Call a shell command from Python

[Wildman]

On Tue, 01 Nov 2016 16:52:18 +1100, Steve D'Aprano wrote:

> On Tue, 1 Nov 2016 04:00 pm, Wildman wrote:
> 
>> You are correct about that but, in this case grep never "sees" the '$'
>> sign.  Bash expands $USER to the actual user name beforehand.  If you
>> are on a Linux system, enter this into a terminal to illustrate:
>> 
>> sudo grep ^$USER\: /etc/shadow
> 
> Bash is not involved here. Python is calling grep directly.
> 
> 
> You don't have to believe us, you can test this yourself. Create a simple
> text file with a single line containing your username, and a simple Python
> script that calls grep as you have been:
> 
> 
> [steve at ando ~]$ echo $USER
> steve
> [steve at ando ~]$ cat foo.txt
> blah blah steve blah blah
> [steve at ando ~]$ cat greptest.py
> import subprocess
> cmdlist = ['grep', '$USER', 'foo.txt']
> p = subprocess.Popen(cmdlist, stdout=subprocess.PIPE,stderr=subprocess.PIPE)
> line, err = p.communicate()
> print err, line
> 
> [steve at ando ~]$ python2.7 greptest.py
> 
> [steve at ando ~]$
> 
> 
> 
> So there you have it: categorical proof that bash does not expand the
> string '$USER'. It cannot: bash is not involved in the subprocess call.
> Python calls grep directly.
> 
> If you want to expand the '$USER' string from Python, do it yourself:

I understand now.  That explains more clearly why my original
code did not work.  Thank you.

-- 
<Wildman> GNU/Linux user #557453
The cow died so I don't need your bull!