Simple implementation of the tail command in Python

tail.py

'''
Basic tail command implementation
Usage:

tail.py filename numlines

'''

import sys
import linecache

if len(sys.argv) !=3:
    print 'Usage: tail.py <file> <nlines>'
    sys.exit(1)

# filename and number of lines requested
fname, nlines = sys.argv[1:]
nlines = int(nlines)

# count the total number of lines
tot_lines = len(open(fname).readlines())

# use line cache module to read the lines
for i in range(tot_lines - nlines + 1, tot_lines+1):
    print linecache.getline(sys.argv[1],i),

tail_2.py

""" This is a more efficient version, since it does not read the entire
file
"""

import sys
import os

bufsize = 8192

lines = int(sys.argv[1])
fname = sys.argv[2]
fsize = os.stat(fname).st_size

iter = 0
with open(sys.argv[2]) as f:
    if bufsize > fsize:
        bufsize = fsize-1
    data = []
    while True:
        iter +=1
        f.seek(fsize-bufsize*iter)
        data.extend(f.readlines())
        if len(data) >= lines or f.tell() == 0:
            print(''.join(data[-lines:]))
            break

I'm an oldie, and the thought of reading a sequential file, one character at a time, backwards, from the end, just makes me shudder.

I've done more reading this thread, and it appears to me there are problems with all the implementations. The very first one ends up reading the file twice, to no point. It reads the file to count the lines, you can just print from that read buffer, instead of just throwing it away and using "linecache" whatever that is. You could simply use:

file_lines = open(fname).readlines()
# count the total number of lines
tot_lines = len(file_lines)

print '\n'.join(file_lines[-(tot_lines if tot_lines < nlines else nlines):])

Also, the one with the "bufsize" moves back in the file by "bufsize" increments, but actually ends up extending the data by the entire file each jump back! To see the problems, use a line numbered file, and set a small buffsize.

Quick quest for all: Does anyone know whether the solution is for Version 2.7 or for Version 3 ? Let me know when you read this post please. Thank you -

Second implementation breaks for empty files, causing infinite looping. Solved by putting if fsize < 1: return '' before opening file

Implementation of tail -n k. This uses offset and doesn't read the whole line. Imagine the line is 10GB large...

def tail(filename, n):
    stat = os.stat(filename)
    if stat.st_size == 0 or n == 0:
        yield ''
        return

    page_size = 5
    offsets = []
    count = _n = n if n >= 0 else -n

    last_byte_read = last_nl_byte = starting_offset = stat.st_size - 1

    with open(filename, 'r') as f:
        while count > 0:
            starting_byte = last_byte_read - page_size
            if last_byte_read == 0:
                offsets.append(0)
                break
            elif starting_byte < 0:
                f.seek(0)
                text = f.read(last_byte_read)
            else:
                f.seek(starting_byte)
                text = f.read(page_size)

            for i in range(-1, -1*len(text)-1, -1):
                last_byte_read -= 1
                if text[i] == '\n':
                    last_nl_byte = last_byte_read
                    starting_offset = last_nl_byte + 1
                    offsets.append(starting_offset)
            count -= 1

    offsets = offsets[len(offsets)-_n:]
    offsets.reverse()

    with open(filename, 'r') as f:
        for i, offset in enumerate(offsets):
            f.seek(offset)

            if i == len(offsets) - 1:
                yield f.read()
            else:
                bytes_to_read = offsets[i+1] - offset
                yield f.read(bytes_to_read)

filename = '/tmp/test.txt'
for x in tail(filename, 10):
    print(x.strip())

@amit, There is a small flaw here. If bufsizeiter returns pointer on the first line in the N lines to be printed, then there is a possibility that first item in data[-lines:] can start from anywhere between first and last line of that line ?
Say if line is " John Johny yes pappa" , data[-lines:][0] can be "ny yes pappa" since f.seek(fsize-bufsizeiter) can return anywhere ?
Changing if len(data) >= lines or f.tell() == 0: to if len(data) > lines or f.tell() == 0: Should fix the issue

Python 3.8 asyncio version,

#! /usr/bin/env python
# -*- coding: utf-8 -*-
#
# >>
#   Blake VandeMerwe, LiveViewTech
# <<

import os
import io
import asyncio
from functools import partial
from typing import AsyncIterator

LINE_BUFFER = 1

async def tail(
    filename: str,
    last_lines: int = 10,
    non_exist_max_secs: float = 30.0,
    fp_poll_secs: float = 0.125
) -> AsyncIterator[str]:
    """Continuously tail a file pointer yielding one line at a time."""

    async def wait_exists() -> bool:
        """Wait for a file to exist, the return statement reflects
        whether or not the file existed when the timeout limits were reached."""
        bail_at: float = time.monotonic() + non_exist_max_secs
        while not os.path.exists(filename):
            if time.monotonic() >= bail_at:
                return False
            await asyncio.sleep(fp_poll_secs)
        return True

    async def check_rotate(_fp) -> io.TextIOBase:
        """Determine if the file rotated in place; same name different inode."""
        nonlocal fino
        if os.stat(filename).st_ino != fino:
            new_fp = open(filename, 'r')
            _fp.close()
            new_fp.seek(0, os.SEEK_SET)
            fino = os.fstat(new_fp.fileno()).st_ino
            return new_fp
        return _fp

    # ~~
    if not await wait_exists():
        return

    buff = io.StringIO()
    stat = os.stat(filename)

    fino: int = stat.st_ino
    size: int = stat.st_size
    blocksize: int = os.statvfs(filename).f_bsize

    fp = open(filename, 'r', LINE_BUFFER)

    if last_lines > 0:
        if stat.st_size <= blocksize:
            # if the file is smaller than 8kb, read all the lines
            for line in fp.readlines()[-last_lines::]:
                yield line.rstrip()
        else:
            # if the file is larger than 8kb, seek 8kb from the end
            #  and return all the lines except the (potential) half-line
            # first element and the null-terminated extra line at the end.
            fp.seek(os.stat(fp.fileno()).st_size - blocksize)
            for line in fp.readlines()[1:-1][-last_lines::]:
                yield line.rstrip()

    # seek to the end of the file for tailing
    #  given the above operations we should already be there.
    fp.seek(0, os.SEEK_END)

    try:
        while True:
            # wait for the file to exist -- generously
            if not os.path.exists(filename):
                if not await wait_exists():
                    return

            fp = await check_rotate(fp)
            n_stat = os.fstat(fp.fileno())
            n_size = n_stat.st_size

            # if the file is the same size, churn
            #  .. this could be error-prone on small files that
            # rotate VERY fast, but that's an edge case for
            #  tailing a persistent log file.
            if n_size == size:
                await asyncio.sleep(fp_poll_secs)
                continue

            # if the file shrank, seek to the beginning
            if n_size < size:
                fp.seek(0, os.SEEK_SET)

            size = n_size
            for chunk in iter(partial(fp.read, blocksize), ''):
                buff.write(chunk)

            buff.seek(0, os.SEEK_SET)

            for line in buff.readlines():
                yield line.rstrip()

            # resize our string buffer
            buff.truncate(0)

    except IOError:
        buff.close()
        fp.close()


if __name__ == '__main__':

    async def main():
        async for line in tail(r'/etc/foldingathome/log.txt'):
            print(line)

    try:
        loop = asyncio.get_event_loop()
        loop.run_until_complete(main())
    except KeyboardInterrupt:
        pass
    loop.stop()
    loop.close()

This is great, but you don't need the # -*- coding: utf-8 -*- line in Python 3

Python 3.8 asyncio version,

#! /usr/bin/env python
# -*- coding: utf-8 -*-
...

@therumbler TIL, thanks! I've been using the same "new file" template for years. Time to update!

Tail in Lua: https://gist.github.com/blasti/a1f2fd90f0f6933fe5583ea6777a374a