我使用过的Linux命令之if - Bash中的条件判断语句

本文链接：http://codingstandards.iteye.com/blog/780156 ?? （转载请注明出处）

用途说明

Shell中的条件判断语句，与其他编程语言类似。

如果需要知道有哪些条件判断方式，通过man test就可以得到帮助。

常用格式

格式一

if 条件; then

??? 语句

fi

格式二

if 条件; then

??? 语句

else

??? 语句

fi

格式三

if 条件; then

??? 语句

elif 条件; then

??? 语句

fi

格式四

if 条件; then

??? 语句

elif 条件; then

??? 语句

else

??? 语句

fi

使用示例

示例一

if [ "foo" = "foo" ]; then
    echo expression evaluated as true
fi

?

[root@jfht ~]# if [ "foo" = "foo" ]; then
>???? echo expression evaluated as true
> fi
expression evaluated as true
[root@jfht ~]#

示例二

if [ "foo" = "foo" ]; then
    echo expression evaluated as true
else
    echo expression evaluated as false
fi

?

[root@jfht ~]# if [ "foo" = "foo" ]; then
>???? echo expression evaluated as true
> else
>???? echo expression evaluated as false
> fi
expression evaluated as true
[root@jfht ~]#

示例三

T1="foo"
T2="bar"
if [ "$T1" = "$T2" ]; then
    echo expression evaluated as true
else
    echo expression evaluated as false
fi

?

[root@jfht ~]# T1="foo"
[root@jfht ~]# T2="bar"
[root@jfht ~]# if [ "$T1" = "$T2" ]; then
>???? echo expression evaluated as true
> else
>???? echo expression evaluated as false
> fi
expression evaluated as false
[root@jfht ~]#

示例四 判断命令行参数数量

文件 if_4.sh

#!/bin/sh

if [ "$#" != "1" ]; then
    echo "usage: $0 <file>"
    exit 1
fi

?

[root@smsgw root]# cat if_4.sh
#!/bin/sh

if [ "$#" != "1" ]; then
??? echo "usage: $0 <file>"
??? exit 1
fi

[root@smsgw root]# chmod +x if_4.sh
[root@smsgw root]# ./if_4.sh
usage: ./if_4.sh <file>
[root@smsgw root]# ./if_4.sh hello
[root@smsgw root]#

?

示例五 判断文件中是否包含某个字符串

if grep -q root /etc/passwd; then
    echo account root exists
else
    echo account root not exist
fi

?

[root@jfht ~]# if grep -q root /etc/passwd; then
>???? echo account root exists
> else
>???? echo account root not exist
> fi
account root exists
[root@jfht ~]#

?

示例六 判断文件是否存在

if [ -e myfile ]; then
    echo myfile exists
else
    touch myfile
    echo myfile created
fi

?

[root@jfht ~]# if [ -e myfile ]; then
>???? echo myfile exists
> else
>???? touch myfile