Shell记录-Shell脚本基础(二)

Shell 基本运算符

算术运算符:

运算符描述例子
+ Addition - Adds values on either side of the operator `expr $a + $b` will give 30
- Subtraction - Subtracts right hand operand from left hand operand `expr $a - $b` will give -10
* Multiplication - Multiplies values on either side of the operator `expr $a * $b` will give 200
/ Division - Divides left hand operand by right hand operand `expr $b / $a` will give 2
% Modulus - Divides left hand operand by right hand operand and returns remainder `expr $b % $a` will give 0
= Assignment - Assign right operand in left operand a=$b would assign value of b into a
== Equality - Compares two numbers, if both are same then returns true. [ $a == $b ] would return false.
!= Not Equality - Compares two numbers, if both are different then returns true. [ $a != $b ] would return true.

这是非常重要的,这里要注意,所有的条件式将放在方括号内,他们身边有一个空格,例如 [ $a == $b ]是正确的,为[$a==$b] 是不正确的。

所有的算术计算,使用长整数。

关系运算符:

Bourne Shell的支持,关系运算符的具体数值。这些运算符不能使用字符串值,除非它们的值是数字。

例如,运算符将努力检查10和20之间的关系,以及在“10”和“20”,但不是“10”和“21”之间。

假设变量a=10,变量b=20:

运算符描述示例
-eq Checks if the value of two operands are equal or not, if yes then condition becomes true. [ $a -eq $b ] is not true.
-ne Checks if the value of two operands are equal or not, if values are not equal then condition becomes true. [ $a -ne $b ] is true.
-gt Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. [ $a -gt $b ] is not true.
-lt Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. [ $a -lt $b ] is true.
-ge Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true. [ $a -ge $b ] is not true.
-le Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true. [ $a -le $b ] is true.

这里要注意,所有的条件式将放在方括号内,他们周围有一个空格,这是非常重要的,例如 [ $a <= $b ]是正确的, [$a <= $b]是不正确的。

布尔运算:

布尔运算符有以下Bourne Shell的支持。

假设变量一个变量b=10,然后变量b=20:

运算符描述示例
! This is logical negation. This inverts a true condition into false and vice versa. [ ! false ] is true.
-o This is logical OR. If one of the operands is true then condition would be true. [ $a -lt 20 -o $b -gt 100 ] is true.
-a This is logical AND. If both the operands are true then condition would be true otherwise it would be false. [ $a -lt 20 -a $b -gt 100 ] is false.

字符串运算符:

有下列字符串运算由Bourne Shell支持。

假设变量a=“abc”和变量b=“efg”:

运算符描述例子
= Checks if the value of two operands are equal or not, if yes then condition becomes true. [ $a = $b ] is not true.
!= Checks if the value of two operands are equal or not, if values are not equal then condition becomes true. [ $a != $b ] is true.
-z Checks if the given string operand size is zero. If it is zero length then it returns true. [ -z $a ] is not true.
-n Checks if the given string operand size is non-zero. If it is non-zero length then it returns true. [ -z $a ] is not false.
str Check if str is not the empty string. If it is empty then it returns false. [ $a ] is not false.

文件测试操作:

有以下是操作测试Unix文件相关联的各种属性。

假设一个的变量文件保存现有文件名“test”,其大小为100字节,有读,写和执行权限:

操作符描述示例
-b file Checks if file is a block special file if yes then condition becomes true. [ -b $file ] is false.
-c file Checks if file is a character special file if yes then condition becomes true. [ -b $file ] is false.
-d file Check if file is a directory if yes then condition becomes true. [ -d $file ] is not true.
-f file Check if file is an ordinary file as opposed to a directory or special file if yes then condition becomes true. [ -f $file ] is true.
-g file Checks if file has its set group ID (SGID) bit set if yes then condition becomes true. [ -g $file ] is false.
-k file Checks if file has its sticky bit set if yes then condition becomes true. [ -k $file ] is false.
-p file Checks if file is a named pipe if yes then condition becomes true. [ -p $file ] is false.
-t file Checks if file descriptor is open and associated with a terminal if yes then condition becomes true. [ -t $file ] is false.
-u file Checks if file has its set user id (SUID) bit set if yes then condition becomes true. [ -u $file ] is false.
-r file Checks if file is readable if yes then condition becomes true. [ -r $file ] is true.
-w file Check if file is writable if yes then condition becomes true. [ -w $file ] is true.
-x file Check if file is execute if yes then condition becomes true. [ -x $file ] is true.
-s file Check if file has size greater than 0 if yes then condition becomes true. [ -s $file ] is true.
-e file Check if file exists. Is true even if file is a directory but exists. [ -e $file ] is true.
#!/bin/sh

a=10
b=20
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

记下有以下几点:

  • 运算符和表达式之间必须有空格,例如2+2是不正确的,因为它应该写成 2 + 2。

  • ``,称为倒逗号之间应包含完整的表达。

  • 应该用*符号的乘法。

    #!/bin/sh
    
    a=10
    b=20
    
    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

 

#!/bin/sh

a=10
b=20

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


#!/bin/sh

a="abc"
b="efg"

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


#!/bin/sh

file="/var/www/yiibai/unix/test.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 zero"
else
   echo "File size is not zero"
fi

if [ -e $file ]
then
   echo "File exists"
else
   echo "File does not exist"
fi

 

posted @ 2017-10-24 16:17  信方  阅读(151)  评论(0编辑  收藏  举报