Convert a Apptainer/Singularity SIF file to a Docker image.

sif2docker.sh

#!/bin/bash

set -euo pipefail

function show_usage() {
    echo "Usage: $0 <input.sif> [docker_image_name:tag]"
    echo
    echo "Converts a Apptainer/Singularity SIF file to a Docker image."
    echo
    echo "Arguments:"
    echo "  input.sif             Input Apptainer/Singularity image file"
    echo "  docker_image_name:tag Optional Docker image name and tag"
    echo "                        (defaults to basename of input file)"
    exit 1
}

# Check for required commands
for cmd in apptainer unsquashfs docker; do
    if ! command -v $cmd &> /dev/null; then
        echo "Error: Required command '$cmd' not found"
        exit 1
    fi
done

# Check arguments
if [ $# -lt 1 ] || [ $# -gt 2 ]; then
    show_usage
fi

input_sif="$1"
if [ ! -f "$input_sif" ]; then
    echo "Error: Input file '$input_sif' not found"
    exit 1
fi

# Set docker image name
if [ $# -eq 2 ]; then
    docker_image="$2"
else
    docker_image="$(basename "$input_sif" .sif):latest"
fi

# Create temporary working directory
work_dir=$(mktemp -d)
trap 'rm -rf "$work_dir"' EXIT

echo "Converting '$input_sif' to Docker image '$docker_image'..."

# Find the squashfs partition ID
squashfs_id=$(apptainer sif list "$input_sif" | grep -i 'Squashfs' | awk '{print $1}')
if [ -z "$squashfs_id" ]; then
    echo "Error: Could not find Squashfs partition in SIF file"
    exit 1
fi

# Extract the squashfs filesystem
echo "Extracting squashfs filesystem..."
apptainer sif dump "$squashfs_id" "$input_sif" > "$work_dir/data.squash"
unsquashfs -dest "$work_dir/data" "$work_dir/data.squash"

# Create Dockerfile
cat > "$work_dir/Dockerfile" << 'EOF'
FROM scratch
COPY data /
ENV PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
CMD ["/bin/bash"]
EOF

# Build Docker image
echo "Building Docker image..."
(cd "$work_dir" && docker build --tag "$docker_image" .)

echo "Successfully created Docker image: $docker_image"