Thx for reply ! =) . Sorry I have made my question unclear to you. What I mean  is how to read the output in filename.txt. In order to able to analyst the output in filename.txt, what I am trying to do is by recognizing some keyword, for example to get the IOS version from the output of sh ver, I will try to find the line which contains the keyword "Cisco IOS Software" at front. So I need to know the format/pattern of the output in the filename.txt. Is there any references or example output of show commands for all the Cisco switches and router ? 

In order to able to analyst the output in filename.txt, what I am trying to do is by recognizing some keyword, for example to get the IOS version from the output of sh ver, I will try to find the line which contains the keyword "Cisco IOS Software" at front. 

Command:  sh version | include Cisco IOS Software

You can use the command references for each IOS and/or device where they give examples of outputs.

These can be found by a simple internet search eg "IOS and/or device command reference".

A lot of them will be the same but as there are variants of IOS and NXOS and different devices, especially relevant to switches, the output may differ.

In addition a lot of devices can use modules and so the output can differ greatly depending on what is and isn't installed.

In effect your application needs a sort of template which matches IOS or variant and model of device and then it imports this so it knows where to look in the output for the required commands.

It can be a trial and error procedure and when I was doing automation a tedious process. I often had to log into every different type of device and record where the fields were and what to look for and then, as I say, record all this in a template file that the script would load on startup.

However that was a while back so you may find standardised templates that have done all the hard work for you.

Or you may not :-)

Jon