IF语句有三种格式：
第一种：if ... fi statement
下面是一个实例：
cat if1.sh #!/bin/sh a=10 b=20 #① if [ $a -eq $b ]; then echo "a is equal to b"; fi if [ $a -gt $b ]; then echo "a is great than b"; fi #② if [ $a -lt $b ] then echo "a is less than b"; fi # the EOF注意：
①条件和处理命令分开的一种写法：
if 条件; then
处理命令
fi
②条件和处理命令分开的另一种写法：
if 条件
then
处理命令
fi
这里需要根据个人习惯去选择。
上面的例子中，变量的值是赋死了的，若要给此脚本传递两个参数，可做如下修改：
cat if1.sh #!/bin/sh # a=10 # b=20 if [ $1 -eq $2 ]; then echo "the first number is equal to the second"; fi if [ $1 -gt $2 ]; then echo "the first number is great than the second"; fi if [ $1 -lt $2 ] then echo "the first number is less than the second"; fi # the EOF
给脚本传递参数，只需要在sh命令后面添加即可，使用空格隔开：
sh if1.sh 1 2 the first number is less than the second sh if1.sh 12 1 the first number is great than the second sh if1.sh 1 1 the first number is equal to the second
第二种：if ... else ... fi，具体如下：
if [ expression ]
then
statement(s) to be sxecuted if expression is true
else
statement(s) to be sxecuted if expression is not true
fi
一个简单的实例
cat ifparam.sh #!/bin/sh if [ $# -lt 3 ]; then echo "Usage:`basename $0` arg1 arg2 arg3" >&2 exit 1 fi #EOFecho "arg1:$1"
echo "arg1:$2"
echo "arg1:$3"
脚本注解：
1.$#表示参数输入的个数
2.basename $0打印文件的名称
3.若输入小于三个参数则，将输出一个信息，这个信息被当做是错误信息（>&2）。
执行脚本：
sh ifparam.sh scott tom
Usage:ifparam.sh arg1 arg2 arg3
sh ifparam.sh scott tom jim
arg1:scott
arg1:tom
arg1:jim
再来看一个测试：
cat ifeditor.sh #!/bin/csh #if [ -z $EDITOR ]① #if [ -z "`echo $EDITOR`" ]③ #下面这种写法不能正确的计算出环境的值 #因为wc -c计算包括新的空的行 #if [ `echo $EDITOR | wc -c` -eq 0 ]② if [ -z "`echo $EDITOR`" ] then echo "Your EDITOR environment is not set" else echo "Using $EDITOR as the default editor" fi #EOFsh ifeditor.sh
Your EDITOR environment is not set
这里使用三种方式去检测环境变量是否设置；
①直接报错：语法错误
②正确写法，使用test检测，-z表示如果为设置则长度为0返回值为true
③wc -c计算包括了空行，所以不准确
第三种：if ... elif ... fi，具体如下：
if [ expression 1 ]; then statement(s) to be sxecuted if expression 1 is true elif [ expression 2 ]; then statement(s) to be sxecuted if expression 2 is true elif [ expression 3 ]; then statement(s) to be sxecuted if expression 3 is true else statement(s) to be sxecuted if no expression is true fi下面是一个比较两个数字大小的例子：
cat elif.sh #!/bin/sh if [ $1 == $2 ]; then echo "the first number is equal to the next" elif [ $1 -gt $2 ]; then echo "the first number is great than the next" elif [ $1 -lt $2 ]; then echo "the first number is great than the next" else echo "None of the condition met" fi #EOF当不输入任何数字的时候，也就是为空，结果如下：
sh elif.sh
the first number is equal to the next
当输入数字的时候，如下：
sh elif.sh 10 20
elif.sh: test: unknown operator ==
这种情况，我们可以将其错误信息输入到一个文件当中，如下：
sh elif.sh 10 20 > log.txt 2>&1
cat log.txt
elif.sh: test: unknown operator ==
当将“==”号修改为-eq，结果如下：
sh elif.sh 10 20
the first number is less than the next
下面是一个if的实例，包括这三种命令格式；
脚本的作用是创建一个目录，如果不输入任何值，则打印脚本的作用说明；
输入则提示是否创建，输入非提示，则报错误，否则按提示走。
#!/bin/sh
DIR=$1
if [ "$DIR" = "" ]; then
echo "Usage:`basename $0` directory to create" >&2
exit 1
fi
if [ -d $DIR ]; then
echo "Directory $DIR exists"
else
echo "The Directory does exist"
echo -n "Create it now?[y..n]:"
read ANS
if [ "$ANS" = "y" ] || [ "$ANS" = "Y" ]; then
echo "creating now"
mkdir $DIR > log.txt 2>&1
if [ $? -ne 0 ]; then
echo "Errors creating the directory $DIR" >&2
exit 1
fi
echo "Creating successful"
elif [ "$ANS" = "n" ] || [ "$ANS" = "N" ]; then
echo "Giving up creating directory $DIR"
else
echo "Bad input"
fi
fi
#EOF1.不输入参数
sh ifmkdir.sh
Usage:ifmkdir.sh directory to create
2.输入一个存在的目录，提示目录已经存在：
sh ifmkdir.sh test
Directory test exists
查看确实有test目录：
[ -d test ]
echo $?
0
创建test1目录：sh ifmkdir.sh test1
The Directory does exist
Create it now?[y..n]:y
creating now
Creating successful
3.执行脚本，但不想创建目录：
sh ifmkdir.sh test2
The Directory does exist
Create it now?[y..n]:n
Giving up creating directory test2
4.执行脚本时，不按照提示输入：
sh ifmkdir.sh test2
The Directory does exist
Create it now?[y..n]:d
Bad input
更多信息，参考：Decision making