diff --git a/mfbatch/__main__.py b/mfbatch/__main__.py
index eec94d2..9f4fb07 100644
--- a/mfbatch/__main__.py
+++ b/mfbatch/__main__.py
@@ -1,24 +1,21 @@
 # __main__.py 
 
-import sys
-from subprocess import run
 import os
 from glob import glob
-from re import match
+from subprocess import run
+
 from optparse import OptionParser
 import shlex
 
 
 from mfbatch.util import readline_with_escaped_newlines
+import mfbatch.metaflac as flac
 from mfbatch.commands import BatchfileParser 
 
+
 from tqdm import tqdm
 # import readline
 
-# MFBATCH COMMAND FILE
-
-METAFLAC_PATH = '/opt/homebrew/bin/metaflac'
-
 def execute_batch_list(batch_list_path: str, dry_run: bool):
     with open(batch_list_path, mode='r') as f:
         parser = BatchfileParser()
@@ -30,20 +27,14 @@ def execute_batch_list(batch_list_path: str, dry_run: bool):
 
 
 def create_batch_list(command_file: str):
-    metadatums = {}
+
     with open(command_file, mode='w') as f:
         f.write("# mfbatch\n\n")
-        metaflac_command = [METAFLAC_PATH, '--list']
+        metadatums = {}
         flac_files = glob('./**/*.flac', recursive=True)
         flac_files = sorted(flac_files)
         for path in tqdm(flac_files, unit='File', desc='Scanning FLAC files'):
-            result = run(metaflac_command + [path], capture_output=True)
-            this_file_metadata = {}
-            for line in result.stdout.decode('utf-8').splitlines():
-                m = match(r'^\s+comment\[\d\]: ([A-Za-z]+)=(.*)$', line)
-                if m is not None:
-                    this_file_metadata[m[1]] = m[2]
-
+            this_file_metadata = flac.read_metadata(path) 
             for this_key in this_file_metadata.keys():
                 if this_key not in metadatums:
                     f.write(f":set {this_key} "
diff --git a/mfbatch/commands.py b/mfbatch/commands.py
new file mode 100644
index 0000000..3a9b011
--- /dev/null
+++ b/mfbatch/commands.py
@@ -0,0 +1,226 @@
+# commands.py 
+# mfbatch 
+
+# from string import Template
+import sys
+import shlex
+import shutil
+import re
+
+import mfbatch.metaflac as flac
+
+from typing import Dict, Tuple
+
+
+class UnrecognizedCommandError(Exception):
+    command: str
+    line: int 
+
+    def __init__(self, command, line) -> None:
+        self.command = command 
+        self.line = line
+
+class CommandArgumentError(Exception):
+    command: str
+    line: int 
+
+    def __init__(self, command, line) -> None:
+        self.command = command 
+        self.line = line
+
+
+class CommandEnv:
+    metadatums: Dict[str, str]
+    incr: Dict[str, str]
+    patterns: Dict[str, Tuple[str, str, str]]
+
+    def __init__(self) -> None:
+        self.metadatums = dict()
+        self.incr = dict()
+        self.patterns = dict()
+
+    def unset_key(self, k):
+        del self.metadatums[k]
+
+        self.incr.pop(k, None)
+        self.patterns.pop(k, None)
+
+    def set_pattern(self, to: str, frm: str, pattern: str, repl: str):
+        self.patterns[to] = (frm, pattern, repl)
+
+
+    def evaluate_patterns(self):
+        for to_key in self.patterns.keys():
+            from_key, pattern, replacement = self.patterns[to_key]
+            from_value = self.metadatums[from_key]
+            self.metadatums[to_key] = re.sub(pattern, replacement, from_value)
+
+    def increment_all(self):
+        for k in self.incr.keys():
+            v = int(self.metadatums[k])
+            self.metadatums[k] = self.incr[k] % (v + 1)
+            
+    
+
+
+class BatchfileParser:
+    """
+A batchfile is a text file of lines. Lines either begin with a '#' to denote a
+comment, a ':' to denote a Command, and if neither of these are present, the
+are interpreted as a file path to act upon. Empty lines are ignored.
+
+If a line ends with a backslash '\\', the backslash is deleted and the contents
+of the following line are appended to the end of the present line.
+
+Commands precede the files that they act upon, and manipulate the values of
+"keys" in an internal dictionary. These keys are what are written to files as
+they appear in the batchfile.
+    """
+
+    dry_run: bool
+    env: CommandEnv
+    
+    COMMAND_LEADER = ':'
+    COMMENT_LEADER = '#'
+
+
+    def __init__(self):
+        self.dry_run = True 
+        self.env = CommandEnv()
+
+    def _handle_line(self, line:str, lineno: int, interactive: bool = True):
+        if line.startswith(self.COMMAND_LEADER):
+            self._handle_command(line.lstrip(self.COMMAND_LEADER), lineno)
+        elif line.startswith(self.COMMENT_LEADER):
+            self._handle_comment(line.lstrip(self.COMMENT_LEADER))
+        else:
+            self._handle_file(line, interactive)
+
+    def _handle_command(self, line, lineno):
+        args = shlex.split(line)
+        actions = [member for member in dir(self) \
+                if not member.startswith('_')]
+
+        if args[0] in actions:
+            try:
+                self.__getattribute__(args[0])(args[1:])
+            except KeyError:
+                raise CommandArgumentError(command=args[0], line=lineno)
+        else:
+            raise UnrecognizedCommandError(command=args[0], line=lineno)
+
+    def _handle_comment(self, _):
+        pass
+
+    def _handle_file(self, line, interactive): 
+        while True:
+            self.env.evaluate_patterns()
+
+            if self.dry_run:
+                sys.stdout.write(f"\nDRY RUN File: \033[1m{line}\033[0m\n")
+            else:
+                sys.stdout.write(f"\nFile: \033[1m{line}\033[0m\n")            
+
+            for key in self.env.metadatums.keys():
+
+                if key.startswith('_'):
+                    continue 
+
+                value = self.env.metadatums[key]
+
+                LINE_LEN = int(shutil.get_terminal_size()[0]) - 32
+                value_lines = [value[i:i+LINE_LEN] for i in \
+                        range(0,len(value), LINE_LEN)]
+                
+                for l in value_lines:
+                    if key:
+                        sys.stdout.write(f"{key:.<30}  \033[4m{l}\033[0m\n")
+                        key = None
+                    else:
+                        sys.stdout.write(f"{' ' * 30}  \033[4m{l}\033[0m\n")
+            
+            if interactive:
+                    val = input('Write? [Y/n/:] > ')
+                    if val == '' or val.startswith('Y') or val.startswith('y'):
+                        if self.dry_run:
+                            print("DRY RUN would write metadata here.")
+                        else:
+                            flac.write_metadata(line, self.env.metadatums)
+
+                        self.env.increment_all()
+
+                    elif val.startswith(self.COMMAND_LEADER):
+                        self._handle_command(val.lstrip(self.COMMAND_LEADER), 
+                                             lineno=-1)
+                        continue
+
+                    break
+            else:
+                break
+
+
+    def set(self, args):
+        """
+        set KEY VALUE
+        KEY in each subsequent file appearing in the transcript will be set to
+        VAL. If KEY begins with an underscore, it will be set in the internal
+        environment but will not be written to the file. 
+        """
+        key = args[0]
+        value = args[1]
+        self.env.metadatums[key] = value 
+
+    def unset(self, args):
+        """
+        unset KEY 
+        KEY in each subsequent file will not be set, the existing value for KEY
+        is deleted.
+        """
+        key = args[0]
+        self.env.unset_key(key)
+
+    def reset(self, _):
+        """
+        reset 
+        All keys in the environment will be reset, subsequent files will have 
+        no keys set.
+        """
+        all_keys = list(self.env.metadatums.keys())
+        for k in all_keys:
+            self.env.unset_key(k)
+
+    def seti(self, args):
+        """
+        seti KEY INITIAL [FORMAT]
+        KEY in the next file appearing in the batchfile will be set to INITIAL,
+        which must be an integer. INITIAL will then be incremented by one and
+        this process will be repeated for each subsequent file in the
+        batchfile. If FORMAT is given, it will be used to format the the
+        integer value when setting, FORMAT is a python printf-style string and
+        the default is "%i".
+        """
+        key = args[0]
+        initial = args[1]
+        fmt = '%i'
+        if len(args) > 2:
+            fmt = args[2]
+
+        self.env.metadatums[key] = initial
+        self.env.incr[key] = fmt
+
+    def setp(self, args):
+        """
+        setp KEY INPUT PATTERN REPL
+        KEY will be set to the result of re.sub(PATTERN, REPL, INPUT). Patterns
+        are evaluated in the order they appear in the batchfile, once for each 
+        file that appears in the batchfile before writing.
+        """
+        key = args[0]
+        inp = args[1]
+        pattern = args[2]
+        repl = args[3]
+        self.env.set_pattern(key,inp,pattern, repl)
+        
+    
+    
+
diff --git a/mfbatch/metaflac.py b/mfbatch/metaflac.py
new file mode 100644
index 0000000..f257c26
--- /dev/null
+++ b/mfbatch/metaflac.py
@@ -0,0 +1,69 @@
+# metaflac.py 
+# mfbatch
+
+from subprocess import run
+from re import match
+from io import StringIO
+
+from typing import Dict 
+
+METAFLAC_PATH = '/opt/homebrew/bin/metaflac'
+
+FlacMetadata = Dict[str, str]
+
+
+def sanatize_key(k: str) -> str:
+    """
+    Enforces the VORBIS_COMMENT spec with regard to keys
+    """
+    k = k.upper()
+    rval = ''
+    for c in k:
+        v = ord(c)
+        if 0x20 <= v <= 0x7D and v != 0x3D:
+            rval = rval + c
+        else:
+            rval = rval + '_'
+
+    return rval
+
+
+def sanatize_value(v: str) -> str: 
+    """
+    Enforces the VORBIS_COMMENT spec with regard to values. Also removes 
+    newlines, which are not supported by this tool.
+    """
+
+    return v.translate(str.maketrans('\n',' '))
+
+
+def read_metadata(path: str, metaflac_path=METAFLAC_PATH) -> FlacMetadata:
+    metaflac_command = [metaflac_path, '--list']
+    result = run(metaflac_command + [path], capture_output=True)
+    result.check_returncode()
+
+    file_metadata = {}
+    for line in result.stdout.decode('utf-8').splitlines():
+        m = match(r'^\s+comment\[\d\]: ([A-Za-z]+)=(.*)$', line)
+        if m is not None:
+            file_metadata[m[1]] = m[2]
+
+    return file_metadata
+
+
+def write_metadata(path: str, data: FlacMetadata, 
+                   metaflac_path=METAFLAC_PATH):
+    remove_job = run([metaflac_path, '--remove-all-tags', path])
+    remove_job.check_returncode()
+
+    metadatum_f = "" 
+
+    for k in data.keys():
+        key = sanatize_key(k)
+        val = sanatize_value(data[k])
+        metadatum_f  = metadatum_f + f"{key}={val}\n"
+
+    insert_job = run([metaflac_path, "--import-tags-from=-", path], 
+                     input=metadatum_f.encode('utf-8'))
+    insert_job.check_returncode()
+