initial release

README.md

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+# newcat
+
+print (cat) only new lines in files (after previous run of newcat). This is useful to parse recent log records.
+
+## Install
+~~~
+pipx install newcat
+~~~
+
+## Example
+~~~bash
+$ echo This is line 1 > /tmp/test.log
+
+# works like normal /bin/cat now
+$ newcat /tmp/test.log 
+This is line 1
+
+# only new lines are printed
+$ echo This is line 2 >> /tmp/test.log
+$ newcat /tmp/test.log 
+This is line 2
+
+# no new lines => nothing on stderr
+$ newcat /tmp/test.log 
+
+# despite file is having two lines
+$ cat /tmp/test.log 
+This is line 1
+This is line 2
+~~~
+
+## How does this magic work?
+newcat stores some info about file (including position in file) in *state file*. Default state file is `/tmp/.newcat.state`, but can be overriden with `NEWCAT_STATE` environment variable or `-s` / `--state` option. Normally, newcat prints file content starting from this position (which was saved before), thus lines which were already printed before, will not be printed on new newcat run.
+
+Sometime file content it rewritten (e.g. after `echo New content > /tmp/test.log`). To detect this situation newcat checks two things:
+- file inode changed
+- file size is less then old printed position
+
+if any of these conditions are met, file considered as rewritten and newcat prints it from the beginning.
+Note: this is not absolutely reliable, and there is a chance that sometimes newcat will not detect when file is overwritten.
+
+

newcat

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+#!/usr/bin/env python
+
+import argparse
+import json
+import os
+import sys
+import shutil
+
+__version__ = '0.0.1'
+
+state = dict()
+
+def get_args():
+    def_state = os.getenv('NEWCAT_STATE', '/tmp/.newcat.state')
+    parser = argparse.ArgumentParser(description=f'newcat version {__version__}. print / cat only new lines in log files')
+    parser.add_argument('-s','--state',metavar='PATH', default=def_state, help=f'path to state file. default: {def_state}')
+    parser.add_argument('files', metavar='LOGFILE', nargs='+', help='log file(s) to read')
+    return parser.parse_args()
+
+def cat(path: str, filestate: dict) -> None:
+    with open(path ,"r") as fh:
+
+        if not file_rewritten(path, filestate):
+            fh.seek(filestate['pos'])
+
+        shutil.copyfileobj(fsrc=fh, fdst=sys.stdout)
+        filestate['pos'] = fh.tell()
+
+def create_state(path):
+    s = dict()
+    stat = os.stat(path)
+    s['pos'] = 0
+    s['st_ino'] = stat.st_ino
+    return s
+
+def file_rewritten(path: str, oldstate: dict) -> bool:
+
+    stat = os.stat(path)
+
+    if oldstate['st_ino'] != stat.st_ino:
+        return True
+
+    if oldstate['pos'] > stat.st_size:
+        return True
+
+    return False
+
+def main():
+    global state
+    args=get_args()
+
+    try:
+        with open(args.state) as fh:
+            state = json.load(fh)
+    except FileNotFoundError:
+        state = dict()
+    for file in args.files:
+        file = os.path.realpath(file)
+
+        if file not in state or file_rewritten(file, state[file]):
+            state[file] = create_state(file)
+
+        cat(file, state[file])
+
+
+    with open(args.state, "w") as fh:
+        json.dump(state, fh, indent=4, sort_keys=True)
+
+if __name__ == '__main__':
+    main()

setup.py

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+#!/usr/bin/env python3
+
+from setuptools import setup
+import os
+import sys
+
+from importlib.machinery import SourceFileLoader
+
+def read(fname):
+    return open(os.path.join(os.path.dirname(__file__), fname)).read()
+
+def get_version(path):
+    foo = SourceFileLoader(os.path.basename(path), path).load_module()
+    return foo.__version__
+
+setup(
+    name='newcat',
+    version=get_version('newcat'),
+    packages=[],
+    scripts=['newcat'],
+
+    install_requires=[],
+
+    url='https://github.com/yaroslaff/newcat',
+    license='MIT',
+    author='Yaroslav Polyakov',
+    long_description=read('README.md'),
+    long_description_content_type='text/markdown',
+    author_email='[email protected]',
+    description='print / cat only new lines in log files',
+    python_requires='>=3.6',
+    classifiers=[
+        'Development Status :: 3 - Alpha',
+
+        # Indicate who your project is intended for
+        'Intended Audience :: Developers',
+
+        # Pick your license as you wish (should match "license" above)
+         'License :: OSI Approved :: MIT License',
+
+        # Specify the Python versions you support here. In particular, ensure
+        # that you indicate whether you support Python 2, Python 3 or both.
+        'Programming Language :: Python :: 3.6',
+    ],
+)