否可执行
then
echo "You can run $HOME/test1";
./test1
else
echo "You can not run $HOME/test1";
fi

if [ -o /etc/passwd ]  #判断是否为文件的属主
then
echo "You are the owner of /etc/passwd file";
else
echo "You are not the owner of /etc/passwd file";
fi
touch test1
touch test2
touch test3
if [ ./test2 -nt ./test1 ] #判断test2是否比test1新
then
echo "test2 is newer than test1";
else
echo "test2 is older than test1";
fi
if [ ./test2 -ot ./test3 ] #判断test2是否比test3旧
then
echo "test2 is older than test3";
else
echo "test2 is newer than test3";
fi

复合条件测试

可以使用

&&

和

||

组合复合条件

#!/bin/bash
if [ -e $HOME/testing ] && [ -w $HOME/testing ] #文件存在并且可写
then
echo "You can write to the file";
fi

if-then的高级特性

双圆括号，允许将高级数学表达式放入比较重，也可以在脚本中的普通命令里使用来赋值

#!/bin/bash

val1=10
if (( $val1**2 > 90 ))
then
(( val2 = $val1 ** 2 ));
echo "The square of $val1 is $val2";
fi

双方括号命令提供了针对字符串比较的高级特性，可以使用正则表达式

#!/bin/bash

if [[ $USER == c* ]]
then
echo "Hello $USER";
else
echo "Sorry, i do not know you ";
fi

case命令

#!/bin/bash

case $USER in
rich | chenhong)   #值为rich或 chenhong
echo "Welcome, dear $USER";;
testing)           #值为testing
echo "Welcome, test";;
*)                      #不满足上面两个的其他值
echo "Welcome ,everyone";;
esac

for命令

#!/bin/bash

for test in Alabama Alaks Alibaba Arizona
do
echo The next state is $test;
done
echo The last state is $test; #The last state is Arizona

for test in I don\'t know if this\'ll work #转义
do
echo The word is $test;
done

for test in Nevada "New Hampshire" "New York"
do
echo going to $test
done

list="Alabama Alaks Alibaba Arizona";
for state in $list
do
echo "The next state $state";
done

默认情况下,bash shell会将下列字符当作字段分隔符: 空格、制表符、换行符。 如果bash shell在数据中看到这些字符中的任意一个，它就会假定你在列表中开始了一个新的数据段。可以修改

ISF

的值来修改字段分割符，如

ISF=$'\n'  #修改字段分割符为换行符

C语言风格的for命令

#!/bin/bash

for (( i=1; i <= 10; i++ ))
do
echo "The next number is $i";
done

while命令

#!/bin/bash

var1=10
while [ $var1 -gt 0 ]
do
echo $var1;
var1=$[ $var1 - 1 ];
done

在含有多个命令的while语句中，在每次迭代中所有的测试命令都会被执行

until命令

#!/bin/bash

var1=100
until [ $var1 -eq 0 ]
do
echo $var1;
var1=$[ $var1 - 25 ];
done

嵌套循环

#!/bin/bash

for (( a=1; a <= 3 ; a++ ))
do
echo "a : $a";
for (( b=1; b <= 3 ; b++ ))
do
echo "  b : $b";
done
done

var1=3
until [ $var1 -eq 0 ]
do
echo "Outer loop:$var1"
var2=1
while [ $var2 -lt 5 ]
do
var3=`echo "scale=4; $var1 / $var2 " | bc`
echo "  Inner loop: $var1 / $var2 = $var3"
var2=$[ $var2 + 1 ]
done
var1=$[ $var1 - 1 ];
done

循环处理文件数据

#!/bin/bash

IFS.OLD=$IFS;
IFS=$'\n';
for entry in `cat /etc/passwd`
do
echo "the entry: - ";
IFS=":";
for item in $entry
do
echo "  $item";
done
done
IFS=IFS.OLD;

控制循环

break 命令

#!/bin/bash

for (( a=1 ; a < 4 ; a++ ))
do
echo "Outer loop $a";
for (( b=1; b < 100 ; b++ ))
do
echo "  Inner loop $b";
if (( $b >4 ))
then
break 2 #跳出两层循环
fi
done
done

continue 命令

处理循环的输出

for循环处理结果重定向到 output.txt

“`

!/bin/bash

for file in HOME/∗doif[−d“file” ]

then

echo “’file′isadirectory“;elif[−f“file” ]

then

echo “’file′isafile”;fidone>HOME/output.txt