UNIX菜菜鸟：一个非常简单的shell script

我写得不知道为什么总也通不过，不知道有哪位可以帮帮忙写出来让我看看，我自己错在什么地方了，多谢。虽然我知道这个很简单，但是我现在搞不掂，只好请大家帮帮忙拉。


1. Check if the file exists
2. Check if the input argument is a Directory, instead of a file.
3. Check if right number of argument is entered.

Here are a few sample runs of the shell script chex2.
这是这个scripts运行时的几个输入例子。

$ ls -l
-rwxrwxr-x 1 ngyat ngyat 293 Oct 3 22:22 chex2*
-rw-rw-r-- 1 ngyat ngyat 8 Oct 3 22:36 test123
drwxrwxr-x 2 ngyat ngyat 4096 Oct 3 22:20 try/

$ chex2 test123
test123 is now executable:
-rwxrw-r-- 1 ngyat ngyat 8 Oct 3 22:36 test123


$ chex2 try
try is a Directory.


$ chex2 file_no_exits
file_no_exits does not exist.


$ chex2 test123 extra_arg
usage: chex2 <infile>

$ chex2 test123 extra_arg extra_arg2
usage: chex2 <infile>

$ chex2
usage: chex2 <infile>




以下是老师的Hints,高手不看也罢（我是傻的看了都没用）。
Hints:


1. Argument Variables:

$n [where n = 1, 2, ... n] The argument value follows the command.
e.g. if we type: my_command a1 a2


then, $0 = my_command ,$1 = a1 and $2 = a2

$#argv report the number of words in the input list

e.g. if we type: my_command a1 a2
we will have $#argv equals 2



2. To test attribute of a file, you may want to use the following table:

Argument Test if it is a ...
-d file file is a directory
-f file file is an ordinary file
-r file file is reable
-w file file is writable
-x file file is executable

Note: you only need to use TWO of the above arguments, NOT all.



The following are two examples of using the above attributes:


a) to check if a file is readable,

if (-r $filename) then
...
endif

b) to check if a file is NOT writable,

if (! -w $filename) then
...
endif

3. Use exit command to get out of the if loop and the script

4. Here is one sample portion of logic of your shell script. [Hint: There should be three of this if part].

if no input file name then

print error message

exit ## this command will terminate the script and lets you get back to the shell prompt

endif