Develop Reference

Python os.system or subprocess calls for command line automation

I would like to be able to call some executables that take in parameters and then dump the output to a file. I've attempted to use both os.system and subprocess calls to no avail. Here is a sample of what I'd like python to execute for me...
c:\directory\executable_program.exe -f w:\directory\input_file.txt > Z\directory\output_file.txt
Notice the absolute paths as I will be traversing hundreds of various directories to act on files etc..
Many thanks ahead of time!
Some examples that I've tried:
subprocess.run(['c:\directory\executable_program.exe -f w:\directory\input_file.txt > Z\directory\output_file.txt']
subprocess.call(r'"c:\directory\executable_program.exe -f w:\directory\input_file.txt > Z\directory\output_file.txt"']
subprocess.call(r'"c:\directory\executable_program.exe" -f "w:\directory\input_file.txt > Z\directory\output_file.txt"']

Your attempts contain various amounts of quoting errors.
subprocess.run(r'c:\directory\executable_program.exe -f w:\directory\input_file.txt > Z\directory\output_file.txt', shell=True)
should work, where the r prefix protects the backslashes from being interpreted and removed by Python before the subprocess runs, and the absence of [...] around the value passes it verbatim to the shell (hence, shell=True).
On Windows you could get away with putting the command in square brackets even though it's not a list, and omitting shell=True in some circumstances.
If you wanted to avoid the shell, try
with open(r'Z\directory\output_file.txt', 'wb') as dest:
subprocess.run(
[r'c:\directory\executable_program.exe', '-f', r'w:\directory\input_file.txt'],
stdout=dest)
which also illustrates how to properly pass a list of strings in square brackets as the first argument to subprocess.run.

python: How does subprocess.check_output create it's calls?

I'm trying to read the duration of video files using mediainfo. This shell command works
mediainfo --Inform="Video;%Duration/String3%" file
and produces an output like
00:00:33.600
But when I try to run it in python with this line
subprocess.check_output(['mediainfo', '--Inform="Video;%Duration/String3%"', file])
the whole --Inform thing is ignored and I get the full mediainfo output instead.
Is there a way to see the command constructed by subprocess to see what's wrong?
Or can anybody just tell what's wrong?

Try:
subprocess.check_output(['mediainfo', '--Inform=Video;%Duration/String3%', file])
The " in your python string are likely passed on to mediainfo, which can't parse them and will ignore the option.
These kind of problems are often caused by shell commands requiring/swallowing various special characters. Quotes such as " are often removed by bash due to shell magic. In contrast, python does not require them for magic, and will thus replicate them the way you used them. Why would you use them if you wouldn't need them? (Well, d'uh, because bash makes you believe you need them).
For example, in bash I can do
$ dd of="foobar"
and it will write to a file named foobar, swallowing the quotes.
In python, if I do
subprocess.check_output(["dd", 'of="barfoo"', 'if=foobar'])
it will write to a file named "barfoo", keeping the quotes.

Redirecting CGI error output from STDERR to a file (python AND perl)

I'm moving a website to Hostmonster and asked where the server log is located so I can automatically scan it for CGI errors. I was told, "We're sorry, but we do not have cgi errors go to any files that you have access to."
For organizational reasons I'm stuck with Hostmonster and this awful policy, so as a workaround I thought maybe I'd modify the CGI scripts to redirect STDERR to a custom log file.
I have a lot of scripts (269) so I need an easy way in both Python and Perl to redirect STDERR to a custom log file.
Something that accounts for file locking either explicitly or implicitly would be great, since a shared CGI error log file could theoretically be written to by more than one script at once if more than one script fails at the same time.
(I want to use a shared error log so I can email its contents to myself nightly and then archive or delete it.)
I know I may have to modify each file (grrr), that's why I'm looking for something elegant that will only be a few lines of code. Thanks.

For Perl, just close and re-open STDERR to point to a file of your choice.
close STDERR;
open STDERR, '>>', '/path/to/your/log.txt'
or die "Couldn't redirect STDERR: $!";
warn "this will go to log.txt";
Alternatively, you could look into a filehandle multiplexer like File::Tee.

Python: cgitb. At the top of your script, before other imports:
import cgitb
cgitb.enable(False, '/home/me/www/myapp/logs/errors')
(‘errors’ being a directory the web server user has write-access to.)

In Perl try CGI::Carp
BEGIN {
use CGI::Carp qw(carpout);
use diagnostics;
open(LOG, ">errors.txt");
carpout(LOG);
close(LOG);
}
use CGI::Carp qw(fatalsToBrowser);

The solution I finally went with was similar to the following, near the top of all my scripts:
Perl:
open(STDERR,">>","/path/to/my/cgi-error.log")
or die "Could not redirect STDERR: $OS_ERROR";
Python:
sys.stderr = open("/path/to/my/cgi-error.log", "a")
Apparently in Perl you don't need to close the STDERR handle before reopening it.
Normally I would close it anyway as a best practice, but as I said in the question, I have 269 scripts and I'm trying to minimize the changes. (Plus it seems more Perlish to just re-open the open filehandle, as awful as that sounds.)
In case anyone else has something similar in the future, here's what I'm going to do for updating all my scripts at once:
Perl:
find . -type f -name "*.pl" -exec perl -pi.bak -e 's%/usr/bin/perl%/usr/bin/perl\nopen(STDERR,">>","/path/to/my/cgi-error.log")\n or die "Could not redirect STDERR: \$OS_ERROR";%' {} \;
Python:
find . -type f -name "*.py" -exec perl -pi.bak -e 's%^(import os, sys.*)%$1\nsys.stderr = open("/path/to/my/cgi-error.log", "a")%' {} \;
The reason I'm posting these commands is that it took me quite a lot of syntactical massaging to get those commands to work (e.g., changing Couldn't to Could not, changing #!/usr/bin/perl to just /usr/bin/perl so the shell wouldn't interpret ! as a history character, using $OS_ERROR instead of $!, etc.)
Thanks to everyone who commented. Since no one answered for both Perl and Python I couldn't really "accept" any of the given answers, but I did give votes to the ones which led me in the right direction. Thanks again!

python:
import sys
sys.stderr = open('file_path_with_write_permission/filename', 'a')

Python has the sys.stderr module that you might want to look into.
>>>help(sys.__stderr__.read)
Help on built-in function read:
read(...)
read([size]) -> read at most size bytes, returned as a string.
If the size argument is negative or omitted, read until EOF is reached.
Notice that when in non-blocking mode, less data than what was requested
may be returned, even if no size parameter was given.
You can store the output of this in a string and write that string to a file.
Hope this helps

In my Perl CGI programs, I usually have
BEGIN {
open(STDERR,'>>','stderr.log');
}
right after shebang line and "use strict;use warnings;". If you want, you may append $0 to file name. But this will not solve multiple programs problem, as several copies of one programs may be run simultaneously. I usually just have several output files, for every program group.

Passing a multi-line string as an argument to a script in Windows

I have a simple python script like so:
import sys
lines = sys.argv[1]
for line in lines.splitlines():
print line
I want to call it from the command line (or a .bat file) but the first argument may (and probably will) be a string with multiple lines in it. How does one do this?
Of course, this works:
import sys
lines = """This is a string
It has multiple lines
there are three total"""
for line in lines.splitlines():
print line
But I need to be able to process an argument line-by-line.
EDIT: This is probably more of a Windows command-line problem than a Python problem.
EDIT 2: Thanks for all of the good suggestions. It doesn't look like it's possible. I can't use another shell because I'm actually trying to invoke the script from another program which seems to use the Windows command-line behind the scenes.

I know this thread is pretty old, but I came across it while trying to solve a similar problem, and others might as well, so let me show you how I solved it.
This works at least on Windows XP Pro, with Zack's code in a file called
"C:\Scratch\test.py":
C:\Scratch>test.py "This is a string"^
More?
More? "It has multiple lines"^
More?
More? "There are three total"
This is a string
It has multiple lines
There are three total
C:\Scratch>
This is a little more readable than Romulo's solution above.

Just enclose the argument in quotes:
$ python args.py "This is a string
> It has multiple lines
> there are three total"
This is a string
It has multiple lines
there are three total

The following might work:
C:\> python something.py "This is a string^
More?
More? It has multiple lines^
More?
More? There are three total"

This is the only thing which worked for me:
C:\> python a.py This" "is" "a" "string^
More?
More? It" "has" "multiple" "lines^
More?
More? There" "are" "three" "total
For me Johannes' solution invokes the python interpreter at the end of the first line, so I don't have the chance to pass additional lines.
But you said you are calling the python script from another process, not from the command line. Then why don't you use dbr' solution? This worked for me as a Ruby script:
puts `python a.py "This is a string\nIt has multiple lines\nThere are three total"`
And in what language are you writing the program which calls the python script? The issue you have is with argument passing, not with the windows shell, not with Python...
Finally, as mattkemp said, I also suggest you use the standard input to read your multi-line argument, avoiding command line magic.

Not sure about the Windows command-line, but would the following work?
> python myscript.py "This is a string\nIt has multiple lines\there are three total"
..or..
> python myscript.py "This is a string\
It has [...]\
there are [...]"
If not, I would suggest installing Cygwin and using a sane shell!

Have you tried setting you multiline text as a variable and then passing the expansion of that into your script. For example:
set Text="This is a string
It has multiple lines
there are three total"
python args.py %Text%
Alternatively, instead of reading an argument you could read from standard in.
import sys
for line in iter(sys.stdin.readline, ''):
print line
On Linux you would pipe the multiline text to the standard input of args.py.
$ <command-that-produces-text> | python args.py

Python subprocess issue with ampersands

I'm currently having a major issue with a python script. The script runs arbitrary commands through a handler to convert incorrect error reporting into correct error reporting.
The issue I'm having is getting the script to work correctly on windows with a command that contains ampersands in it's path. I've attempted quoting the command, escaping the ampersand with ^ and neither works. I'm now out of ideas. Any suggestions?
To clarify from current responses:
I am using the subprocess module
I am passing the command line + arguments in as a list
The issue is with the path to the command itself, not any of the arguments
I've tried quoting the command. It causes a [Error 123] The filename, directory name, or volume label syntax is incorrect error
I'm using no shell argument (so shell=false)
In case it matters, I'm grabbing a pipe to stderr for processing it, but ignoring stdout and stdin
It is only for use on Windows currently, and works as expected in all other cases that I've tested so far.
The command that is failing is:
p = subprocess.Popen(prog, stderr = subprocess.PIPE, bufsize=-1)
when the first element of the list 'prog' contains any ampersands. Quoting this first string does not work.

Make sure you are using lists and no shell expansion:
subprocess.Popen(['command', 'argument1', 'argument2'], shell=False)

Try quoting the argument that contains the &
wget "http://foo.com/?bar=baz&baz=bar"
Is usually what has to be done in a Linux shell

To answer my own question:
Quoting the actual command when passing the parameters as a list doesn't work correctly (command is first item of list) so to solve the issue I turned the list into a space separated string and passed that into subprocess instead.
Better solutions still welcomed.

"escaping the ampersand with ^"
Are you sure ^ is an escape character to Windows? Shouldn't you use \?

I try a situation as following:
exe = 'C:/Program Files (x86)/VideoLAN/VLC/VLC.exe'
url = 'http://translate.google.com/translate_tts?tl=en&q=hello+world'
subprocess.Popen([exe, url.replace("&","^&")],shell=True)
This does work.