UNIX Shell控制结构—IF

流控制（Decision Making）
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/sha=10b=20#①if [ $a -eq $b ]; then  echo "a is equal to b";fiif [ $a -gt $b ]; then  echo "a is great than b";fi#②if [ $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=20if [ $1 -eq $2 ]; then  echo "the first number is equal to the second";fiif [ $1 -gt $2 ]; then  echo "the first number is great than the second";fiif [ $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 2the first number is less than the secondsh if1.sh 12 1the first number is great than the secondsh if1.sh 1 1the 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/shif [ $# -lt 3 ]; then   echo "Usage:`basename $0` arg1 arg2 arg3" >&2   exit 1fi#EOF

echo "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 ]②if [ -z "`echo $EDITOR`" ] then   echo "Your EDITOR environment is not set"else   echo "Using $EDITOR as the default editor"fi#EOF

sh 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 trueelif [ expression 2 ]; then   statement(s) to be sxecuted if expression 2 is trueelif [ expression 3 ]; then   statement(s) to be sxecuted if expression 3 is trueelse   statement(s) to be sxecuted if no expression is truefi

下面是一个比较两个数字大小的例子：

cat elif.sh#!/bin/shif [ $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/shDIR=$1if [ "$DIR" = "" ]; then   echo "Usage:`basename $0` directory to create" >&2   exit 1fiif [ -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"   fifi#EOF

1.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