Bash使用示例(5) – 条件表达式

文件类型测试



-e条件运算符用来测试一个文件是否存在(包括所有文件类型,目录等)

  1. if [[ -e $filename ]]; then
  2.   echo "$filename exists"
  3. fi

也可以测试指定类型的文件

  1. if [[ -f $filename ]]; then
  2.   echo "$filename is a regular file"
  3. elif [[ -d $filename ]]; then
  4.   echo "$filename is a directory"
  5. elif [[ -p $filename ]]; then
  6.   echo "$filename is a named pipe"
  7. elif [[ -S $filename ]]; then
  8.   echo "$filename is a named socket"
  9. elif [[ -b $filename ]]; then
  10.   echo "$filename is a block device"
  11. elif [[ -c $filename ]]; then
  12.   echo "$filename is a character device"
  13. fi
  14. if [[ -L $filename ]]; then
  15.   echo "$filename is a symbolic link (to any file type)"
  16. fi

对于软链接，使用-L测试时，当链接指向的目标文件不存在时会返回false

  1. if [[ -L $filename || -e $filename ]]; then
  2.   echo "$filename exists (but may be a broken symbolic link)"
  3. fi
  4.  
  5. if [[ -L $filename && ! -e $filename ]]; then
  6.   echo "$filename is a broken symbolic link"
  7. fi

字符串比较和匹配



在两个带引号的字符串之间比较时使用==操作符。!=操作符则是否则的比较

  1. if [[ "$string1" == "$string2" ]]; then
  2.   echo "\$string1 and \$string2 are identical"
  3. fi
  4. if [[ "$string1" == "$string2" ]]; then
  5.   echo "\$string1 and \$string2 are identical"
  6. fi

如果右则没有引号，那么它是以通配符的模式作比较。

  1. string1='abc'
  2. pattern='a*'
  3. if [[ "$string1" == $pattern ]]; then
  4.   # the test is true
  5.   echo "The string $string1 matches the pattern $pattern"
  6. fi
  7. if [[ "$string1" != "$pattern" ]]; then
  8.   echo "The string $string1 is not equal to the string $pattern"
  9. fi

操作符以字典顺序来比较字符串(它们没有小或等于或大于的说法)
可以测试空字符串

  1. if [[ -n $string ]]; then
  2.   echo "$string is non-empty"
  3. fi
  4. if [[ -z "${string// }" ]]; then
  5.   echo "$string is empty or contains only spaces"
  6. fi
  7. if [[ -z $string ]]; then
  8.   echo "$string is empty"
  9. fi

以上的-z检查可能表示$string没有被设置或者已经设置了但为空字符。为了区分空字符和未设置，使用：

  1. if [[ -n ${string+x} ]]; then
  2.     echo "$string is set, possibly to the empty string"
  3. fi
  4. if [[ -n ${string-x} ]]; then
  5.     echo "$string is either unset or set to a non-empty string"
  6. fi
  7. if [[ -z ${string+x} ]]; then
  8.     echo "$string is unset"
  9. fi
  10. if [[ -z ${string-x} ]]; then
  11.     echo "$string is set to an empty string"
  12. fi

其中x是任意的，以表格形式如下：

  1. +-------+-------+-----------+
  2.             $string is: | unset | empty | non-empty |
  3. +-----------------------+-------+-------+-----------+
  4. | [[ -z ${string} ]]    | true  | true  | false     |
  5. | [[ -z ${string+x} ]]  | true  | false | false     |
  6. | [[ -z ${string-x} ]]  | false | true  | false     |
  7. | [[ -n ${string} ]]    | false | false | true      |
  8. | [[ -n ${string+x} ]]  | false | true  | true      |
  9. | [[ -n ${string-x} ]]  | true  | false | true      |
  10. +-----------------------+-------+-------+-----------+

文件权限测试


  1. if [[ -r $filename ]]; then
  2.   echo "$filename is a readable file"
  3. fi
  4. if [[ -w $filename ]]; then
  5.   echo "$filename is a writable file"
  6. fi
  7. if [[ -x $filename ]]; then
  8.   echo "$filename is an executable file"
  9. fi
标签：Bash 发布于：2019-11-21 01:48:02
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