AWK - Delete all blank lines from a file

awk NF file.txt

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awk '/./' file.txt

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Assuming a file, file.txt, exists with the following contents:

file.txt

aaaaa
bbbbb
 
ccccc
 
 
 
ddddd
 
eeeee

returns:

aaaaa
bbbbb
ccccc
ddddd
eeeee