How to reverse the order of multiline text sections separated by blank newlines/line breaks?

Question

How to reverse the order of multiline text sections separated by blank newlines/line breaks...

..., with a simple command or application, preferably without programming?

I don't want a sort, but a reverse order of the text sections only.

For example:

A
B

0
1

X
YX
Y

HTTP
FTP

becomes

HTTP
FTP

X
YX
Y

0
1

A
B

Answer 1

tac accepts a separator, so in e.g. bash you can do:

tac -s $'\n\n' foo

which will produce slightly incorrect output because the last paragraph isn't terminated by a double newline. So,

(cat foo; echo) | tac -s $'\n\n' | sed '$d'

is probably what you'd want. (Pipe contents of foo and newline through tac, then sed to strip the last line.)

Note: Do not redirect this into > foo again as you could end up with an empty file.

Answer 2

Although the author doesn't prefer programming solutions, using Python would be fairly easy in 1 line:

open('output.txt', 'w').write('\n\n'.join(reversed(open('input.txt', 'r').read().split('\n\n'))))

Obviously replace input.txt and output.txt with the desired files, and change \n to \r\n for Windows-formatted text files.

Put this in a .py file and call it via python myscript.py.

Answer 3

If you're using Windows then you could simply use Powershell. This reverses the contents of test.txt and saves the result to test2.txt:

$f=Get-Content c:\folder\test.txt
[array]::Reverse($f)
$f | out-file c:\folder\test2.txt

Update:

This should do the trick (reads all the lines from test.txt and rearranges them to test2.txt file the way you wanted):

$content = [System.IO.File]::ReadAllLines("C:\Folder\test.txt")
$a = New-Object System.Collections.ArrayList
$i=0
ForEach ($line in $content) {If ($line -ne "") {$a.insert($i++, $line)} else {$a.insert(0, $line); $i=0}}
$a | out-file C:\Folder\test2.txt

Probably not the most elegant way but it did the job with your test case.