== code and data ==

$ rm a b c; echo a > a; echo b > b; touch c; tar -cf a.tar; sleep 1
$ rm a b c; echo a > a; echo b > b; touch c; tar -cf aa.tar; sleep 1
$ sha1sum a.tar aa.tar # different

$ grep -oba $'\x75\x73\x74\x61\x72\x00' a.tar | cut -d: -f1 | awk '{ print (int($1/512)*512) }' | sort -n -u > prs1.txt # force to previous 512 boundary as header start

$ truncate -s "$(stat --format=%s a.tar)" framework.tar

$ cat prs1.txt | xargs -n1 sh -c 'off="$1"; dd if=a.tar of=framework.tar bs=512 skip="$((off/512))" seek="$((off/512))" count=1 conv=notrunc' sh
$ # when working on a 52-GB TAR file, this used a lot of CPU in my experience, didn't use lots of RAM; it ran without the below \n -> \

$ cat prs1.txt | xargs -n1 sh -c 'off="$1"; dd if=framework.tar of=aa.tar bs=512 skip="$((off/512))" seek="$((off/512))" count=1 conv=notrunc' sh
$ # or: [dd command] 2>&1 | perl -pE "s/\n/\\\\/g"
$ # is this [...] perl [...] version slower? IDK.
$ # or: [dd command] 2>&1 >dd-log.txt

$ sha1sum a.tar aa.tar # same

$ du -sh /path/to/folder.tar.txt.gz # contains some type of list of files and folders inside of folder.tar

$ # test if it works
$ # utc; find /path/to/folder/ -type f | xargs -d "\n" touch; utc # set mtime of all files to now
$ # utc; 7z a /path/to/folder.sync.tar /path/to/folder; utc # vs. an older "folder.tar"
$ # [above commands]

$ # Example *.sha1.txt file:
$ cat /mnt/p4/b/zd/4changif/4chan_gif_2026_06.tar.sha1.txt
$ head -c 411222999 /mnt/p4/b/zd/4changif/4chan_gif_2026_06.tar | sha1sum
c506b05196addbb9f8cd8afb11143df4f0c0af69  -
$ tail -c 411222999 /mnt/p4/b/zd/4changif/4chan_gif_2026_06.tar | sha1sum
a59bc260060f7799cb1899dea8876d2a43087a62  -
$ [...]
41acb816623f5472abd84459e7b7fdb6f505c2e7 /mnt/p4/b/zd/4changif/4chan_gif_2026_06.tar

== test on 52-GB .tar file ==

Worked:
$ [...]
$ utc; cat /mnt/p4/b/zd/4changif/4chan_gif_2026_06.framework.txt | xargs -n1 sh -c 'off="$1"; dd if=/mnt/p4/b/zd/4changif/4chan_gif_2026_06.framework.tar of=/mnt/p4/b/zd/4changif/4chan_gif_2026_06.sync.tar bs=512 skip="$((off/512))" seek="$((off/512))" count=1 conv=notrunc 2>&1 | perl -pE "s/\n/\\\\/g"' sh; echo; utc

It ran from 2026-08-14T05:22:32.181772157Z to 2026-08-14T07:34:36.552445643Z. Hashes match:
$ cat /mnt/p4/b/zd/4changif/4chan_gif_2026_06.tar.sha1.txt
$ head -c 411222999 /mnt/p4/b/zd/4changif/4chan_gif_2026_06.tar | sha1sum
c506b05196addbb9f8cd8afb11143df4f0c0af69 [...]
$ tail -c 411222999 /mnt/p4/b/zd/4changif/4chan_gif_2026_06.tar | sha1sum
a59bc260060f7799cb1899dea8876d2a43087a62 [...]
41acb816623f5472abd84459e7b7fdb6f505c2e7 /mnt/p4/b/zd/4changif/4chan_gif_2026_06.tar

Compared to:
$ head -c 411222999 /mnt/p4/b/zd/4changif/4chan_gif_2026_06.sync.tar | sha1sum
c506b05196addbb9f8cd8afb11143df4f0c0af69  -
$ tail -c 411222999 /mnt/p4/b/zd/4changif/4chan_gif_2026_06.sync.tar | sha1sum
a59bc260060f7799cb1899dea8876d2a43087a62  -
$ utc; cat /mnt/p4/b/zd/4changif/4chan_gif_2026_06.sync.tar | pv | sha1sum; utc