Halting tar after locating entry or EOF in an actual tape archive

Question

I've just been handed a pile of LTO-5s and a tape library, and the exciting task of extracting a subset of files.

Order of magnitude is 60 tapes from said pile, around 10,000 files of 70,000 per tape (in maybe half a dozen or a dozen asstd directories) that I need to pull. Tape library is running tar 1.23

I do have pre-existing dumps of each tape's content as per tar --list so i'm happy to reconcile this with my list of files I need to pull so I can feed in a proper stream of records to grab, but

tar -xvf /dev/nst0 -b $file_to_pull

doesn't halt until i get to the end of the tape. Which makes sense - tar might be fed a wildcard pattern so it just goes to end of archive.

I guess I could solve this by just feeding in the directory glob and purging unwanted files once they're off tape, but I can't help but think there's got to be a way to halt tar at the EOF boundary and skip the purge step. Right? In all my hunting however, I've not found anything of that nature though..

So, questions:

• Is there a way to get tar (or heck, anything else) to pull just one file from wherever the tape head is, and then stop? Or to signal tar once it's gotten an EOF and stop?

• Alternately, am I mentally attacking this an odd way? Happy to take alt suggestions if anyone's got them.

Noting of course, this isn't a problem that's unsolvable right now, just my current options seem to be really awkward - this is the first time I've really had to deal with tape at this magnitude (and our other tapes are all LTFS)

Answer 1

You can ask tar to run an action every few blocks at what they call checkpoints. This action can test if the wanted file has been extracted and if so kill the tar. I tried it using a tar file and it seems to work ok.

Here's my example script to do my test, tarring /usr/bin and extracting usr/bin/bash into /tmp/usr/bin/bash. The default --checkpoint is 10 blocks.

#!/bin/bash

cat <<\! >/tmp/checkdone
#!/bin/bash
# env has TAR_CHECKPOINT TAR_ARCHIVE TAR_VERSION TAR_BLOCKING_FACTOR
# tar -C directory is NOT used for checkpoint action!
want=$1

if size=$(stat --printf='%s\n' "$want" 2>&1)
then if [ "$(</tmp/lastsize)" = "$size" -a -s /tmp/pid ]
    then  echo "same size $size. time to stop"
          ls -l "$want"
          >/tmp/lastsize
          kill -1 $(</tmp/pid)
    else  echo "partial size $size"
          echo "$size" >/tmp/lastsize
    fi
else echo -n "."
fi
!
chmod +x /tmp/checkdone
>/tmp/lastsize
>/tmp/pid

tar -cf /tmp/tar /usr/bin/ # create example tar file
# wanted file. must be in current dir
want=usr/bin/bash
cd /tmp || exit # dont use tar -C dir

tar -xvf /tmp/tar "$want" --checkpoint=10 --checkpoint-action=exec="/tmp/checkdone $want" &
echo $! >/tmp/pid
wait

rm /tmp/tar /tmp/pid /tmp/lastsize /tmp/checkdone
rm -fr /tmp/usr