比如比较字符串、判断文件是否存在及是否可读等，通常用”[]”来表示条件测试。
注意：这里的空格很重要。要确保方括号的空格。笔者就曾因为空格缺少或位置不对，而浪费好多宝贵的时间。
if ….; then
….
elif ….; then
….
else
….
fi
[ -f “somefile” ] ：判断是否是一个文件
[ -x “/bin/ls” ] ：判断/bin/ls是否存在并有可执行权限
[ -n “var” ] ：判断var变量是否有值
[ “a” = “b” ] ：判断a和b是否相等
-r file 用户可读为真
-w file 用户可写为真
-x file 用户可执行为真
-f file 文件为正规文件为真
-d file 文件为目录为真
-c file 文件为字符特殊文件为真
-b file 文件为块特殊文件为真
-s file 文件大小非0时为真
-t file 当文件描述符(默认为1)指定的设备为终端时为真
**
str1 = str2 当两个串有相同内容、长度时为真
str1 != str2 当串str1和str2不等时为真
-n str1 当串的长度大于0时为真(串非空)
-z str1 当串的长度为0时为真(空串)
str1
含条件选择的shell脚本 对于不含变量的任务简单shell脚本一般能胜任。但在执行一些决策任务时，就需要包含if/then的条件判断了。shell脚本编程支持此类运算，包括比较运算、判断文件是否存在等。
基本的if条件命令选项有： – eq —比较两个参数是否相等（例如，if [ 2 –eq 5 ]）
-ne —比较两个参数是否不相等
-lt —参数1是否小于参数2
-le —参数1是否小于等于参数2
-gt —参数1是否大于参数2
-ge —参数1是否大于等于参数2
-f — 检查某文件是否存在（例如，if [ -f “filename” ]）
-d — 检查目录是否存在
几乎所有的判断都可以用这些比较运算符实现。脚本中常用-f命令选项在执行某一文件之前检查它是否存在。
——————————————————————————————-
由于程序需要，我要判断一个浮点数是否大于另一个浮点数。
大概情况描述如下：
变量 mya的值为一个两位小数，这个值是这么取的：
- [nigelzeng@ubuntu <wbr />~]$ <wbr />df <wbr />-h <wbr /> <wbr />
- Filesystem <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr />Size <wbr /> <wbr />Used <wbr />Avail <wbr />Use% <wbr />Mounted <wbr />on <wbr /> <wbr />
- /dev/sda1 <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr /> <wbr />5.7G <wbr /> <wbr />3.0G <wbr /> <wbr />2.5G <wbr /> <wbr />55% <wbr />/ <wbr /> <wbr />
- <wbr /> <wbr />
- [nigelzeng@ubuntu <wbr />~]$df <wbr />-h <wbr />| <wbr />grep <wbr />xvda2 <wbr />| <wbr />awk <wbr />‘{print <wbr />$2}’ <wbr />| <wbr />sed <wbr />‘s/G//’ <wbr /> <wbr />
- 5.7 <wbr /> <wbr />
然后我希望让它跟4进行判断，
一开始是想利用整数来进行判断，但是不行：
- [nigelzeng@ubuntu <wbr />~]$if <wbr />[ <wbr />$mya <wbr />-le <wbr />4 <wbr />]; <wbr />then <wbr />echo <wbr />“ok”;else <wbr />echo <wbr />“fail”; <wbr />fi <wbr /> <wbr />
- -bash: <wbr />[: <wbr />5.7: <wbr />integer <wbr />expression <wbr />expected <wbr /> <wbr />
只能判断整数，而不是5.7这个浮点数。
但是shell里面是没有变量类型的，所以需要想个别的办法。
参考了CU里的大牛们的建议，这样写这个比较就可以了：
- [nigelzeng@ubuntu <wbr />~]$<span style="color: #fe4c09;">if <wbr />[ <wbr />$(echo <wbr /></span><span style="color: #fe4c09;">“$mya <wbr /><= <wbr />4″|bc) <wbr />= <wbr />1 <wbr />]; <wbr />then <wbr />echo <wbr />“ok”;else <wbr />echo <wbr />“fail”</span><span style="color: #fe4c09;">;fi <wbr /></span> <wbr />
这里借助了bc这个命令（bc是一个计算器，Bash内置了对整数四则运算的支持，但是并不支持浮点运算，而bc命令可以很方便的进行浮点运算，当然整数运算也不再话下。）
shell脚本—-if（数字条件，字符串条件，字符串为空）
二元比较操作符,比较变量或者比较数字.
注意数字与字符串的区别.
1.整数比较
- -eq <wbr />等于,如:if <wbr />[ <wbr />“$a” <wbr />-eq <wbr />“$b” <wbr />] <wbr /> <wbr /> <wbr />
- -ne <wbr />不等于,如:if <wbr />[ <wbr />“$a” <wbr />-ne <wbr />“$b” <wbr />] <wbr /> <wbr /> <wbr />
- -gt <wbr />大于,如:if <wbr />[ <wbr />“$a” <wbr />-gt <wbr />“$b” <wbr />] <wbr /> <wbr /> <wbr />
- -ge <wbr />大于等于,如:if <wbr />[ <wbr />“$a” <wbr />-ge <wbr />“$b” <wbr />] <wbr /> <wbr /> <wbr />
- -lt <wbr />小于,如:if <wbr />[ <wbr />“$a” <wbr />-lt <wbr />“$b” <wbr />] <wbr /> <wbr /> <wbr />
- -le <wbr />小于等于,如:if <wbr />[ <wbr />“$a” <wbr />-le <wbr />“$b” <wbr />] <wbr /> <wbr /> <wbr />
- < <wbr /> <wbr /> <wbr />小于(需要双括号),如:((“$a” <wbr />< <wbr />“$b”)) <wbr /> <wbr /> <wbr />
- <= <wbr /> <wbr />小于等于(需要双括号),如:((“$a” <wbr /><= <wbr />“$b”)) <wbr /> <wbr /> <wbr />
- > <wbr /> <wbr /> <wbr />大于(需要双括号),如:((“$a” <wbr />> <wbr />“$b”)) <wbr /> <wbr /> <wbr />
- >= <wbr /> <wbr />大于等于(需要双括号),如:((“$a” <wbr />>= <wbr />“$b”)) <wbr /> <wbr /> <wbr />
小数据比较可使用AWK
2.字符串比较
- = <wbr />等于,如:if <wbr />[ <wbr />“$a” <wbr />= <wbr />“$b” <wbr />] <wbr /> <wbr /> <wbr />
- == <wbr />等于,如:if <wbr />[ <wbr />“$a” <wbr />== <wbr />“$b” <wbr />],与=等价 <wbr /> <wbr /> <wbr />
注意：
注意:==的功能在[[]]和[]中的行为是不同的,如下:
- [[ <wbr />$a <wbr />== <wbr />z* <wbr />]] <wbr /> <wbr /> <wbr /># <wbr />如果$a以”z”开头(模式匹配)那么将为true <wbr /> <wbr /> <wbr />
- [[ <wbr />$a <wbr />== <wbr />“z*” <wbr />]] <wbr /># <wbr />如果$a等于z*(字符匹配),那么结果为true <wbr /> <wbr /> <wbr />
- <wbr /> <wbr />
- [ <wbr />$a <wbr />== <wbr />z* <wbr />] <wbr /> <wbr /> <wbr /> <wbr /> <wbr /># <wbr />File <wbr />globbing <wbr />和word <wbr />splitting将会发生 <wbr /> <wbr /> <wbr />
- [ <wbr />“$a” <wbr />== <wbr />“z*” <wbr />] <wbr /># <wbr />如果$a等于z*(字符匹配),那么结果为true <wbr /> <wbr /> <wbr />
一点解释,关于File globbing是一种关于文件的速记法,比如”*.c”就是,再如~也是.
但是file globbing并不是严格的正则表达式,虽然绝大多数情况下结构比较像.
!= 不等于,如:if [ “a” != “b” ]
这个操作符将在[[]]结构中使用模式匹配.
< 小于,在ASCII字母顺序下.如:
if [[ “a”<"b” ]]
if [ “a” < “b” ]
注意:在[]结构中”<“需要被转义.
大于,在ASCII字母顺序下.如:
if [[ “a”>”b” ]]
if [ “a” > “b” ]
注意:在[]结构中”>”需要被转义.
具体参考Example 26-11来查看这个操作符应用的例子.
-z 字符串为”null”.就是长度为0.
-n 字符串不为”null”
注意:
使用-n在[]结构中测试必须要用””把变量引起来.使用一个未被””的字符串来使用! -z
或者就是未用””引用的字符串本身,放到[]结构中。虽然一般情况下可
以工作,但这是不安全的.习惯于使用””来测试字符串是一种好习惯.
举例：
1.数字比较
- #!/bin/bash <wbr /> <wbr />
- <wbr /> <wbr />
- i=6 <wbr /> <wbr />
- a=10 <wbr /> <wbr />
- <wbr /> <wbr />
- if <wbr />[ <wbr />$a <wbr />-eq <wbr />10 <wbr />] <wbr /> <wbr />
- then <wbr /> <wbr />
- <wbr /> <wbr /> <wbr /> <wbr />echo <wbr />“a <wbr />= <wbr />10″ <wbr /> <wbr />
- fi <wbr /> <wbr />
- <wbr /> <wbr />
- if <wbr />[ <wbr />$a <wbr />-ne <wbr />$i <wbr />] <wbr /> <wbr />
- then <wbr /> <wbr />
- <wbr /> <wbr /> <wbr /> <wbr />echo <wbr />“a <wbr />!= <wbr />$i” <wbr /> <wbr />
- fi <wbr /> <wbr />
- <wbr /> <wbr />
- if <wbr />[ <wbr />$a <wbr />-gt <wbr />$i <wbr />] <wbr /> <wbr />
- then <wbr /> <wbr /> <wbr />
- <wbr /> <wbr /> <wbr /> <wbr />echo <wbr />“a <wbr />> <wbr />i” <wbr /> <wbr />
- fi <wbr /> <wbr />
- <wbr /> <wbr />
- if <wbr />[ <wbr />$a <wbr />-lt <wbr />$i <wbr />] <wbr /> <wbr />
- then <wbr /> <wbr />
- <wbr /> <wbr /> <wbr /> <wbr />echo <wbr />“a <wbr />< <wbr />i” <wbr /> <wbr />
- else <wbr /> <wbr />
- <wbr /> <wbr /> <wbr /> <wbr />echo <wbr />“a <wbr />> <wbr />i” <wbr /> <wbr />
- fi <wbr /> <wbr />
- <wbr /> <wbr />
- if((“$a” <wbr />> <wbr />“$i”)) <wbr /> <wbr />
- then <wbr /> <wbr />
- <wbr /> <wbr /> <wbr /> <wbr />echo <wbr />“(())a>i” <wbr /> <wbr />
- fi <wbr /> <wbr />
- <wbr /> <wbr />
- if(($a <wbr />!= <wbr />$i)) <wbr /> <wbr />
- then <wbr /> <wbr />
- <wbr /> <wbr /> <wbr /> <wbr />echo <wbr />“(())a!=i” <wbr /> <wbr />
- fi <wbr /> <wbr />
备注：通过sh运行脚本，[ ]运算是可以的，而（（））运行出错
chmod 777 后，直接./ 运行，都可以
2.字符串比较
- #!/bin/bash <wbr /> <wbr />
- <wbr /> <wbr />
- a=”123″ <wbr /> <wbr />
- b=”1234″ <wbr /> <wbr />
- c=”123″ <wbr /> <wbr />
- <wbr /> <wbr />
- if <wbr />[ <wbr />“$a”x <wbr />!= <wbr />“$b”x <wbr />] <wbr /> <wbr />
- then <wbr /> <wbr />
- <wbr /> <wbr /> <wbr /> <wbr />echo <wbr />“a <wbr />!= <wbr />b” <wbr /> <wbr />
- fi <wbr /> <wbr />
- <wbr /> <wbr />
- if <wbr />[ <wbr />“$a”x <wbr />= <wbr />“$c”x <wbr />] <wbr /> <wbr />
- then <wbr /> <wbr />
- <wbr /> <wbr /> <wbr /> <wbr />echo <wbr />“a <wbr />== <wbr />c” <wbr /> <wbr />
- fi <wbr /> <wbr />
判断字符串为空
- if <wbr />[ <wbr />-z <wbr />“$d” <wbr />] <wbr /> <wbr />
- then <wbr /> <wbr />
- <wbr /> <wbr /> <wbr /> <wbr />echo <wbr />“d <wbr />is <wbr />empty” <wbr /> <wbr />
- fi <wbr /> <wbr />
备注：
-e
-a
-f
-s
-d
-b
-c
-p
-h
-L
-S(大写)