The -H/--format option allows one to choose an archive format. GNU tar also provides direct options to select a format. e.g. --posix as an alias for --format=posix or --portability for --format=v7. I ran into this issue in a GitHub workflow in an Alpine docker container, which uses busybox by default. The actions/cache action requires a "Posix-compliant tar". It looks like busybox already generates posix archives. I have pasted the error message below. The error is because of an --posix option. ``` /usr/bin/docker exec fe3da0b58a8d73eeeaa346c3db67e6e1f7f0bf9896d148552db4c9077d01788d sh -c "cat /etc/*release | grep ^ID" /bin/tar --posix -cf cache.tgz --exclude cache.tgz -P -C /__w/SDL/SDL --files-from manifest.txt -z /bin/tar: unrecognized option: posix BusyBox v1.35.0 (2022-11-19 10:13:10 UTC) multi-call binary. Usage: tar c|x|t [-ZzJjahmvokO] [-f TARFILE] [-C DIR] [-T FILE] [-X FILE] [LONGOPT]... [FILE]... Create, extract, or list files from a tar file c Create x Extract t List -f FILE Name of TARFILE ('-' for stdin/out) -C DIR Change to DIR before operation -v Verbose -O Extract to stdout -m Don't restore mtime -o Don't restore user:group -k Don't replace existing files -Z (De)compress using compress -z (De)compress using gzip -J (De)compress using xz -j (De)compress using bzip2 --lzma (De)compress using lzma -a (De)compress based on extension -h Follow symlinks -T FILE File with names to include -X FILE File with glob patterns to exclude --exclude PATTERN Glob pattern to exclude --overwrite Replace existing files --strip-components NUM NUM of leading components to strip --no-recursion Don't descend in directories --numeric-owner Use numeric user:group --no-same-permissions Don't restore access permissions Warning: Failed to save: "/bin/tar" failed with error: The process '/bin/tar' failed with exit code 1 Warning: Cache save failed. ```