This is not a duplicate of How to verify a backup file from ADB backup. The poster there probably didn't know that an unrooted phone requires -nosystem (ref) and ended with a 0 byte file, where my backup file is almost 8GB.

Given that adb gives almost zero feedback on it's backup progress and success or failure. How can I be confident that the backup I've just completed is a good one? and that it contains what I think it does? and that it will (probably) work to restore or extract data from?

I have the one android device (Nexus 5), access to Win7 PCs, and ubuntu virtual machines if needed.

Though not directly related to your question, this does answer it in a very round-about way. I found this blog which gives directions on how to open the file and modify contents. My reasoning is that if the file opens then it should probably be ok.
Here's the link: http://nelenkov.blogspot.in/2012/06/unpacking-android-backups.html
I'd really love if anyone finds a more straight forward foolproof approach and post it here.

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    Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. Otherwise, your answer becomes useless in case the link dies. – Izzy Dec 17 '14 at 17:36

If you're on Linux, Adebar contains a tiny little shell script to convert ADB Backups (.ab) to tar files, which you then easily can investigate with any archive manager. Only limitation is: it doesn't work on password protected backup files.

The basic command it runs (everything else mainly is a wrapper for command line options and syntax) is:

dd if=$1 bs=24 skip=1 | openssl zlib -d >${1%%.ab}.tar && gzip -9 ${1%%.ab}.tar

A command I've found here on SE some time in the past (full disclosure: I'm the author of Adebar) – see here.

As you can see in the quoted command, it requires openssl and zlib to be available.

  • note: On Ubuntu, the 'openssl zlib -d' will give you an error message. You can use 'zlib-flate -uncompress' instead. – jelle foks Feb 17 '15 at 6:43
  • Umm? I use it that way on Ubuntu. Adebar is being developed/tested on Ubuntu even. But it's always good to have alternatives, so thanks for the note, @jellefoks! – Izzy Feb 17 '15 at 12:45

For unencrypted backups, it is possible to list the content using the command

dd if=backup-file2.ab bs=24 skip=1 | pigz -d | tar -tvf - > backup-file.ab.list

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