5. ch-convert

Convert an image from one format to another.

5.1. Synopsis

$ ch-convert [-i FMT] [-o FMT] [OPTION ...] IN OUT

5.2. Description

Copy image IN to OUT and convert its format. Replace OUT if it already exists, unless --no-clobber is specified. It is an error if IN and OUT have the same format; use the format’s own tools for that case.

ch-run can run container images that are plain directories or (optionally) SquashFS archives. However, images can take on a variety of other formats as well. The main purpose of this tool is to make images in those other formats available to ch-run.

For best performance, ch-convert should be invoked only once, producing the final format actually needed.

IN

Descriptor for the input image. For image builders, this is an image reference; otherwise, it’s a filesystem path.

OUT

Descriptor for the output image.

-h, --help

Print help and exit.

-i, --in-fmt FMT

Input image format is FMT. If omitted, inferred as described below.

-n, --dry-run

Don’t read the input or write the output. Useful for testing format inference.

--no-clobber

Error if OUT already exists, rather than replacing it.

--no-xattrs

Ignore xattrs and ACLs when converting. Overrides $CH_XATTRS.

-o, --out-fmt FMT

Output image format is FMT; inferred if omitted.

-q, --quiet

Be quieter; can be repeated. Incompatible with -v. See the FAQ entry on verbosity for details.

-s, --storage DIR

Set the storage directory. Equivalent to the same option for ch-image(1) and ch-run(1).

--tmp DIR

A sub-directory for temporary storage is created in DIR and removed at the end of a successful conversion. If this script crashes or errors out, the temporary directory is left behind to assist in debugging. Storage may be needed up to twice the uncompressed size of the image, depending on the input and output formats. Default: $TMPDIR if specified; otherwise /var/tmp.

-v, --verbose

Print extra chatter. Can be repeated.

--xattrs

Preserve xattrs and ACLs when converting.

5.3. Image formats

ch-convert knows about these values of FMT:

ch-image

Internal storage for Charliecloud’s unprivileged image builder (Dockerfile interpreter) ch-image.

dir

Ordinary filesystem directory (i.e., not a mount point) containing an unpacked image. Output directories that already exist are replaced if they look like an image. If the output directory is empty, the conversion should use the directory without overwriting it. If the directory doesn’t look like an image and isn’t empty, exit with an error.

docker

Internal storage for Docker.

podman

Internal storage for Podman.

squash

SquashFS filesystem archive containing the flattened image. SquashFS archives are much like tar archives but are mountable, including by ch-run’s internal SquashFUSE mounting. Most systems have at least the SquashFS-Tools installed which allows unpacking into a directory, just like tar. Due to this greater flexibility, SquashFS is preferred to tar.

Note: Conversions to and from SquashFS are quite noisy due to the verbosity of the underlying mksquashfs(1) and unsquashfs(1) tools.

tar

Tar archive containing the flattened image with no layer sub-archives; i.e., the output of docker export works but the output of docker save does not. Output tarballs are always gzipped and must end in .tar.gz; input tarballs can be any compression acceptable to tar(1).

All of these are local formats; ch-convert does not know how to push or pull images.

5.4. Format inference

ch-convert tries to save typing by guessing formats when they are reasonably clear. This is done against filenames, rather than file contents, so the rules are the same for output descriptors that do not yet exist.

Format inference is done for both IN and OUT. The first matching glob below yields the inferred format. Paths need not exist in the filesystem.

  1. *.sqfs, *.squash, *.squashfs: SquashFS.

  2. *.tar, *.t?z, *.tar.?, *.tar.??: Tarball.

  3. /*, ./*, i.e. absolute path or relative path with explicit dot: Directory.

  4. If ch-image is installed: ch-image internal storage.

  5. If Podman is installed: Podman internal storage.

  6. If Docker is installed: Docker internal storage.

  7. Otherwise: No format inference.

5.5. Examples

Typical build-to-run sequence for image foo/bar using ch-run’s internal SquashFUSE code, inferring the output format:

$ sudo docker build -t foo/bar -f Dockerfile .
[...]
$ ch-convert foo/bar:latest /var/tmp/foobar.sqfs
input:   docker    foo/bar:latest
output:  squashfs  /var/tmp/foobar.sqfs
copying ...
done
$ ch-run /var/tmp/foobar.sqfs -- echo hello
hello

Same conversion, but no format inference:

$ ch-convert -i ch-image -o squash foo/bar:latest /var/tmp/foobar.sqfs
input:   docker    foo/bar:latest
output:  squashfs  /var/tmp/foobar.sqfs
copying ...
done