Read a file from a position in Robot Framework - file-io

Question

How can I read a file from a specific byte position in Robot Framework?
Let's say I have a process running for a long time writing a long log file. I want to get the current file size, then I execute something that affects the behaviour of the process and I wait until some message appears in the log file. I want to read only the portion of the file starting from the previous file size.
I am new to Robot Framework. I think this is a very common scenario, but I haven't found how to do it.

Answer 1

There are no built-in keywords to do this, but writing one in python is pretty simple.
For example, create a file named "readmore.py" with the following:
from robot.libraries.BuiltIn import BuiltIn
class readmore(object):
ROBOT_LIBRARY_SCOPE = "TEST SUITE"
def __init__(self):
self.fp = {}
def read_more(self, path):
# if we don't already know about this file,
# set the file pointer to zero
if path not in self.fp:
BuiltIn().log("setting fp to zero", "DEBUG")
self.fp[path] = 0
# open the file, move the pointer to the stored
# position, read the file, and reset the pointer
with open(path) as f:
BuiltIn().log("seeking to %s" % self.fp[path], "DEBUG")
f.seek(self.fp[path])
data = f.read()
self.fp[path] = f.tell()
BuiltIn().log("resetting fp to %s" % self.fp[path], "DEBUG")
return data
You can then use it like this:
*** Settings ***
| Library | readmore.py
| Library | OperatingSystem
*** test cases ***
| Example of "tail-like" reading of a file
| | # read the current contents of the file
| | ${original}= | read more | /tmp/junk.txt
| | # do something to add more data to the file
| | Append to file | /tmp/junk.txt | this is new content\n
| | # read the new data
| | ${new}= | Read more | /tmp/junk.txt
| | Should be equal | ${new.strip()} | this is new content