How do I execute a dry run of a tar compression?

Question

I want to execute a dry run of a tar compression and print the entries to stdout without actually creating the tar.

So far I've tried:

// just spins
$ tar t -O Downloads

$ tar c -O Downloads
tar: Option -O is not permitted in mode -c

tar --help gives the following:

⋊> ~ tar --help                                                                                                                                                                                 08:06:22
tar(bsdtar): manipulate archive files
First option must be a mode specifier:
  -c Create  -r Add/Replace  -t List  -u Update  -x Extract
Common Options:
  -b #  Use # 512-byte records per I/O block
  -f <filename>  Location of archive
  -v    Verbose
  -w    Interactive
Create: tar -c [options] [<file> | <dir> | @<archive> | -C <dir> ]
  <file>, <dir>  add these items to archive
  -z, -j, -J, --lzma  Compress archive with gzip/bzip2/xz/lzma
  --format {ustar|pax|cpio|shar}  Select archive format
  --exclude <pattern>  Skip files that match pattern
  -C <dir>  Change to <dir> before processing remaining files
  @<archive>  Add entries from <archive> to output
List: tar -t [options] [<patterns>]
  <patterns>  If specified, list only entries that match
Extract: tar -x [options] [<patterns>]
  <patterns>  If specified, extract only entries that match
  -k    Keep (don't overwrite) existing files
  -m    Don't restore modification times
  -O    Write entries to stdout, don't restore to disk
  -p    Restore permissions (including ACLs, owner, file flags)
bsdtar 3.3.2 - libarchive 3.3.2 zlib/1.2.11 liblzma/5.0.5 bz2lib/1.0.6

Answer 1

To do a dry-run just listing files, use -v for verbose mode to list the files and use /dev/null as output to avoid the time/space overhead of writing an acutal file. Exclude -v if you just want to see tar errors.

tar -cvf /dev/null mydir 

Answer 2

tar t lists an existing archive. You haven't provided any as an argument (-f), so it's waiting for it on stdin. Humans are no good at typing tar archives on a keyboard, so it's not very useful.

tar c doesn't accept an -O option. It's an option to x as indicated in the help message.

You can combine those two to get expected result:

tar c Downloads | tar t

There's no -f argument to tar c, so it will write compressed output to stdout. We're then piping it to tar t's stdin, using the mode in which it was "stuck" previously.