journal_print.py allow user passing fields

Allow users to pass fields among other options.

tools/journal_print.py

@@ -1,13 +1,12 @@
 #!/usr/bin/env python3
-#
-# file: print_journal_events.py
 # -*- coding: utf-8 -*-
+# vim: set fileencoding=utf-8 :
+# file: print_journal_events.py
 #
 # helper tool to double-check .journal file entries found using
 # python-systemd:journal.Reader
 
-__doc__: str = \
-"""\
+__doc__: str = """\
 Print all events from .journal file in the order returned by
 `python-systemd:journal.Reader.get_next`, which itself calls
 `libsytemd` API `sd_journal_next`.
@@ -16,22 +15,11 @@
 `libsystemd.so` is used.
 """
 
+import argparse
 import os
-from typing import Optional
+from typing import List, Optional
 import sys
-try:
-    from systemd import journal
-except ImportError as err:
-    print(err, file=sys.stderr)
-    print("Install on Ubuntu with:\n    sudo apt install python3-systemd", file=sys.stderr)
-    print("Or with pip:\n    pip install systemd-python", file=sys.stderr)
-    sys.exit(1)
-
-# TODO allow passing field names to print, overrides default field name tuple
-#      use argparse to do this.
 
-# default Journal File
-JOURNAL_FILE = "./logs/programs/journal/user-1000.journal"
 KEY_MONOTONIC_TIMESTAMP = "__MONOTONIC_TIMESTAMP"
 KEY_SOURCE_REALTIME_TIMESTAMP = "_SOURCE_REALTIME_TIMESTAMP"
 KEY__REALTIME_TIMESTAMP = "__REALTIME_TIMESTAMP"
@@ -40,29 +28,89 @@
 SEP = "|"
 
 if __name__ == "__main__":
-    file_: str = JOURNAL_FILE
-    time_: Optional[str] = None
-    # primitive argument parsing
-    do_help = False
-    if len(sys.argv) > 1:
-        file_ = sys.argv[1]
-        if file_ in ("-h", "--help", "-?"):
-            do_help = True
-    if len(sys.argv) > 2:
-        time_ = sys.argv[2]
-    if len(sys.argv) > 3 or len(sys.argv) <= 1 or do_help:
-        print(f"""Usage:
-    {os.path.basename(__file__ )} journal_file [time]
+    parser = argparse.ArgumentParser(
+        description=__doc__,
+        prog=os.path.basename(__file__),
+        formatter_class=argparse.ArgumentDefaultsHelpFormatter,
+        # TODO: how to both show defaults and preserve newlines in help message?
+        epilog=(
+            """Works well when piped to column.
 
-where journal_file is the path to a .journal file.
-where time is from Unix epoch in microseconds.
+"""
+            + os.path.basename(__file__)
+            + """ -j /var/log/journal/$(hostname)/system.journal | column -t -s '|'"""
+        ),
+    )
+    parser.add_argument(
+        "--journal-file",
+        "-j",
+        type=str,
+        required=True,
+    )
+    parser.add_argument(
+        "fields",
+        metavar="field",
+        type=str,
+        nargs="*",
+        default=",".join(
+            (
+                KEY_MONOTONIC_TIMESTAMP,
+                KEY_SOURCE_REALTIME_TIMESTAMP,
+                KEY__REALTIME_TIMESTAMP,
+                KEY_MESSAGE_ID,
+                KEY_MESSAGE,
+            )
+        ),
+        help="print these journal entry fields separated by commas",
+    )
+    parser.add_argument(
+        "--time",
+        "-t",
+        type=int,
+        default=None,
+        help="Unix epoch in microseconds to start printing from",
+    )
+    parser.add_argument(
+        "--oneline",
+        "-o",
+        type=bool,
+        default=False,
+        help="Strip newlines from MESSAGE field",
+    )
+    parser.add_argument(
+        "--separator",
+        "-s",
+        type=str,
+        default=SEP,
+        help="Separator between fields",
+    )
+
+    args = parser.parse_args()
 
-About:
-""", file=sys.stderr)
-        print(__doc__, file=sys.stderr)
+    # late import to allow user to see `--help` message without this error
+    try:
+        from systemd import journal
+    except ImportError as err:
+        print(err, file=sys.stderr)
+        print(
+            "Install on Ubuntu with:\n    sudo apt install python3-systemd",
+            file=sys.stderr,
+        )
+        print("Or with pip:\n    pip install systemd-python", file=sys.stderr)
         sys.exit(1)
 
-    reader = journal.Reader(files=[file_])
+    journal_file: str = args.journal_file
+    fields: List = []
+    if isinstance(args.fields, str):
+        fields += args.fields.split(",")
+    elif isinstance(args.fields, list):
+        for field in args.fields:
+            fields += field.split(",")
+    time_: Optional[str] = args.time
+    oneline: bool = args.oneline
+    sep = args.separator
+
+    reader = journal.Reader(files=[journal_file])
 
     if time_ is not None:
         t: int = int(time_)
@@ -72,29 +120,32 @@
 
     try:
         print(f"index", end="")
-        print(f"{SEP}{KEY_MONOTONIC_TIMESTAMP}", end="")
-        print(f"{SEP}{KEY_SOURCE_REALTIME_TIMESTAMP}", end="")
-        print(f"{SEP}{KEY__REALTIME_TIMESTAMP}", end="")
-        print(f"{SEP}{KEY_MESSAGE_ID}", end="")
-        print(f"{SEP}{KEY_MESSAGE}")
+        for field in fields:
+            print(f"{sep}{field}", end="")
+        print()
         sys.stdout.flush()
         i = 0
         while entry := reader.get_next():
             i += 1
-            mt_ts = entry.get(KEY_MONOTONIC_TIMESTAMP, "")
-            mt_ts = str(mt_ts.timestamp)
-            srt_ts = entry.get(KEY_SOURCE_REALTIME_TIMESTAMP, " ")
-            rt_ts = entry.get(KEY__REALTIME_TIMESTAMP, " ")
-            mesg_id = entry.get(KEY_MESSAGE_ID, " ")
-            mesg_id = str(mesg_id).replace("-", "")
-            message = entry.get(KEY_MESSAGE, " ")
-            message = message.replace("\n", " ")
             print(f"{i}", end="")
-            print(f"{SEP}{mt_ts}", end="")
-            print(f"{SEP}{srt_ts}", end="")
-            print(f"{SEP}{rt_ts}", end="")
-            print(f"""{SEP}{mesg_id}""", end="")
-            print(f"{SEP}{message}")
+            for field in fields:
+                # special handling for monotonic timestamp
+                if field == KEY_MONOTONIC_TIMESTAMP:
+                    mt_ts = entry.get(KEY_MONOTONIC_TIMESTAMP, " ")
+                    mt_ts = str(mt_ts.timestamp)
+                    print(f"{sep}{mt_ts}", end="")
+                else:
+                    # defalt print at least a space to piping stdout to
+                    # `column -t -s '|'` will correctly parse
+                    value = entry.get(field, " ")
+                    # replace some control characters with glyph representations
+                    if oneline:
+                        value = value.replace("\0", "␀")
+                        value = value.replace("\n", "␤")
+                        value = value.replace("\1", "␁")
+                        value = value.replace("\r", "␊")
+                    print(f"{sep}{value}", end="")
+            print()
             sys.stdout.flush()
     except BrokenPipeError:
         # this occurs when piping to `head`