一个计算机技术爱好者与学习者

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好好学Shell：Shell脚本编程入门篇（下）

发表于 更新于 分类于 ，

1. 运算符

1.1. 算术运算符

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#!/bin/sh

a=10
b=20
echo "a=$a, b=$b"
val=`expr $a + $b`
echo "a + b : $val"
val=`expr $a - $b`
echo "a - b : $val"
val=`expr $a \* $b`
echo "a * b : $val"
val=`expr $b / $a`
echo "b / a : $val"
val=`expr $b % $a`
echo "b % a : $val"
if [ $a == $b ]
then
   echo "a is equal to b"
fi
if [ $a != $b ]
then
   echo "a is not equal to b"
fi

1.2. 关系运算符

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#!/bin/sh

a=10
b=20
echo "a=$a, b=$b"
if [ $a -eq $b ]
then
   echo "$a -eq $b : a is equal to b"
else
   echo "$a -eq $b: a is not equal to b"
fi
if [ $a -ne $b ]
then
   echo "$a -ne $b: a is not equal to b"
else
   echo "$a -ne $b : a is equal to b"
fi
if [ $a -gt $b ]
then
   echo "$a -gt $b: a is greater than b"
else
   echo "$a -gt $b: a is not greater than b"
fi
if [ $a -lt $b ]
then
   echo "$a -lt $b: a is less than b"
else
   echo "$a -lt $b: a is not less than b"
fi
if [ $a -ge $b ]
then
   echo "$a -ge $b: a is greater or  equal to b"
else
   echo "$a -ge $b: a is not greater or equal to b"
fi
if [ $a -le $b ]
then
   echo "$a -le $b: a is less or  equal to b"
else
   echo "$a -le $b: a is not less or equal to b"
fi

1.3. 布尔运算符

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#!/bin/sh

a=10
b=20
echo "a=$a, b=$b"
if [ $a != $b ]
then
   echo "$a != $b : a is not equal to b"
else
   echo "$a != $b: a is equal to b"
fi
if [ $a -lt 100 -a $b -gt 15 ]
then
   echo "$a -lt 100 -a $b -gt 15 : returns true"
else
   echo "$a -lt 100 -a $b -gt 15 : returns false"
fi
if [ $a -lt 100 -o $b -gt 100 ]
then
   echo "$a -lt 100 -o $b -gt 100 : returns true"
else
   echo "$a -lt 100 -o $b -gt 100 : returns false"
fi
if [ $a -lt 5 -o $b -gt 100 ]
then
   echo "$a -lt 100 -o $b -gt 100 : returns true"
else
   echo "$a -lt 100 -o $b -gt 100 : returns false"
fi

1.4. 字符串运算符

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#!/bin/sh

a="abc"
b="efg"
echo "a=$a, b=$b"
if [ $a = $b ]
then
   echo "$a = $b : a is equal to b"
else
   echo "$a = $b: a is not equal to b"
fi
if [ $a != $b ]
then
   echo "$a != $b : a is not equal to b"
else
   echo "$a != $b: a is equal to b"
fi
if [ -z $a ]
then
   echo "-z $a : string length is zero"
else
   echo "-z $a : string length is not zero"
fi
if [ -n $a ]
then
   echo "-n $a : string length is not zero"
else
   echo "-n $a : string length is zero"
fi
if [ $a ]
then
   echo "$a : string is not empty"
else
   echo "$a : string is empty"
fi

1.5. 文件测试运算符

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#!/bin/sh

file="../start/helloworld.sh"
if [ -r $file ]
then
   echo "File has read access"
else
   echo "File does not have read access"
fi
if [ -w $file ]
then
   echo "File has write permission"
else
   echo "File does not have write permission"
fi
if [ -x $file ]
then
   echo "File has execute permission"
else
   echo "File does not have execute permission"
fi
if [ -f $file ]
then
   echo "File is an ordinary file"
else
   echo "This is sepcial file"
fi
if [ -d $file ]
then
   echo "File is a directory"
else
   echo "This is not a directory"
fi
if [ -s $file ]
then
   echo "File size is not zero"
else
   echo "File size is zero"
fi
if [ -e $file ]
then
   echo "File exists"
else
   echo "File does not exist"
fi

2. 条件

2.1. if…fi

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#!/bin/sh

a=10
b=20
echo "a=$a, b=$b"
if [ $a == $b ]
then
   echo "a is equal to b"
fi
if [ $a != $b ]
then
   echo "a is not equal to b"
fi

2.2. if…else…fi

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#!/bin/sh

a=10
b=20
echo "a=$a, b=$b"
if [ $a == $b ]
then
   echo "a is equal to b"
else
   echo "a is not equal to b"
fi

2.3. if…elif…fi

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#!/bin/sh

a=10
b=20
echo "a=$a, b=$b"
if [ $a == $b ]
then
   echo "a is equal to b"
elif [ $a -gt $b ]
then
   echo "a is greater than b"
elif [ $a -lt $b ]
then
   echo "a is less than b"
else
   echo "None of the condition met"
fi

2.4. test

test 命令用于检查某个条件是否成立，与方括号[]类似。

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#!/bin/sh

num1=$[2*3]
num2=$[1+5]
if test $[num1] -eq $[num2]
then
    echo 'The two numbers are equal!'
else
    echo 'The two numbers are not equal!'
fi

2.5. case

case … esac 与其他语言中的 switch … case 语句类似，是一种多分枝选择结构。

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#!/bin/sh

echo -e "Input a number between 1 to 4: \c"
read aNum
case $aNum in
    1)  echo 'You select 1'
    ;;
    2)  echo 'You select 2'
    ;;
    3)  echo 'You select 3'
    ;;
    4)  echo 'You select 4'
    ;;
    *)  echo 'You do not select a number between 1 to 4'
    ;;
esac
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#!/bin/bash

option="${1}"
case ${option} in
    -f) FILE="${2}"
        echo "File name is $FILE"
    ;;
    -d) DIR="${2}"
        echo "Dir name is $DIR"
    ;;
    *) 
        echo "`basename ${0}`:usage: [-f file] | [-d directory]"
        exit 1 # Command to come out of the program with status 1
    ;;
esac

3. 循环

3.1. for

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#!/bin/bash

for loop in 1 2 3 4 5
do
    echo "The value is: $loop"
done

for str in 'This is a string'
do
    echo $str
done

for FILE in $HOME/.bash*
do
   echo $FILE
done

3.2. while

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#!/bin/bash

COUNTER=0
while ( $COUNTER <= 5 )
do
    COUNTER=`expr $COUNTER + 1`
    echo $COUNTER
done

while [ $COUNTER -lt 10 ]
do
    COUNTER=`expr $COUNTER + 1`
    echo $COUNTER
done

echo 'type <CTRL-D> to terminate'
echo -n 'enter your most liked film: '
while read FILM
do
    echo "Yeah! great film the $FILM"
done

3.3. until

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#!/bin/bash

a=0
until [ ! $a -lt 10 ]
do
    echo $a
    a=`expr $a + 1`
done

3.4. for_break

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#!/bin/bash

for var1 in 1 2 3
do
    for var2 in 0 5
    do
        if [ $var1 -eq 2 -a $var2 -eq 0 ]
        then
            break 2
        else
            echo "$var1 $var2"
        fi
    done
done

3.5. while_break

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#!/bin/bash

while :
do
    echo -n "Input a number between 1 to 5: "
    read aNum
    case $aNum in
        1|2|3|4|5) echo "Your number is $aNum!"
        ;;
        *) echo "You do not select a number between 1 to 5, game is over!"
            break
        ;;
    esac
done

3.6. for_continue

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#!/bin/bash

NUMS="1 2 3 4 5 6 7"
for NUM in $NUMS
do
    Q=`expr $NUM % 2`
    if [ $Q -eq 0 ]
    then
        echo "Number is an even number!!"
        continue
    fi
    echo "Found odd number"
done

3.7. while_continue

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#!/bin/bash

while :
do
    echo -n "Input a number between 1 to 5: "
    read aNum
    case $aNum in
        1|2|3|4|5) echo "Your number is $aNum!"
        ;;
        *) echo "You do not select a number between 1 to 5!"
            continue
            echo "Game is over!"
        ;;
    esac
done

4. 函数

4.1. hello

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#!/bin/bash

# Define your function here
Hello () {
   echo "http://www.voidking.com"
}

# Invoke your function
Hello

4.2. return

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#!/bin/bash

funWithReturn(){
    echo "The function is to get the sum of two numbers..."
    echo -n "Input first number: "
    read aNum
    echo -n "Input another number: "
    read anotherNum
    echo "The two numbers are $aNum and $anotherNum !"
    return $(($aNum+$anotherNum))
}

funWithReturn

# Capture value returnd by last command
ret=$?
echo "The sum of two numbers is $ret !"

4.3. two_function

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#!/bin/bash

# Calling one function from another
number_one () {
    echo "Url_1 is http://www.voidking.com"
    number_two
}
number_two () {
    echo "Url_2 is http://www.baidu.com"
}
number_one

number_three () {
    echo "Url_3 is http://www.baidu.com"
}

unset -f number_three

4.4. 参数

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#!/bin/bash

funWithParam(){
    echo "The value of the first parameter is $1 !"
    echo "The value of the second parameter is $2 !"
    echo "The value of the tenth parameter is $4 !"
    # 参数个数
    echo "The amount of the parameters is $# !"
    # 传递给函数的所有参数  
    echo "The string of the parameters is $* !"  
}
funWithParam 1 2 3 4

5. 获取脚本绝对路径

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scriptpath=$(cd "$(dirname "$0")"; pwd)

6. 非零状态立即停止

set -e 设置脚本在执行过程中任何命令返回非零退出状态时立即停止。

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#!/bin/bash
set -e

echo "This is a successful command."
ls non-existent-directory  # 这个命令会失败，导致脚本停止执行
echo "This command will not be executed."

7. 源码分享

https://github.com/voidking/shell.git