In this post I will show you that how you can exactly implemented system command using python.Here we use for example ls -l command. When I use this command in terminal it gives me following output

Harshs-MacBook-Pro:scripts harshkothari$ ls -l
total 32
-rw-r--r--  1 harshkothari  staff    13 Jul 23 02:33 demo.ini
-rw-r--r--@ 1 harshkothari  staff  1864 Jul 23 04:30 filesread.py
-rw-r--r--  1 harshkothari  staff  4121 Jul 23 04:30 langtofontmap.json

Now this command we can implement by following methods using python 🙂

• os.system("command with args") passes the command and arguments to your system’s shell. This is nice because you can actually run multiple commands at once in this manner and set up pipes and input/output redirection.
  os.system("command < input_file | anothercommand > output_file")
  However, while this is convenient, you have to manually handle the escaping of shell characters such as spaces, etc. On the other hand, this also lets you run commands which are simply shell commands and not actually external programs. http://docs.python.org/lib/os-process.html

  >>> import os
  >>> os.system('ls -l')
  total 32
  -rw-r--r--  1 harshkothari  staff    13 Jul 23 02:33 demo.ini
  -rw-r--r--@ 1 harshkothari  staff  1864 Jul 23 04:30 filesread.py
  -rw-r--r--  1 harshkothari  staff  4121 Jul 23 04:30 langtofontmap.json
  0

• stream = os.popen("command with args") will do the same thing as os.systemexcept that it gives you a file-like object that you can use to access standard input/output for that process. There are 3 other variants of popen that all handle the i/o slightly differently. If you pass everything as a string, then your command is passed to the shell; if you pass them as a list then you don’t need to worry about escaping anything. http://docs.python.org/lib/os-newstreams.html

  >>> import os
  >>> print os.popen('ls -l').read()
  total 32
  -rw-r--r--  1 harshkothari  staff    13 Jul 23 02:33 demo.ini
  -rw-r--r--@ 1 harshkothari  staff  1864 Jul 23 04:30 filesread.py
  -rw-r--r--  1 harshkothari  staff  4121 Jul 23 04:30 langtofontmap.json

• The Popen class of the subprocess module. This is intended as a replacement for os.popenbut has the downside of being slightly more complicated by virtue of being so comprehensive. For example, you’d say
  print Popen("ls -l", stdout=PIPE, shell=True).stdout.read()
  instead of
  print os.popen("ls -l").read()
  but it is nice to have all of the options there in one unified class instead of 4 different popen functions. http://docs.python.org/lib/node528.html

  >>> from subprocess import Popen, PIPE
  >>> print Popen("ls -l", stdout=PIPE, shell=True).stdout.read()
  total 32
  -rw-r--r--  1 harshkothari  staff    13 Jul 23 02:33 demo.ini
  -rw-r--r--@ 1 harshkothari  staff  1864 Jul 23 04:30 filesread.py
  -rw-r--r--  1 harshkothari  staff  4121 Jul 23 04:30 langtofontmap.json
  

• The call function from the subprocess module. This is basically just like the Popen class and takes all of the same arguments, but it simply wait until the command completes and gives you the return code. For example:
  call(["ls", "-l"], shell=True) http://docs.python.org/lib/node529.html

  >>> from subprocess import call
  >>> call(["ls", "-l"], shell=True)
  demo.ini                filesread.py            langtofontmap.json
  0

• Using command module http://docs.python.org/2/library/commands.html

  >>> import commands
  >>> commands.getstatusoutput('ls -l')
  (0, 'total 32\n-rw-r--r--  1 harshkothari  staff    13 Jul 23 02:33 demo.ini\n-rw-r--r--@ 1 harshkothari  staff  1864 Jul 23 04:30 filesread.py\n-rw-r--r--  1 harshkothari  staff  4121 Jul 23 04:30 langtofontmap.json')
  >>> commands.getoutput('ls -l')
  'total 32\n-rw-r--r--  1 harshkothari  staff    13 Jul 23 02:33 demo.ini\n-rw-r--r--@ 1 harshkothari  staff  1864 Jul 23 04:30 filesread.py\n-rw-r--r--  1 harshkothari  staff  4121 Jul 23 04:30 langtofontmap.json'
  

• The os module also has all of the fork/exec/spawn functions that you’d have in a C program

Hope this will help you 🙂