linux中Bash条件表达式使用示例

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)

if [[ -e $filename ]]; then
  echo "$filename exists"
fi

也可以测试指定类型的文件

if [[ -f $filename ]]; then
  echo "$filename is a regular file"
elif [[ -d $filename ]]; then
  echo "$filename is a directory"
elif [[ -p $filename ]]; then
  echo "$filename is a named pipe"
elif [[ -S $filename ]]; then
  echo "$filename is a named socket"
elif [[ -b $filename ]]; then
  echo "$filename is a block device"
elif [[ -c $filename ]]; then
  echo "$filename is a character device"
fi
if [[ -L $filename ]]; then
  echo "$filename is a symbolic link (to any file type)"
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

if [[ -L $filename || -e $filename ]]; then
  echo "$filename exists (but may be a broken symbolic link)"
fi
 
if [[ -L $filename && ! -e $filename ]]; then
  echo "$filename is a broken symbolic link"
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

if [[ "$string1" == "$string2" ]]; then
  echo "\$string1 and \$string2 are identical"
fi
if [[ "$string1" == "$string2" ]]; then
  echo "\$string1 and \$string2 are identical"
fi
如果右则没有引号，那么它是以通配符的模式作比较。

string1='abc'
pattern='a*'
if [[ "$string1" == $pattern ]]; then
  # the test is true
  echo "The string $string1 matches the pattern $pattern"
fi
if [[ "$string1" != "$pattern" ]]; then
  echo "The string $string1 is not equal to the string $pattern"
fi

<和>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

if [[ -n $string ]]; then
  echo "$string is non-empty"
fi
if [[ -z "${string// }" ]]; then
  echo "$string is empty or contains only spaces"
fi
if [[ -z $string ]]; then
  echo "$string is empty"
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：

if [[ -n ${string+x} ]]; then
    echo "$string is set, possibly to the empty string"
fi
if [[ -n ${string-x} ]]; then
    echo "$string is either unset or set to a non-empty string"
fi
if [[ -z ${string+x} ]]; then
    echo "$string is unset"
fi
if [[ -z ${string-x} ]]; then
    echo "$string is set to an empty string"
fi

其中x是任意的，以表格形式如下：

+-------+-------+-----------+
            $string is: | unset | empty | non-empty |
+-----------------------+-------+-------+-----------+
| [[ -z ${string} ]]    | true  | true  | false     |
| [[ -z ${string+x} ]]  | true  | false | false     |
| [[ -z ${string-x} ]]  | false | true  | false     |
| [[ -n ${string} ]]    | false | false | true      |
| [[ -n ${string+x} ]]  | false | true  | true      |
| [[ -n ${string-x} ]]  | true  | false | true      |
+-----------------------+-------+-------+-----------+

文件权限测试

if [[ -r $filename ]]; then
  echo "$filename is a readable file"
fi
if [[ -w $filename ]]; then
  echo "$filename is a writable file"
fi
if [[ -x $filename ]]; then
  echo "$filename is an executable file"
fi