Using tar to update archive with files from a while loop?

Question

I have the problem listed here: Oct 2019 upgrade: pip upgrade failure · Issue #5844 · msys2/MINGW-packages: basically, pip on MSYS2 refuses to update because it sees some files. pacman -Syu basically dumps this:

...
:: Proceed with installation? [Y/n] y
:: Retrieving packages...
 mingw-w64-x86_64-gl...     4.6 MiB  4.97M/s 00:01 [#####################] 100%
 mingw-w64-x86_64-gn...  1942.9 KiB  1045K/s 00:02 [#####################] 100%
(18/18) checking keys in keyring                   [#####################] 100%
(18/18) checking package integrity                 [#####################] 100%
(18/18) loading package files                      [#####################] 100%
(18/18) checking for file conflicts                [#####################] 100%
error: failed to commit transaction (conflicting files)
python2-pip: /c/msys64/usr/lib/python2.7/site-packages/pip/_internal/commands/debug.py exists in filesystem
python2-pip: /c/msys64/usr/lib/python2.7/site-packages/pip/_internal/commands/debug.pyc exists in filesystem
...

I don't want to uninstall pip, and it seems (pacman "exists on filesystem" error) that I can "just" move the files elsewhere, then have the update run. Of course, I'd like to preserve the original file locations, of the files that I'm moving.

So, I thought, maybe I could tar those files in an archive (using --remove-files so the originals are also deleted, so they don't trip the update any more), then restore them after update - if any other packages I need, have installed them.

Unfortunately, I really cannot find the right invocation, so that a tar archive is update with files that come in from a while loop (that is, I don't have a "one-line", space-separated list of files).

This results with only a single file in archive:

$ pacman -Syu | grep exists | awk '{print $2}' | while read f; do tar -czf /tmp/bckp.tar "$f"; done
:: Proceed with installation? [Y/n] y
error: failed to commit transaction (conflicting files)
tar: Removing leading `/' from member names
tar: Removing leading `/' from member names
tar: Removing leading `/' from member names
...

$ tar tzvf /tmp/bckp.tar
-rw-r--r-- user/None       13854 2019-09-02 09:07 c/msys64/usr/lib/python3.7/site-packages/pip/_vendor/urllib3/packages/rfc3986/validators.py

This won't even run:

$ pacman -Syu | grep exists | awk '{print $2}' | while read f; do tar -czvf --strip-components=1 --append /tmp/bckp.tar.gz "$f"; done
:: Proceed with installation? [Y/n] y
error: failed to commit transaction (conflicting files)
tar: You may not specify more than one '-Acdtrux', '--delete' or  '--test-label' option
Try 'tar --help' or 'tar --usage' for more information.
tar: You may not specify more than one '-Acdtrux', '--delete' or  '--test-label' option
Try 'tar --help' or 'tar --usage' for more information.
...

This won't either:

$ pacman -Syu | grep exists | awk '{print $2}' | while read f; do tar -zvf --strip-components=1 --append /tmp/bckp.tar.gz "$f"; done
:: Proceed with installation? [Y/n] y
error: failed to commit transaction (conflicting files)
tar: Cannot update compressed archives
Try 'tar --help' or 'tar --usage' for more information.
tar: Cannot update compressed archives
Try 'tar --help' or 'tar --usage' for more information.
...

This ends up complaining "no such file or directory" for existing files:

$ pacman -Syu | grep exists | awk '{print $2}' | while read f; do tar -vf --strip-components=1 --append /tmp/bckp.tar.gz "$f"; done
:: Proceed with installation? [Y/n] y
error: failed to commit transaction (conflicting files)
tar: Removing leading `/' from member names
tar: /tmp/bckp.tar.gz: Cannot stat: No such file or directory
/c/msys64/usr/lib/python2.7/site-packages/pip/_internal/commands/debug.py
tar: Removing leading `/' from hard link targets
tar: Exiting with failure status due to previous errors
tar: Removing leading `/' from member names
tar: /tmp/bckp.tar.gz: Cannot stat: No such file or directory
/c/msys64/usr/lib/python2.7/site-packages/pip/_internal/commands/debug.pyc
...

So, assuming I have a list of files (absolute filepaths), of the form /c/msys64/usr/lib/python2.7/site-packages/pip/_internal/commands/debug.pyc, in a while loop - how can I add them all to a .tar archive, such that they end up in it with the first part of the absolute path stripped (i.e. mentioned file would end up in tar archive as msys64/usr/lib/python2.7/site-packages/pip/_internal/commands/debug.pyc)?

Answer 1

Ok, I think I finally found the right invocation - note, the sed is added because --strip-components works on extraction only; so here we strip the /c/ manually from the filepath strings, and then instruct tar to change directory to /c/ using the -C switch:

$ pacman -Syu | grep exists | awk '{print $2}' | sed 's_/c/__' | xargs tar -cvf /tmp/bckp.tar -C /c/
:: Proceed with installation? [Y/n] y
error: failed to commit transaction (conflicting files)
msys64/usr/lib/python2.7/site-packages/pip/_internal/commands/debug.py
msys64/usr/lib/python2.7/site-packages/pip/_internal/commands/debug.pyc
msys64/usr/lib/python2.7/site-packages/pip/_internal/distributions/__init__.py
msys64/usr/lib/python2.7/site-packages/pip/_internal/distributions/__init__.pyc
...

... and to double-check:

$ tar tvf /tmp/bckp.tar
-rw-r--r-- user/None        3360 2019-09-02 09:08 msys64/usr/lib/python2.7/site-packages/pip/_internal/commands/debug.py
-rw-r--r-- user/None        4154 2019-09-02 09:08 msys64/usr/lib/python2.7/site-packages/pip/_internal/commands/debug.pyc
-rw-r--r-- user/None         861 2019-09-02 09:08 msys64/usr/lib/python2.7/site-packages/pip/_internal/distributions/__init__.py
-rw-r--r-- user/None         986 2019-09-02 09:08 msys64/usr/lib/python2.7/site-packages/pip/_internal/distributions/__init__.pyc
...