title: OSError: [Errno 2] No such file or directory
body: <p>i have the following code that is attempting to start each of the "commands" below. The module attempts to open a second program and cashes</p>
  
  <pre><code>#!/usr/bin/python
  from sys import argv, exit
  from subprocess import Popen, PIPE
  from collections import defaultdict
  from os import path, remove
  
  def main():
  if len(argv) != 4 or not argv[2] in ['C', 'G', 'A', 'D']:
      print("Cas-OFFinder-bulge v1.1 beta (2015-09-30)")
      print("")
      print("Copyright (c) 2015 Jeongbin Park and Sangsu Bae")
      print("")
      print("Usage: cas-offinder-bulge {input_file} {C|G|A|D} {output_file}")
      print("(C: using CPUs, G: using GPUs, A: using accelerators, D: dry-run")
      print("")
      print("Example input file (DNA bulge 2, RNA bulge 1):")
      print("/var/chromosomes/human_hg19")
      print("NNNNNNNNNNNNNNNNNNNNNRG 2 1")
      print("GGCCGACCTGTCGCTGACGCNNN 5")
      print("CGCCAGCGTCAGCGACAGGTNNN 5")
      print("ACGGCGCCAGCGTCAGCGACNNN 5")
      print("GTCGCTGACGCTGGCGCCGTNNN 5")
      print("")
      p = Popen(['cas-offinder'], stderr=PIPE, stdout=PIPE)
      flag = False
      for line in p.stdout:
          line = line.strip()
          if line == "Available device list:":
              flag = True
          if flag:
              print(line)
  else:
      fnhead = path.join(path.dirname(argv[1]), path.splitext(path.basename(argv[1]))[0])
      with open(argv[1]) as f:
          chrom_path = f.readline()
          pattern, bulge_dna, bulge_rna = f.readline().strip().split()
          isreversed = False
          for i in range(int(len(pattern)/2)):
              if pattern[i] == 'N' and pattern[len(pattern)-i-1] != 'N':
                  isreversed = False
                  break
              elif pattern[i] != 'N' and pattern[len(pattern)-i-1] == 'N':
                  isreversed = True
                  break
  </code></pre>
  
  <p>Upon executing the code the following exception is thrown:</p>
  
  <pre><code>Traceback (most recent call last):
  File "./cas-offinder-bulge", line 154, in &lt;module&gt;
  main()
  File "./cas-offinder-bulge", line 24, in main
  p = Popen(['cas-offinder'], stderr=PIPE, stdout=PIPE)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 710, in __init__
  errread, errwrite)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1335, in _execute_child
  raise child_exception
  OSError: [Errno 2] No such file or directory
  </code></pre>
  
  <p>I think I'm missing something, can anyone point out the mistakes?
  - Thanks</p>