Shell中的条件判断语句，与其他编程语言类似。linux命令
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。
if 条件; then
语句
fi
if 条件; then
语句
else
语句
fi
if 条件; then
语句
elif 条件; then
语句
fi
if 条件; then
语句
elif 条件; then
语句
else
语句
fi
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 ~]#
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 ~]#
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]#
if grep -q root /etc/passwd; then
echo account root exists
else
echo account root not exist
fi
