Popen in main and subprocess

[pistacchio]

the following code (in the main thread) works well, I grep some files
and the search until the first 100 results are found (writing the
results to a file), then exit:

    command = 'grep -F "%s" %s*.txt' % (search_string, DATA_PATH)

    p = Popen(['/bin/bash', '-c', command], stdout = PIPE)
    f = open(output_file, 'w+')
    num_lines = MAX_RESULTS
    while True:
        line = p.stdout.readline()
        print num_lines
        if line != '':
            f.write(line)
        num_lines = num_lines - 1
        if num_lines == 0:
            break
        else:
            break
The very same code used into a Process subclass, always returns grep:
writing output: Broken pipe in the console:

    class Search(Process):
        def __init__(self, search_id, search_string):
            self.search_id = search_id
            self.search_string = search_string
            self.grepped = ''
            Process.__init__(self)

        def run(self):
            output_file = TMP_PATH + self.search_id

            # flag if no regex chars
            flag = '-F' if re.match(r"^[a-zA-Z0\ ]*$",
self.search_string) else '-P'

            command = 'grep %s "%s" %s*.txt' % (flag,
self.search_string, DATA_PATH)

            p = Popen(['/bin/bash', '-c', command], stdout = PIPE)
            f = open(output_file, 'w+')
            num_lines = MAX_RESULTS
            while True:
                line = p.stdout.readline()
                print num_lines
                if line != '':
                    f.write(line)
                num_lines = num_lines - 1
                if num_lines == 0:
                    break
                else:
                    break
How come? How to fix this? Thanks