shell或bash中的 判断文件是否存在的脚本语法

比如这样判断：
for i in 0 1 2; do
    if [ -e /sys/class/net/ech$i ]; then
ifconfig eth$i
break
    fi
done

语法分析--------------------------------

-f 和-e的区别

Conditional Logic on Files

-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.

是用 -s 还是用 -f 这个区别是很大的！

---------------------或者：

#!/bin/sh
# 判断文件是否存在
# link：www.jb51.net
# date：2013/2/28

myPath="/var/log/httpd/"
myFile="/var /log/httpd/access.log"

# 这里的-x 参数判断$myPath是否存在并且是否具有可执行权限
if [ ! -x "$myPath"]; then
  mkdir "$myPath"
fi
# 这里的-d 参数判断$myPath是否存在
if [ ! -d "$myPath"]; then
 mkdir "$myPath"
fi

# 这里的-f参数判断$myFile是否存在
if [ ! -f "$myFile" ]; then
 touch "$myFile"
fi
# 其他参数还有-n,-n是判断一个变量是否是否有值
if [ ! -n "$myVar" ]; then
 echo "$myVar is empty"
 exit 0
fi

# 两个变量判断是否相等
if [ "$var1" = "$var2" ]; then
 echo '$var1 eq $var2'
else
 echo '$var1 not eq $var2'
fi

---------------------其他的：

shell语法好麻烦的，一个循环都弄了一会 ，找了几个不同的方法来实现输出1－100间可以被3整除的数

1.用(())
#!/bin/bash  ///《---------------这里是bash
clear

for((i=1;i<100;i++))
for
        do
        if((i%3==0))
        then
        echo $i
        continue
        fi
        done
2.使用`seq 100`
#!/bin/bash
clear

for i in `seq 100`
        do
        if((i%3==0))
        then
        echo $i
        continue
        fi
        done
3.使用while
#!/bin/bash
clear

i=1
while(($i<100))
do
        if(($i%3==0))
        then
        echo $i
        fi
        i=$(($i+1))
done