[SOLVED] writing a script

kenny53067 · 07-23-2011, 01:49 PM

how do I write a workable script that demonstrates that variable expansion occurs before pathname expansion. Can someone help me, I don't know where or how to start.

grail · 07-23-2011, 02:19 PM

Sounds suspiciously like homework. Have you tried googling for any of this? If it is in fact homework one would think you have probably already been instructed and so it is
in notes somewhere.

kenny53067 · 07-23-2011, 02:37 PM

Quote:

Originally Posted by grail

Sounds suspiciously like homework. Have you tried googling for any of this? If it is in fact homework one would think you have probably already been instructed and so it is
in notes somewhere.

Our instructor gave us this:
$ ls g?

g1 g2 g5 g8

$ a=g?

$ echo "$a"

g?

$ echo $a

g1 g2 g5 g8

but I don't understand what this is.

grail · 07-23-2011, 02:47 PM

Well in your example 'a' is a variable and '?' is being used by the shell to expand into any single character. So I believe this gives you exactly the information you require to answer your question.

kenny53067 · 07-23-2011, 02:51 PM

Quote:

Originally Posted by grail

Well in your example 'a' is a variable and '?' is being used by the shell to expand into any single character. So I believe this gives you exactly the information you require to answer your question.

so without the question mark g would not expand, and this would cause only g8 to show in the output. Is this correct?

grail · 07-23-2011, 04:12 PM

Think about what you are saying, would the following work with your logic:

Code:

$ ls g
g8

kenny53067 · 07-23-2011, 04:13 PM

Quote:

Originally Posted by kenny53067

so without the question mark g would not expand, and this would cause only g8 to show in the output. Is this correct?

$ a=g? Is this an example of variable expansion?

$ echo "$a" Is an example of pathname expansion?

Am I understanding this correctly?

grail · 07-23-2011, 04:25 PM

Quote:

Am I understanding this correctly?

ummm ... not exactly

Code:

$ a=g?

This is an example of variable assignment. Variable expansion is not performed until the variable prefixed with a $

Code:

$ echo "$a"

This is an example of a command acting on a variable after expansion

catkin · 07-23-2011, 04:28 PM

Shell operations, including their order is explained here. "Not a lot of people know that"! But it is key to a full understanding of how bash works. Time for bed now; will answer your specific question -- if not answered already -- tomorrow,

kenny53067 · 07-23-2011, 04:30 PM

Quote:

Originally Posted by grail

Think about what you are saying, would the following work with your logic:

Code:

$ ls g
g8

$ ls g This command would list all files that contain g Is this correct?

markush · 07-23-2011, 04:42 PM

Hello kenny53067,

you should write some code by yourself and try it out. Learning programming doesn't work without experience. Just work in it.

Note that it is much easier for the advanced members here at LQ to help you with some code you've written.

Markus

grail · 07-23-2011, 04:48 PM

Quote:

$ ls g This command would list all files that contain g Is this correct?

markush's response is the correct one here. Try it for yourself and see what happens. Create three files starting with the letter g and see what
happens when you issue the command.

Diantre · 07-23-2011, 04:54 PM

Quote:

Originally Posted by kenny53067

$ ls g This command would list all files that contain g Is this correct?

No. Actually that command will only list a file or directory named "g", if exists. You need to use wildcards in your command so the shell expands them.

Have a look at the bash manpage, section EXPANSION. You'll find there the types of expansion the shell performs, and their explanation.

markush · 07-23-2011, 05:00 PM

Hello,

@kenny53067, it seems you're posting from a Windows-computer. If you don't have Linux running, you should either boot a live-CD or use for example Virtualbox and install a Linux-distribution as a virtual machine. Then you can try out the code.

Markus

kenny53067 · 07-23-2011, 05:06 PM

Quote:

Originally Posted by markush

Hello kenny53067,

you should write some code by yourself and try it out. Learning programming doesn't work without experience. Just work in it.

Note that it is much easier for the advanced members here at LQ to help you with some code you've written.

Markus

I understand this 100%
I'm trying to understand what things do so that I will be able to write code.

example: I didn't know that ls Memo would not work, but ls Memo? would list all files with Memo in the name. Now I know.