[Linux 常見問題] How to print a file, excluding comments and blank lines, using grep/sed?

Source From Here 
Question 
As title with text file as below: 
- test.txt 

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1. This is line1  
2. # This is the line being commented  
3.   
4. This is line2  

The expected output is: 

This is line1
This is line2

How-To 
Try command egrep or grep with argument '-E': 

// -E: Interpret PATTERN as an extended regular expression
// -v, --invert-match: Invert the sense of matching, to select non-matching lines.
# grep -E -v '^$|^\s*#' test.txt
This is line1
This is line2

# egrep -v '^$|^\s*#' test.txt
This is line1
This is line2