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python and shell scripting

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Join Date: Aug 2005

Posts: 18

Reputation: Avner .H. is an unknown quantity at this point

Solved Threads: 0

Avner .H.

Offline

Newbie Poster

python and shell scripting

Aug 9th, 2005

Hello everyone!
I want to use bash from python, however the os.system(command) seems to act only on one command at a time.

I want to use complete commands, also containing control flow statements, like If and While, and to call them from python.

something like this code in bash:

T1="foo"
T2="bar"
if [ "$T1" == "$T2" ]; then
echo even
else
echo not even
fi

I tried to use os.system and separate the commands with ';' , and it works only on simple one line commands, but not on control flow commands.

I also want to get the command output back to my python program somehow. the os.system function returns the exit status of the command but not its output.

any suggestions?
thanks!

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Join Date: Jun 2005

Posts: 146

Reputation: G-Do is an unknown quantity at this point

Solved Threads: 29

G-Do

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Junior Poster

Re: python and shell scripting

Aug 9th, 2005

Hi Avner .H.,

I can't help you with your first problem, other than to say "why not put the Bash code in a Bash script?" which I guess is not an option for you, or else you would have done it.

However, I can help with the second item: grabbing STDOUT from the execution of external commands. Rather than use os.system(), use os.popen(), like so:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
import os
stdout_handle = os.popen("echo something", "r")
text = stdout_handle.read()
import os
stdout_handle = os.popen("echo something", "r")
text = stdout_handle.read()

At this point, the variable 'text' will contain any data that `echo something` sent to STDOUT (namely, the string "something"). Not sure how this works with STDERR, though - you'll have to do some digging to figure that out.

Hope that helps!

EDIT: also, with respect, why not just write the whole thing in Python? Or in Bash, if the project is simple enough?

In my limited experience, Frankenscripts are great as quick and dirty solutions to one-time problems, but for a larger project or a re-occurring problem it's better to stick to one programming paradigm. And truly, I don't know that it's worth learning how to do these kinds of programmatic gymnastics when the whole thing can likely be written in Python.

Just my $0.02, anyway.

Vi veri veniversum vivus vici

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Join Date: May 2005

Posts: 215

Reputation: shanenin is an unknown quantity at this point

Solved Threads: 16

shanenin

Offline

Posting Whiz in Training

Re: python and shell scripting

Aug 9th, 2005

What commands are you trying to get output with. I have not tested alot, but I get output with both cat and grep

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
>>> os.system('cat testfile')
this is
the output
I am getting, I think you
even get
error output
0
>>> os.system('cat testfile | grep error')
error output
0
>>> os.system('cat testfile')
this is
the output
I am getting, I think you
even get
error output
0
>>> os.system('cat testfile | grep error')
error output
0

If you want to use the output for a condidtion or some other reason, you can use the commands.getoutput() function

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
>>> import commands
>>> output = commands.getoutput('cat testfile')
>>> print output
this is
the output
I am getting, I think you
even get
error output
>>> import commands
>>> output = commands.getoutput('cat testfile')
>>> print output
this is
the output
I am getting, I think you
even get
error output

as to running multiple command like a loop. I would recommend using triple quotes to confine the command. This way the python interperter will not use them as delimiters

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
>>> command = """ VAR=even ; sed -i "s/$VAR/odd/" testfile; for i in `cat testfile` ; do echo $i; done; echo "now the tr command is removing the vowels" ; cat testfile |tr 'aeiou' ' ' """
>>> os.system(command) 
this
is
the
output
I
am
getting,
I
think
you
odd
get
error
output
now the tr command is removing the vowels
th s  s
th    tp t
I  m g tt ng, I th nk y
 dd g t
 rr r   tp t
0
>>> command = """ VAR=even ; sed -i "s/$VAR/odd/" testfile; for i in `cat testfile` ; do echo $i; done; echo "now the tr command is removing the vowels" ; cat testfile |tr 'aeiou' ' ' """
>>> os.system(command) 
this
is
the
output
I
am
getting,
I
think
you
odd
get
error
output
now the tr command is removing the vowels
th s  s
th    tp t
I  m g tt ng, I th nk y
 dd g t
 rr r   tp t
0