Do a compression dry run (i.e., no file output)

Question

Is there any way to do a dry run compression to RAR or 7-Zip format? (Command line is fine.) What I'd like to get is an accurate compressed size estimate.

I tried to find an option for it in WinRAR and 7-Zip, even looked at their CL switches, but no luck.

Answer 1

You can use gzip to preview zip compression (deflate) and xz to preview 7zip compression (lzma) like this:

gzip -c $1  | wc -c
xz --format=lzma -c $1  | wc -c

Optionally pipe to numfmt --to=iec to make the bytes count easily readable.

Note that it works only for single files and the count is missing the archive header.

You can find all the binaries needed for Windows in the git sdk package.

Answer 2

I'm not familiar with WinRAR or 7-Zip but a quick look at WinRAR's online help suggests you can use the -P and -inul flags to direct compressed output to stdout. You can pipe this to the wc utility which is standard on Linux and MacOS and available for Windows as part of the Windows 10 Windows Subsystem for Linux or the GnuWin32 utils (see the textutils package).

For example:

rar -P -inul <rest of WinRAR parameters...> | wc -c

This will return the number of characters (bytes) that the compressed file would have been if it was written to disk.