Python vs. BASH/Perl/PHP -- backticks

[altonbr]

In BASH/Perl/PHP I can run a command like so:

#BASH
USER=`whoami`
#Perl/PHP
$user = `whoami`; # or...
$user = shell_exec('whoami');

How do I load a variable in Python by running a shell command?

I read that this is the only way:

import os
user = os.popen('whoami').read()

Is that true?

I guess the whole purpose is that python is cross-platform and should try not to rely on shell commands, am I correct?

[Majorix]

Have you already read the library manual for the os module on Python's official docs? They are a good source when it comes to stuff like this.

[ghostdog74]

I read that this is the only way:

import os
user = os.popen('whoami').read()

Is that true?

of course not. Read the docs, esp the section called Process Management in the library reference.


I guess the whole purpose is that python is cross-platform and should try not to rely on shell commands, am I correct?

yes, but Python is not everything. Sometimes, we really do need to call shell commands.

[rolando2424]

There is the os.system() function, that allows you to run the commands in a subshell, but it only returns the exit code (0 if all went well, other numbers if there was an error).

What I usually do is a redirect the output of the command to a file and then open it in python.

Example:



#!/usr/bin/env python
import os

os.system("cat file > output.txt")

content = file("output.txt", "r").readlines()

[ghostdog74]

os.system("cat file > output.txt")
[/code]

I hope this is just an example, and not what you normally do every time.

[Wybiral]

There is the os.system() function, that allows you to run the commands in a subshell, but it only returns the exit code (0 if all went well, other numbers if there was an error).

Do some research on pipes. The subprocess module offers some really easy routines for handling pipes and processes.



import subprocess as sp
proc = sp.Popen("bc", shell=True, stdout=sp.PIPE, stdin=sp.PIPE)
proc.stdin.write("10+2 \n 5*5 \n quit \n")
print proc.communicate()[0]

[rolando2424]

I hope this is just an example, and not what you normally do every time.

Sometimes I think it's easier to just do os.system("cat onlineInvoice.aspx.html | grep KB | sed 's/internet.vodafone.pt//g' | sed 's/\t/ /g' > output.txt") (taken from a script I wrote a few days ago) and then process the output.txt file in python, instead of writing the code to cat, grep and sed.

Isn't it the Unix way (I think)? To write one program that does one thing well, and then mixing them with redirects, pipes, etc. to make your program.

[ghostdog74]

Sometimes I think it's easier to just do os.system("cat onlineInvoice.aspx.html | | sed 's/internet.vodafone.pt//g' | sed 's/\t/ /g' > output.txt") (taken from a script I wrote a few days ago) and then process the output.txt file in python, instead of writing the code to cat, grep and sed.

Noooooooo.... you can do that purely in Python.. who taught you that? If you want to do that , you might as well use shell programming.

cat onlineInvoice.aspx.html
is equivalent to


for line in open("onlineInvoice.aspx.html"):
....

or


data = open("onlineInvoice.aspx.html").read()





grep KB


This is the same as :


...
if "KB" in line :
...





sed 's/internet.vodafone.pt//g' | sed 's/\t/ /g'


This practically says to substitute "internet.vodafone.pt" with a blank and then substitute tabs with space. therefore the Python equivalent might be


...
line.replace("internet.vodafone.pt","").replace("\t","") #not tested
...

And I am sure for "> output.txt", you know what to do in Python.



Isn't it the Unix way (I think)? To write one program that does one thing well, and then mixing them with redirects, pipes, etc. to make your program.
No..definitely not. Mixing too many redirects and pipes makes your code not easy to read and prone to errors. Just the right amount to do the job right will do. For the record, this


cat onlineInvoice.aspx.html | | sed 's/internet.vodafone.pt//g' | sed 's/\t/ /g'

is useless use of cat and pipes. Here's one way it can be done more "nicely"


# tr -s ' ' < onlineInvoice.aspx.html | sed 's/internet.vodafone.pt//g' > output.txt

[ozgurk]

I am trying to learn Python and not sure for the equivalent of "> output.txt" usage..
Is there a way in Python that bypasses the need for output.txt? Can the output be redirected to an array internally?

[tom66]

This writes to a file called output.txt.



f = open("output.txt", "w")
f.write("text to write\n")
f.close()

[slavik]

here's a nice Perl example:


open CMD, "|somecommand|command2|" or die $!;
print CMD "here is data to compute"
while ($result = <CMD>) {
print "Result is" . $result . "\n";
print CMD "here is more data";
}


or you can split the above into two threads that truly run concurrently. :)

[ozgurk]

Thanks :)