有一个需求是判断某一个目录下文件是否存在，如果该文件不存在，则从当前目录拷贝文件到该目录。
由于没有学习写过shell脚本，晚上下班回来搜索后写出了如下的脚本，这里shell的if语法格式与C语言大致相同，语法格式：
if [ 条件 ]; then
条件成立
else
条件不成立
fi
fi是结束if语句。

version1.0

#!/bin/sh
myFile="/root/apue/example.xml "

# 这里的-f参数判断$myFile是否存在
if [ ! -f "$myFile" ]; then
cp  ./example.xml  /root/apue/
fi

echo "success"

在执行了多次后发现无论文件example.xml是否存在，都会将当前目录下的example.xml拷贝到/root/apue/目录下。
原因困扰我达半小时之久直到我在洗澡时才发现原来myFile=”/root/apue/example.xml “最后双引号前面多了空格，去掉空格后验证，目的达到,
附shell中常用的其它判断形式：

-a file exists. 
-b file exists and is a block special file. 
-c file exists and is a character special file. 
-d file exists and is a directory. 
-e file exists (just the same as -a). 
-f file exists and is a regular file. 
-g file exists and has its setgid(2) bit set. 
-G file exists and has the same group ID as this process. 
-k file exists and has its sticky bit set. 
-L file exists and is a symbolic link. 
-n string length is not zero. 
-o Named option is set on. 
-O file exists and is owned by the user ID of this process. 
-p file exists and is a first in,first out (FIFO) special file or 
named pipe. 
-r file exists and is readable by the current process. 
-s file exists and has a size greater than zero. 
-S file exists and is a socket. 
-t file descriptor number fildes is open and associated with a 
terminal device. 
-u file exists and has its setuid(2) bit set. 
-w file exists and is writable by the current process. 
-x file exists and is executable by the current process. 
-z string length is zero.