Here is a test file :
$ cat test
Now, if you want to remove all the blank lines from a file , you can do it as follows:
$ cat test | grep -v '^$' > test1
$ mv test1 test
But , here we are creating a temporary file "test1"
Now , if you want to remove all the blank lines from a file without creating temporary file, it can be done as follows:
$ sed -i '/^$/d' test
"sed -i" overwrites the file itself
so it will remove blank lines and override the file.
Now, let's run the cat command on test