feat: add dependency check

README.md

@@ -60,10 +60,12 @@ Using UDIB to create ISOs requires the following software:
 - `xorriso` *(1.5.4 known to work)*
   - **Debian (bullseye):** [xorriso](https://packages.debian.org/bullseye/xorriso)
   - **Arch Linux:** [extra/libisoburn](extra/libisoburn)
-- GNU `gpg` *(2.2.32 known to work)*
+- GNU `gpg`
   - preinstalled on most distributions
 - GNU `cpio`
   - preinstalled on most distributions
+- GNU `sha512sum`
+  - preinstalled on most distributions
 
 Internet access is (obviously) required if you want to fetch any files using UDIB.
 

core/exceptions.py

@@ -0,0 +1,7 @@
+"""Some general exception classes used throughout the program."""
+
+
+class MissingDependencyError(RuntimeError):
+    """Raised if a required system dependency is missing."""
+
+    pass

core/utils.py

@@ -1,19 +1,20 @@
 """A collection of general utilities, not specific to any module."""
 
-from pathlib import Path
-from tempfile import TemporaryDirectory
-from sys import exit
-from os import remove, rename
-from subprocess import run, STDOUT, PIPE
-
-from cli.clibella import Printer
-from net.download import download_file
-from net.scrape import get_debian_iso_urls
-from gpg.verify import assert_detached_signature_is_valid
-from gpg.exceptions import VerificationFailedError
-from gpg.keystore import debian_signing_key_is_imported, import_debian_signing_key
 from crypt import crypt, METHOD_SHA512
 from getpass import getpass
+from os import remove, rename
+from pathlib import Path
+from subprocess import run, STDOUT, PIPE
+from sys import exit
+from tempfile import TemporaryDirectory
+
+from cli.clibella import Printer
+from exceptions import MissingDependencyError
+from gpg.exceptions import VerificationFailedError
+from gpg.keystore import debian_signing_key_is_imported, import_debian_signing_key
+from gpg.verify import assert_detached_signature_is_valid
+from net.download import download_file
+from net.scrape import get_debian_iso_urls
 
 
 def hash_user_password(printer=None):
@@ -38,6 +39,31 @@ def hash_user_password(printer=None):
     p.info("Password hash:")
     p.info(crypt(password, crypt.METHOD_SHA512))
 
+def assert_system_dependencies_installed():
+    """Checks whether all system dependencies required by udib are installed.
+
+    The programs used by udib must be accessible within the system's PATH
+    environment variable.
+
+    Raises
+    ------
+    MissingDependencyError
+        If a required dependency is not installed.
+    """
+
+    _REQUIRED_PROGRAMS = [
+        "xorriso", "gpg", "cpio", "sha512sum",
+    ]
+
+    for program in _REQUIRED_PROGRAMS:
+        try:
+            run(["command", "-v", program], shell=True, check=True)
+        except subprocess.CalledProcessError:
+            raise MissingDependencyError(
+                f"Program not installed or not in $PATH: "
+                f"'{program}'."
+            )
+
 def find_all_files_under(parent_dir):
     """Recursively finds all files anywhere under the specified directory.
 

udib.py

@@ -7,7 +7,7 @@ from tempfile import TemporaryDirectory
 
 from cli.clibella import Printer
 from cli.parser import get_argument_parser
-from core.utils import download_and_verify_debian_iso
+from core.utils import assert_system_dependencies_installed, download_and_verify_debian_iso
 from iso.injection import inject_preseed_file_into_iso
 from net.download import download_file
 from net.scrape import get_debian_preseed_file_urls, get_debian_iso_urls
@@ -15,9 +15,10 @@ from net.scrape import get_debian_preseed_file_urls, get_debian_iso_urls
 
 def main():
 
+    # check for dependencies
+    assert_system_dependencies_installed()
     # create a CLI printer
     p = Printer()
-
     # create an argument parser and read arguments
     parser = get_argument_parser()
     args = parser.parse_args()