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how to delete blank lines
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First make sure that your file actually has genuine "null" lines (blank lines
could include blank spaces). So use "cat -e"

$ cat -e infile
1$
$
2$
$
3$

$ sed '/^$/d' infile      #method 1
1
2
3

$ awk '$0' infile         #method 2
$ awk 'NF' infile         
$ awk NF   infile         #save two key strokes

$ grep '.' infile         #method 3
$ grep . infile           #save two key stroke