writing backup program in python

Question

I found this tutorial in A Byte of Python, but do not understand it:

import os
import time

source = [r'C:\Users\Desktop\Stuff']

target_dir = 'C:\Users\Backup'

target = target_dir + time.strftime('%Y%m%d%H%M%S') + '.zip'

zip_command = 'zip -qr "%s" %s' % (target, ' '.join(source))

if os.system(zip_command) == 0:
    print 'Successful backup to', target
else:
    print 'Backup failed!'

After checking help(os) I do not understand why os.system(zip_command) would ever be zero. .system() does not return a Boolean, does it?

thanks.

juliomalegria · Accepted Answer · 2011-12-28 04:55:26Z

3

help(os.system) says:

system(...)
    system(command) -> exit_status

    Execute the command (a string) in a subshell.

So, it executes the command you pass as a parameter and returns the exit_status of that command.

When a program returns 0 as result, it means it was executed succesfully and if it returns anything else, it probably means there was an error somewhere.

So in this line:

if os.system(zip_command) == 0:

you are actually asking: if the command line was executed succesfully then ... else ...

answered Dec 28, 2011 at 4:55

juliomalegria

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David Hu Over a year ago

Also worth reading is the online Python docs which explains os.system in more detail.

harish.venkat · Accepted Answer · 2011-12-28 04:58:06Z

0

On Windows, the return value is that returned by the system shell after running command, given by the Windows environment variable COMSPEC: on command.com systems (Windows 95, 98 and ME) this is always 0; on cmd.exe systems (Windows NT, 2000 and XP) this is the exit status of the command run; on systems using a non-native shell, consult your shell documentation. see this.

answered Dec 28, 2011 at 4:58

harish.venkat

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caiocpricci2 · Accepted Answer · 2011-12-28 04:58:20Z

0

The os.system(command) runs a command in a subshell, and returns an int.

Taken from python docs:

"On Windows, the return value is that returned by the system shell after running command, given by the Windows environment variable COMSPEC: on command.com systems (Windows 95, 98 and ME) this is always 0; on cmd.exe systems (Windows NT, 2000 and XP) this is the exit status of the command run; on systems using a non-native shell, consult your shell documentation."

So it returns 0 if the command was successful, and something else if it wasn't.

Source: http://docs.python.org/library/os.html#os.system

answered Dec 28, 2011 at 4:58

caiocpricci2

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