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I have a tar file that has been tar-ed many times. It follows a similar structure like this:

1000.tar
 |
 --- filler.txt (random text)
 999.tar
 |
 --- filler.txt
 998.tar

 ...

How can I write a command/piece of code to get the final .txt of the tar structure?
I can do this by hand tar -xf 1000.tar, but it's more efficient to use code.

I'm thinking the code should go like this:

for i in range 1000, 1:
 tar -xf string(i) + ".tar"

This simple bash for loop should do what you need:

for i in 1000..1; do tar -xf $i.tar; done

Here's a recursive approach that doesn't require you to know the filenames of the tarballs in advance. Put the file in a directory that contains nothing else. Then, assuming all the tarballs contained in the parent archive have a .tar extension, you can simply do:

file=(*tar); while [[ -e $file ]]; do tar xf "$file"; rm "$file"; file=(*tar); done

Explanation

• 
  file=(*tar); : set the variable $file to contain the file name. There should be only one file that matches the *tar glob in the directory you run this in.


• 
  while [[ -e $file ]]; do : while $file exists...


• 
  tar xf $file; rm "$file"; file=(*tar); : untar the current value of $file, then delete the tarball you just extracted, and finally set the $file variable to the name of the new, now only, tar file in the directory.

And here's an even more direct approach (thanks @kusalananda!). I made the tar archive using these commands:

$ cat file 
This is the text!
$ file=file; for i in 1..1000; do tar cf $i.tar $file; file=$i.tar; done

So this is a file called file which contains the text This is the text!n. The file has been tarred 1000 times and we now have 1000.tar. We can print the original text out with:

$ awk -F'' '/[^]/print $(NF)' 1000.tar 
This is the text!

Note that that isn't actually the original text. The original newline was eaten by tar but replaced by awk. To get the real value from the archive, you'd need (the last $ there is my prompt, there was no newline):

$ awk -F'' '/[^]/printf "%s", $NF' 1000.tar 
This is the text!$

In this particular case, I was able to recreate the file name as well by telling awk to print the last field ($NF), and the 438th field before the last one:

awk -F'' '/[^]/print $(NF) >$(NF-438)' 1000.tar 

That created a new file with the contents of the original. So the same thing as extracting. However, I don't know if the -428 is a magic number. tar seems to add multiple NULLs in its archive, so I found that by running:

$ awk -F'' 'for(i=1;i<=NF;i++) if($i ~ /file/)print i,NF-i' 1000.tar 
434674 438

That told me that field 434674 had the contents file and that is 438 fields before the final one.

If your awk supports gsub, you can probably make it more general with:

awk -F'' '/[^]/gsub(/+/,""); print $NF > $(NF-11)' 1000.tar