Add blacklist ability for ddusb script

config/ddusb-exclude.conf

@@ -0,0 +1,3 @@
+## File for excluding particular files from being written to
+## Write one device per line, shell-style globs are accepted i.e. /dev/sda?
+## Lines beginning with "#" and ";" are ignored

ddusb.py

@@ -2,9 +2,54 @@
 """Write an ISO image to a usb drive using dd."""
 
 import argparse
+import glob
 import pathlib
+import re
 import subprocess
 
+# ========== Constants ==========
+COMMENT_PATTERN = "[#;]"
+EXCLUDE_FILE = "/etc/helper-scripts/ddusb-exclude.conf"
+
+
+# ========== Functions ==========
+def read_exclude_file(exclude_file):
+    """Read and return a list of exclusion paths/globs,
+       removing any lines that begin with COMMENT_PATTERN.
+
+    :param exclude_file: path to the exclusion file
+    :type exclude_file: str, bytes, or path-like object
+    :returns: a list containing non-commented lines from the
+        exclusion file
+    :rtype: list
+    """
+    with open(exclude_file, "r") as excludes:
+        lines = [
+            l.strip("\n")
+            for l in excludes.readlines()
+            if not re.match(COMMENT_PATTERN, l)
+        ]
+
+    return lines
+
+
+def expand_globs(*globs):
+    """For each glob given, expand these globs and return a list
+    containing each glob expanded.
+
+    :param globs: patterns to expand
+    :type globs: str
+    :returns: all expanded globs aggregated together
+    :rtype: list
+    """
+    expanded_globs = []
+
+    for line in globs:
+        expanded_globs.extend(glob.glob(line, recursive=True))
+
+    return expanded_globs
+
+
 # ========== Main Script ==========
 parser = argparse.ArgumentParser()
 parser.add_argument("-b", "--bs", default=512, help="block size", metavar="bs")
@@ -17,13 +62,21 @@ input_file = args.input_file
 block_device = args.output_file
 
 if not pathlib.Path(block_device).is_block_device():
-    print(f"Error: {block_device} is not a block device")
+    print(f'Error: "{block_device}" is not a block device')
     exit(1)
 
 print(f"Input file: {input_file}")
 print(f"Block device: {block_device}")
 print(f"Block size: {block_size}")
 
+# Check if block_device is excluded
+exclude_patterns = read_exclude_file(EXCLUDE_FILE)
+device_blacklist = expand_globs(*exclude_patterns)
+
+if block_device in device_blacklist:
+    print(f'Error: "{block_device}" is blacklisted from running dd')
+    exit(2)
+
 try:
     subprocess.run(
         [
@@ -36,6 +89,6 @@ try:
         check=True,
     )
 except subprocess.CalledProcessError:
-    exit(1)
+    exit(3)
 else:
-    subprocess.run(["sync"])
+    subprocess.run("sync")