Use keybase command line utility to sign a file and detach the signature into a .sig file.

sign

#!/bin/sh

usage() {
    echo "sign INPUT_FILE"
}

keybase_not_found() {
    echo >&2 "Keybase command line utilities must be installed to sign files."
    exit 1
}

command -v keybase >/dev/null 2>&1 || keybase_not_found


input_file="$1"
if [ -z "$input_file" ]; then
    usage "$0"
    exit 1
fi

keybase sign --detached --infile "$input_file" --outfile "${input_file}.sig"