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@cbsmith cbsmith/livecat.py
Last active Mar 26, 2017

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What would you like to do?
First pass at doing a live "cat" in python
'''
Live cat of file. Uses mmap to try to be efficient with memory,
though reads a byte at a time, so it burns through CPU by hitting the
python interpreter loop N times. We could add buffering, but that
would just make the code more complex, and who wants that?
stdout gets the default buffering, so consider invoking with
-u flag/PYTHONBUFFERED environment variable if not writing to
a file.
Requires fsmonitor: https://github.com/shaurz/fsmonitor
...which unfortunately lacks a decent monitor for non-Windows/Linux
systems, so burns up CPU unnecessarily on those platforms. Despite its
reputation, python makes it surprisingly difficult to write clean and
correct code.
No, this is *NOT* tail. Tail looks at the end of a file, and just
happens to support live updates to the end of a file.
'''
# all the python things we need
from contextlib import closing
from itertools import chain
from mmap import mmap, ACCESS_READ, ALLOCATIONGRANULARITY
from os import fstat
from sys import argv, exit, stdout
# the magic
from fsmonitor import FSMonitor
def dump_contents(fd, offset=0, output=stdout):
'''
Dump contents attached to a filedescriptor.
fd - a file descriptor, needs to support mmap
offset - offset into the file object to start at
'''
filesize = fstat(fd).st_size
gap = filesize - offset
if gap < 0:
exit(-1) # file shrank, can't do much useful
if gap > 0:
# some logic here to paper over mmap API's inherent ugliness
pageoffset = offset % ALLOCATIONGRANULARITY
mmapoffset = offset - pageoffset
with closing(mmap(fd, filesize - mmapoffset, access=ACCESS_READ, offset=mmapoffset)) as mapped:
mapped.seek(pageoffset)
for _ in xrange(gap):
output.write(mapped.read_byte())
return filesize
def cat(infile=__file__, outfile=None):
m = FSMonitor()
offset = 0
m.add_file_watch(infile)
# arbitary buffering size set to 4096 bytes
with open(outfile, 'wb', 4096) if outfile else stdout as output:
while True:
with open(infile, 'rb') as f:
file_loop = chain((None,), m.read_events()) # prepend initial iteration
for _ in file_loop:
offset = dump_contents(f.fileno(), offset, output)
if __name__ == '__main__':
cat(*argv[1:])
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