bash笔记（三）——条件测试

在if 语句之后除了可以跟命令行之外，还可以用[...]结构，[...]结构和内部命令 test 是等效的，即 [ string1 = string2 ] 和 test string1 = string2 的结果是相同的。另外在bash 2.05之后的版本还可以用[[...]] 结构，[[...]] 和[...]之间的不同可参考: http://stackoverflow.com/a/3427931/1036923 .

注意：在 “[” 之后和 “]” 之前空格是必须的。

通常代码形式如下：

if [ -n "$varname" ]; then
    echo "hello"
else
    echo '$varname' is null!
fi

在条件表达式里始终用双引号将变量引起，有以下两个原因：

1. 将变量解释为一个单词。
2. 如果变量是null, 没有双引号则上例中条件测试将变为[ -n ] ,它总是返回true； 用了双引号则表达式为[ -n "" ] ；

条件组合

if command && [ condition ] && [ condition ] || [ condition ]; then

或

if command && [ \( condition \) -a \( condition \) -o \( condition \) ]; then
...

-a 和 -o 只能用在测试条件表达式中

字符串比较

Operator	True if...
str1 = str2	str1 matches str2
str1 != str2	str1 does not match str2
str1 < str2	str1 is less than str2
str1 > str2	str1 is greater than str2
-n str1	str1 is not null (has length greater than 0)
-z str1	str1 is null (has length 0)

注：bash 没有 >= 和 <= 比较符。

文件测试

下表列出常用的文件测试表达式。

Operator	True if...
-a file	file exists
-d file	file exists and is a directory
-e file	file exists; same as -a
-f file	file exists and is a regular file(i.e., not a directory or other special type of file)
-r file	You have read permission on file
-s file	file exists and is not empty
-w file	You have write permission on file
-x file	You have execute permission on file, or directory search permission if it is a directroy
-N file	file was modified since it was last read
-O file	You own file
-G file	file's group ID matchs yours (or one of yours, if you are in multiple groups)
file1 -nt file2	file1 is newer than file2
file1 -ot file2	file1 is older than file2

算术比较

Test	Comparison
-lt	Less than
-le	Less than or equal
-eq	Equal
-ge	Greater than or equal
-gt	Greater than
-ne	Not equal

注：bash还有针对整数比较的独立语法，效率更高，所以要优先用它而不是上表列出的算术比较运算符。