python variable in command to execute with os.system

Question

Python is driving me crazy; I always have difficulty using variables in PATH. My version is Python 2.4.3

>>> import os
>>> a = "httpd"
>>> cmd = '/etc/init.d/+a restart'
>>> print cmd
/etc/init.d/+a restart
>>>

How do I put a /etc/init.d/httpd in cmd variable so I can use os.system(cmd)?

Answer 1

For python v> 2.7

cmd = '/etc/init.d/{} restart'.format(a)

or

cmd = '/etc/init.d/'+a+' restart'

But you should probably look into using subprocess.

• mgilson: Careful, your second version is missing a space
• askewchan: @mgilson Thanks, fixed. Also noted that .format for newer versions of python, thanks to your answer.
• mgilson: Actually, the way you have it is only python2.7 and newer. with python2.6 you'd need '{0}' rather than '{}'
• Satish: epic win... You are my hero :)

Answer 2

you need something like:

cmd = '/etc/init.d/%s restart' % a

If you need to do multiple substitutions, you could do something like:

cmd = '/etc/init.d/%s %s' % ( 'httpd', 'restart' )

In this form, '%s' is a place holder. Each '%s' gets replaced by an item in the corresponding tuple on the right hand side of the % operator (which is the string interpolation operator I suppose). More details can be found in the String Formatting section of the reference

Starting with python2.6, there's a new way to format strings using the .format method, but I guess that doesn't help you very much.

• Satish: Could you explain those %s and % doing there?
• mgilson: @Satish -- Added some explanation and a link