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Calling a command line utility from Python

I am currently trying to utilize strace to automatically trace a programm 's system calls. To then parse and process the data obtained, I want to use a Python script.
I now wonder, how would I go about calling strace from Python?
Strace is usually called via command line and I don't know of any C library compiled from strace which I could utilize.
What is the general way to simulate an access via command line via Python?
alternatively: are there any tools similar to strace written natively in Python?
I'm thankful for any kind of help.
Nothing, as I'm clueless

You need to use the subprocess module.
It has check_output to read the output and put it in a variable, and check_call to just check the exit code.
If you want to run a shell script you can write it all in a string and set shell=True, otherwise just put the parameters as strings in a list.
import subprocess
# Single process
subprocess.check_output(['fortune', '-m', 'ciao'])
# Run it in a shell
subprocess.check_output('fortune | grep a', shell=True)
Remember that if you run stuff in a shell, if you don't escape properly and allow user data to go in your string, it's easy to make security holes. It is better to not use shell=True.

You can use commands as the following:
import commands
cmd = "strace command"
result = commands.getstatusoutput(cmd)
if result[0] == 0:
print result[1]
else:
print "Something went wrong executing your command"
result[0] contains the return code, and result[1] contains the output.

Python 2 and Python 3 (prior 3.5)
Simply execute:
subprocess.call(["strace", "command"])
Execute and return the output for processing:
output = subprocess.check_output(["strace", "command"])
Reference: https://docs.python.org/2/library/subprocess.html
Python 3.5+
output = subprocess.run(["strace", "command"], caputure_output=True)
Reference: https://docs.python.org/3.7/library/subprocess.html#subprocess.run

Running a command-line command in terminal using python

I have to basically run a terminal-command using python script. General way of doing this would be
In [1]: import subprocess
In [2]: subprocess.Popen(['python some-python-script.py'],shell=True)
Out[2]: <subprocess.Popen at 0x103425358>
Now, my requirement is that this script will be triggered n (<=10) times printing some output. How do i ensure that everytime its triggered using python, it opens in new terminal tab printing the output?
Checked couple of refernce online and tried using
In [11]: os.system("Terminal -x python some-python-script.py")
sh: Terminal: command not found
but doesn't help.
I am on OSx Sierra but a platform independent solution is welcome!!

You can use check_output from the subprocess module. As your command must be executed more than once, you can just call the function inside for loop. The script may look something like this (the upper bound of the range may be variable):
import subprocess
for idx in range(5):
print(subprocess.check_output(['python3', 'full-path-to-script.py'], encoding='utf-8'))
As mentioned in the documentation, the check_output function returns the data as encoded bytes, but as the encoding is specified in my example, the output will be UTF-8 encoding string. Depending on your command the decoding to text may need to be handled separately.

Why doesn't the rest of the code after call() work? [duplicate]

I have a python script which when run, logs information on the terminal, i want to send this logging information to a text file,
To achieve this in the beginning of the file i am inserting
import subprocess
subprocess.call(['script', 'logfile'])
and at the end of the file,i put in,
subprocess.call(['exit'])
The problem with this is when it calls the first commandscript logfile,it terminates the script,
Any suggestions on how i could make this work would be really helpful,Thanks in advance

The problem is that subprocess.call isn't returning until the shell spawned by script exits, at which point your Python script will resume.
The simplest way to do what you want is to call script itself with your Python script as an argument. Instead of
#!/usr/bin/python
import subprocess
subprocess.call(['script', 'logfile'])
# Rest of your Python code
subprocess.call(['exit'])
you will use
#!/usr/bin/python
import os
import sys
if '_underscript' not in os.environ:
os.environ['_underscript'] = "yes"
cmd_args = ['script', 'logfile', 'python'] + sys.argv
os.execvp('script', cmd_args)
# Rest of your Python code
The environment variable prevents your script from entering an infinite loop of re-running itself with script. When you run your Python script, it first checks its environment for a variable that should not yet exist. If it doesn't, it sets that variable, then runs script to re-run the Python script. execvp replaces your script with the call to script; nothing else in the Python script executes. This second time your script runs, the variable _underscript does exist, meaning the if block is skipped and the rest of your script runs as intended.

Seems like that's the expected behaviour of subprocess.call(...). If you want to capture the output of the script to a file, you'll need to open a new file handler in write mode, and tell the subprocess.call where to direct the stdout, which is the terminal output you typically would see.
Try:
import subprocess
f = open('/tmp/mylogfile.log', 'w')
subprocess.call(['/path/to/script'], stdout=f)
f.close()
Then in the terminal you can run tail /tmp/mylogfile.log to confirm.
I'm not sure the last exit call is required for what you're trying to achieve.
You can read more in the python docs, depending which version of Python you're using. https://docs.python.org/2/library/subprocess.html
The file doesn't need to pre-exist. Hope that helps!

What's the best way to execute PowerShell scripts from Python

All the previous posts on this topic deal with specific challenges for their use case. I thought it would be useful to have a post only dealing with the cleanest way to run PowerShell scripts from Python and ask if anyone has an better solution than what I found.
What seems to be the generally accepted solution to get around PowerShell trying to interpret different control characters in your command differently to what's intended is to feed your Powershell command in using a file:
ps = 'powershell.exe -noprofile'
pscommand = 'Invoke-Command -ComputerName serverx -ScriptBlock {cmd.exe \
/c "dir /b C:\}'
psfile = open(pscmdfile.ps1, 'w')
psfile.write(pscommand)
psfile.close()
full_command_string = ps + ' pscmdfile.ps1'
process = subprocess.Popen(full_command_string , shell=True, \
stdout=subprocess.PIPE, stderr=subprocess.PIPE)
When your python code needs to change the parameters for the Powershell command each time you invoke it you end up writing and deleting a lot of temporary files for subprocess.Popen to run. It works perfectly but it's unnecessary and not very clean. It's really nice to be able to tidy up and wanted to get suggestions on any improvements I could make to the solution I found.
Instead of writing a file to disk containing the PS command create a virtual file using the io module. Assuming that the "date" and "server" strings are being fed in as part of a loop or function that contains this code, not including the imports of course:
import subprocess
import io
from string import Template
raw_shellcmd = 'powershell.exe -noprofile '
--start of loop with server and date variables populated--
raw_pslistcmd = r'Invoke-Command -ComputerName $server -ScriptBlock ' \
r'{cmd.exe /c "dir /b C:\folder\$date"}'
pslistcmd_template = Template(raw_pslistcmd)
pslistcmd = pslistcmd_template.substitute(server=server, date=date)
virtualfilepslistcommand = io.BytesIO(pslistcmd)
shellcmd = raw_shellcmd + virtualfilepslistcommand.read()
process = subprocess.Popen(shellcmd, shell=True, stdout=subprocess.PIPE, \
stderr=subprocess.PIPE)
--end of loop--

Arguably the best approach is to use powershell.exe -Command rather than writing the PowerShell command to a file:
pscommand = 'Invoke-Command ...'
process = subprocess.Popen(['powershell.exe', '-NoProfile', '-Command', '"&{' + pscommand + '}"'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
Make sure double quotes in the pscommand string are properly escaped.
Note that shell=True is required only in certain edge cases, and should not be used in your scenario. From the documentation:
On Windows with shell=True, the COMSPEC environment variable specifies the default shell. The only time you need to specify shell=True on Windows is when the command you wish to execute is built into the shell (e.g. dir or copy). You do not need shell=True to run a batch file or console-based executable.

After spending a fair amount of time on this.
I think that running powershell commands from python may not make sense to a lot of people, especially people who work exclusively in windows environments. There are numerous clear advantages to python over powershell however so the ability to do all your business logic in python and then selectively execute powershell on remote servers is truly a great thing.
I've now been through several improvements of my "winrmcntl" module which I can't share due to company policy unfortunately but here is my advice to anyone who would like to do something similar. The module should take as input an unmodified PS command or scriptblock as you'd run it if you were typing directly in PS on the destination box. A few tricks:
To avoid permission difficulties, ensure the user running your python script and hence the one running powershell.exe via process.Popen is the user that has the correct permissions on the windows box you're invoke-command is pointing at. We use an enterprise scheduler which has windows vms as agents on which the python code lives which takes care of that.
You will sometimes rarely but still get the odd esoteric exception from powershell land, if they're anything like the one in particular I saw the odd time, microsoft scratch their heads at a little and get you to do time consuming application stack tracing. This is not only time consuming but very difficult to get right because it's resource intensive and you don't know when the exception will next occur. In my opinion, it's much better and easier to parse the output of the exception and retry up to x number of times if a certain text appears in those exceptions. I keep a list of strings in my winrmcntl module which currently contains a single string.
If you want to not have to "massage" the powershell commands as they traverse the python -> windows -> powershell -> powershell stack to make them work as expected on destination boxes, the most consistent method I've found is to write your one liners and scriptblocks alike into a ps_buffer.ps1 file which you then feed to powershell on the source box so that every process.popen looks exactly the same but the content of ps_buffer.ps1 changes with each execution.
powershell.exe ps_buffer.ps1
To keep your python code nice and clean, it's great having your list of powershell one liners in a json file or similar as well as pointers to scriptblocks you want to run saved into static files. You load up your json file as an ordered dict and cycle through issuing commands based on what you're doing.
Can't be overstated, as far as is possible try to be on the latest stable version of PS but more than that, it's imperative to be on the same version on client and server.
"scriptblock" and "server" are the values fed to this module or function
import subprocess
from string import Template
scriptblock = 'Get-ChildItem' #or a PS scriptblock as elaborate as you need
server = 'serverx'
psbufferfile = os.path.join(tempdir, 'pscmdbufferfile_{}.ps1'.format(server))
fullshellcmd = 'powershell.exe {}'.format(psbufferfile)
raw_pscommad = 'Invoke-Command -ComputerName $server -ScriptBlock {$scriptblock}'
pscmd_template = Template(raw_pscommand)
pscmd = pscmd_template.substitute(server=server, scriptblock=scriptblock)
try:
with open(psbufferfile, 'w') as psbf:
psbf.writelines(pscmd)
....
try:
process = subprocess.Popen(fullshellcmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
output, error = process.communicate()
....

Python script writing results to text file

Today I managed to run my first Python script ever. I'm a newb, on Windows 7 machine.
When I run python.exe and enter following (Python is installed in C:/Python27)
import os
os.chdir('C:\\Pye\\')
from decoder import *
decode("12345")
I get the desired result in the python command prompt window so the code works fine. Then I tried to output those results to a text file, just so I don't have to copy-paste it all manually in the prompt window. After a bit of Googling (again, I'm kinda guessing what I'm doing here) I came up with this;
I wrote "a.py" script in the C:/Pye directory, and it looked like this;
from decoder import *
decode("12345")
And then I wrote a 01.py file that looked like this;
import subprocess
with open("result.txt", "w+") as output:
subprocess.call(["python", "c:/Pye/a.py"], stdout=output);
I see the result.txt gets created in the directory, but 0 bytes. Same happens if I already make an empty result.txt and execute the 01.py (I use Python Launcher).
Any ideas where am I screwing things up?

You didn't print anything in a.py. Change it to this:
from decoder import *
print(decode("12345"))
In the Python shell, it prints it automatically; but the Python shell is just a helper. In a file, you have to tell it explicitly.

When you run python and enter commands, it prints to standard out (the console by default) because you're using the shell. What is printed in the python shell is just a representation of what object is returned by that line of code. It's not actually equivalent to explicitly calling print.
When you run python with a file argument, it executes that script, line by line, without printing any variables to stdout unless you explicitly call "print()" or write directly to stdout.
Consider changing your script to use the print statement.:
print(decode("12345"))