ActiveState Code

Recipe 157035: tail -f in Python

A simple implementation of the standard UNIX utility tail -f in Python.

Python 
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10
import time

while 1:
    where = file.tell()
    line = file.readline()
    if not line:
        time.sleep(1)
        file.seek(where)
    else:
        print line, # already has newline

Discussion

The recipe works by line, and actually differs from standard tail -f in that at startup it does not do the equivalent of a standard tail (that is, it will show what is appended to the file after it starts getting run, but that is all).

Comments

  1. 1. At 9:27 a.m. on 2 jan 2003, Kirk Reeves said:

    How to make it usable.

    import time, os

#Set the filename and open the file
filename = 'security_log'
file = open(filename,'r')

#Find the size of the file and move to the end
st_results = os.stat(filename)
st_size = st_results[6]
file.seek(st_size)

while 1:
    where = file.tell()
    line = file.readline()
    if not line:
        time.sleep(1)
        file.seek(where)
    else:
        print line, # already has newline
  2. 2. At 6:02 a.m. on 26 may 2003, Ed Pascoe said:

    What tail-10 would do. Sometimes you actually just want the last 10 lines of the file. This will work quite happily on 10Meg files.

    I would love to know if there is a more efficient method.

    import sys
def tail_lines(filename,linesback=10,returnlist=0):
    """Does what "tail -10 filename" would have done
       Parameters:
            filename   file to read
            linesback  Number of lines to read from end of file
            returnlist Return a list containing the lines instead of a string

    """
    avgcharsperline=75

    file = open(filename,'r')
    while 1:
        try: file.seek(-1 * avgcharsperline * linesback,2)
        except IOError: file.seek(0)
        if file.tell() == 0: atstart=1
        else: atstart=0

        lines=file.read().split("\n")
        if (len(lines) > (linesback+1)) or atstart: break
        #The lines are bigger than we thought
        avgcharsperline=avgcharsperline * 1.3 #Inc avg for retry
    file.close()

    if len(lines) > linesback: start=len(lines)-linesback -1
    else: start=0
    if returnlist: return lines[start:len(lines)-1]

    out=""
    for l in lines[start:len(lines)-1]: out=out + l + "\n"
    return out

print tail_lines('/etc/hosts',5,1)
sys.stdout.write(tail_lines('/etc/hosts',5))
  3. 3. At 4:10 p.m. on 29 jul 2003, Graham Nicholls said:

    Err, unless I'm missing something. This seems rather inefficient - suppose several lines are added to the file at once - this version will read them a line at a time; pausing for a second between each.

    I'm doing more or less the same, but seeking to EOF then using readlines: infile,seek(0,2) while 1: lines=infile.readlines if not lines: time.sleep(1) else

    print the line, or pattern match in it, etc.

    Better? (New to Python, so I could be way off the mark!)

  4. 4. At 5:40 p.m. on 30 nov 2007, Sam Jansen said:

    A simple addition... A nice addition to this is to use "yield" to make the code generic. That is,

    def tail_f(file):
  interval = 1.0

  while True:
    where = file.tell()
    line = file.readline()
    if not line:
      time.sleep(interval)
      file.seek(where)
    else:
      yield line

    Which then allows you to write code like:

    for line in tail_f(open(sys.argv[1])):
  print line,

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