Develop Reference

How to run advance Linux command on python script

I want to get the string output of the following linux command
systemctl show node_exporter |grep LoadState| awk '{split($0,a,"="); print a[2]}'
I tried with
import subprocess
output = subprocess.check_output("systemctl show node_exporter |grep LoadState| awk '{split($0,a,"="); print a[2]}'", shell=True)
but the output is,
output = subprocess.check_output("systemctl show node_exporter |grep LoadState| awk '{split($0,a,"="); print a[2]}'", shell=True)
SyntaxError: keyword can't be an expression

Well,
First of all, the function takes a list of strings as a command, not a single string. E.g.:
"ls -a -l" - wrong
["ls", "-a", "-l"] - good
Secondly. If the linux command is super complex or contains lots of lines - it makes sense to create a separate bash file e.g. command.sh, put your linux commands there and run the script from python with:
import subprocess
output = subprocess.check_output(["./command.sh"], shell=True)

You need to escape the double quotes (because they indicate the begin/end of the string):
import subprocess
output = subprocess.check_output("systemctl show node_exporter |grep LoadState| awk '{split($0,a,\"=\"); print a[2]}'", shell=True)

Escaping a sed expression to execute over ssh from Python

I have a file on a remote server that needs to have a line updated in it. I am trying to do the update via python but appear to be having a character escape issue.
The line in the file I am trying to update is:
BEGRCVDDATE=02/01/2018 00:00 am
The line of code in python script I am using to try and make the update:
os.popen('ssh %s sed -i s/'BEGRCVDDATE=[0-9][0-9]\/[0-9][0-9]\/[0-9][0-9][0-9][0-9] [0-9][0-9]:[0-9][0-9] [a-z][a-z]'/'BEGRCVDDATE=%s'/ %s' % (ip, rcvdate, file_path))
The above code is throwing the following error:
sed: -e expression #1, char 37: unknown option to `s'
I am however able to run the sed command outside of the python script which leads me to believe this is a character escaping issue. Below works outside of the python script.
ssh <ip> "sed -i s/'BEGRCVDDATE=[0-9][0-9]\/[0-9][0-9]\/[0-9][0-9][0-9][0-9] [0-9][0-9]:[0-9][0-9] [a-z][a-z]'/'BEGRCVDDATE=BEGRCVDDATE=03\\/08\\/2018 00:00 pm'/ /tmp/test.txt"
I have tried various combinations of quotes and back-slashes to try and get around the issue I am seeing without success.
If anyone can help me resolve the issue I am seeing it would be much appreciated.
Note: due to python version in my environment os.popen is being using instead of subprocess.

import subprocess, pipes
# Generate your list of arguments *as a list of Python strings*
rcvdate = '02/01/2018 00:00 am'
cmd=['sed', '-i',
's#BEGRCVDDATE=[0-9][0-9]/[0-9][0-9]/[0-9][0-9][0-9][0-9] [0-9][0-9]:[0-9][0-9] [a-z][a-z]#BEGRCVDDATE=%s#' % (rcvdate,),
file_path,
]
# Ask Python itself to correctly form a shell command from that list
cmd_str = ' '.join([pipes.quote(s) for s in cmd])
# Pass that shell command as an argument to `ssh`.
subprocess.call(['ssh', ip, cmd_str])
...or, if you truly don't have the subprocess module, you'll need a second round of shell escaping:
ssh_cmd = ['ssh', ip, cmd_str]
ssh_cmd_str = ' '.join([pipes.quote(s) for s in ssh_cmd])
os.system(ssh_cmd_str)
Note that I changed your sed expression to use #, not /, as a sigil. This means that literal backslashes are no longer needed.

Quotes without backslash in subprocess command line in python

I'm trying to use ffmpeg from python. The command I need to execute is:
ffmpeg -i test_file-1kB.mp4 -i test_file.mp4 -filter_complex psnr="stats_file=test_file.mp4-1kB.psnr" -f null -
However, my output that is getting passed to subprocess looks like it is escaping the double quotes with backslashes like so:
In[1]: print(subprocess.list2cmdline(psnr_args))
ffmpeg -i test_file-1kB.mp4 -i test_file.mp4 -filter_complex psnr=\"stats_file=test_file.mp4-1kB.psnr\" -f null -
To use subprocess, I build my command line arguments one at a time into a list and then pass the list to subprocess.
psnr_args = []
psnr_args.append("ffmpeg")
#add first input, the encoded video
psnr_args.append("-i")
psnr_args.append(full_output_file_name)
#add second input, the original video
psnr_args.append("-i")
psnr_args.append(video_file)
#Setup the psnr log file
psnr_args.append("-filter_complex")
psnr_args.append('psnr="stats_file=%s.psnr"' % vstats_abs_filename )
#Output the video to null
psnr_args.append("-f")
psnr_args.append("null")
psnr_args.append("-")
print(subprocess.list2cmdline(psnr_args))
run_info_psnr = subprocess.run(psnr_args, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

After more fiddling, I found a solution that works in this case but may not work in all cases. If I use double quotes as the outer quotes and the single quotes as the inner quotes, the output to subprocess uses a single quote at the same location with no backslash. This is acceptable for ffmpeg. However, for others where double quotes are the only solution, it won't be a fix.
psnr_args.append("psnr='stats_file=%s.psnr'" % vstats_abs_filename )
Output to subprocess looks like this:
In[1]: print(subprocess.list2cmdline(psnr_args))
ffmpeg -i test_file-1kB.mp4 -i test_file.mp4 -filter_complex psnr='stats_file=test_file.mp4-1kB.psnr' -f null -

In shell, the argument:
psnr="stats_file=test_file.mp4-1kB.psnr"
Is absolutely identical to:
psnr=stats_file=test_file.mp4-1kB.psnr
The quotes are removed during the shell's own processing. They are not part of the command passed to ffmpeg, which doesn't expect or understand them. Because you're directly telling the Python subprocess module to invoke a literal argument vector, there's no shell involved, so shell syntax shouldn't be present.

This has something to do with ffmpeg AV filter chain syntax too. You need to run the command like xxxx -filter_complex "psnr='stats.txt'" xxxx. To get this, you should ensure the double quote that encapsulate the filter chain reaches inside. subproces expects a flat list as the first argument, where the command is the first entry. So ['ffmpeg', '-i', "t1.mp4", "-filter_compelx", '"psnr=\'stats.txt\'"', .... and so on ].

python subprocess sends backslash before a quote

I have a string, which is a framed command that should be executed by in command line
cmdToExecute = "TRAPTOOL -a STRING "ABC" -o STRING 'XYZ'"
I am considering the string to have the entire command that should be triggered from command prompt. If you take a closer look at the string cmdToExecute, you can see the option o with value XYZ enclosed in SINGLE QUOTE. There is a reason that this needs to be given in single quote orelse my tool TRAPTOOL will not be able to process the command.
I am using subprocess.Popen to execute the entire command. Before executing my command in a shell, I am printing the content
print "Cmd to be exectued: %r" % cmdToExecute
myProcess = subprocess.Popen(cmdToExecute, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=False)
(stdOut, stdErr) = myProcess.communicate()
The output of the above command is,
Cmd to be executed: TRAPTOOL -a STRING "ABC" -o \'XYZ\'.
You can see that the output shows a BACKWARD SLASH added automatically while printing. Actually, the \ is not there in the string, which I tested using a regex. But, when the script is run on my box, the TRAPTOOL truncates the part of the string XYZ on the receiving server. I manually copy pasted the print output and tried sending it, I saw the same error on the receiving server. However, when I removed the backward slash, it sent the trap without any truncation.
Can anyone say why this happens?
Is there anyway where we can see what command is actually executed in subprocess.Popen?
Is there any other way I can execute my command other that subprocess.Popen that might solve this problem?

Try using shlex to split your command string:
>>> import shlex
>>> argv = shlex.split("TRAPTOOL -a STRING \"ABC\" -o STRING 'XYZ'")
>>> myProcess = subprocess.Popen(argv, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=False)
>>> (stdOut, stdErr) = myProcess.communicate()
The first parameter to the Popen constructor can be an argument list for your shell command or a string, but an argument list might be easier to work with because of all the quotes involved. (See the Python subprocess documentation.)
If you want to see the commands being written, you could probably do something like:
>>> argv = shlex.split("bash -x -c 'TRAPTOOL -a STRING \"ABC\" -o STRING \'XYZ\''")
This makes bash echo the commands to the shell by means of the -x option.

You asked for the repr representation of the string, not the str representation. Basically, what would you have to type at the Python interactive interpreter to get the same output? That's what %r displays. Change that to %s to see the value as it's actually stored:
print "Cmd to be exectued: %s" % cmdToExecute

Passing double quote shell commands in python to subprocess.Popen()?

I've been trying to pass a command that works only with literal double quotes in the commandline around the "concat:file1|file2" argument for ffmpeg.
I cant however make this work from python with subprocess.Popen(). Anyone have an idea how one passes quotes into subprocess.Popen?
Here is the code:
command = "ffmpeg -i "concat:1.ts|2.ts" -vcodec copy -acodec copy temp.mp4"
output,error = subprocess.Popen(command, universal_newlines=True,
stdout=subprocess.PIPE,
stderr=subprocess.PIPE).communicate()
When I do this, ffmpeg won't take it any other way other than quotes around the concat segement. Is there a way to successfully pass this line to subprocess.Popen command?

I'd suggest using the list form of invocation rather than the quoted string version:
command = ["ffmpeg", "-i", "concat:1.ts|2.ts", "-vcodec", "copy",
"-acodec", "copy", "temp.mp4"]
output,error = subprocess.Popen(
command, universal_newlines=True,
stdout=subprocess.PIPE, stderr=subprocess.PIPE).communicate()
This more accurately represents the exact set of parameters that are going to be passed to the end process and eliminates the need to mess around with shell quoting.
That said, if you absolutely want to use the plain string version, just use different quotes (and shell=True):
command = 'ffmpeg -i "concat:1.ts|2.ts" -vcodec copy -acodec copy temp.mp4'
output,error = subprocess.Popen(
command, universal_newlines=True, shell=True,
stdout=subprocess.PIPE, stderr=subprocess.PIPE).communicate()

Either use single quotes 'around the "whole pattern"' to automatically escape the doubles or explicitly "escape the \"double quotes\"". Your problem has nothing to do with Popen as such.
Just for the record, I had a problem particularly with a list-based command passed to Popen that would not preserve proper double quotes around a glob pattern (i.e. what was suggested in the accepted answer) under Windows. Joining the list into a string with ' '.join(cmd) before passing it to Popen solved the problem.

This works with python 2.7.3 The command to pipe stderr to stdout has changed since older versions of python:
Put this in a file called test.py:
#!/usr/bin/python
import subprocess
command = 'php -r "echo gethostname();"'
p = subprocess.Popen(command, universal_newlines=True, shell=True,
stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
text = p.stdout.read()
retcode = p.wait()
print text
Invoke it:
python test.py
It prints my hostname, which is apollo:
apollo
Read up on the manual for subprocess: http://docs.python.org/2/library/subprocess.html

I have been working with a similar issue, with running a relatively complex
command over ssh. It also had multiple double quotes and single quotes. Because
I was piping the command through python, ssh, powershell etc.
If you can instead just convert the command into a shell script, and run the
shell script through subprocess.call/Popen/run, these issues will go away.
So depending on whether you are on windows or on linux or mac, put the
following in a shell script either (script.sh or script.bat)
ffmpeg -i "concat:1.ts|2.ts" -vcodec copy -acodec copy temp.mp4
Then you can run
import subprocess; subprocess.call(`./script.sh`; shell=True)
Without having to worry about single quotes, etc.

This line of code in your question isn't valid Python syntax:
command = "ffmpeg -i "concat:1.ts|2.ts" -vcodec copy -acodec copy temp.mp4"
If you had a Python file with just this line in it, you would get a syntax error. A string literal surrounded with double quotes can't have double quotes in them unless they are escaped with a backslash. So you could fix that line by replacing it with:
command = "ffmpeg -i \"concat:1.ts|2.ts\" -vcodec copy -acodec copy temp.mp4"
Another way to fix this line is to use single quotes for the string literal in Python, that way Python is not confused when the string itself contains a double quote:
command = 'ffmpeg -i "concat:1.ts|2.ts" -vcodec copy -acodec copy temp.mp4'
Once you have fixed the syntax error, you can then tackle the issue with using subprocess, as explained in this answer. I also wrote this answer to explain a helpful mental model for subprocess in general.

Also struggling with a string argument containing spaces and not wanting to use the shell=True.
The solution was to use double quotes for the inside strings.
args = ['salt', '-G', 'environment:DEV', 'grains.setvals', '{"man_version": "man-dev-2.3"}']
try:
p = subprocess.Popen(args, stdin=subprocess.PIPE
, stdout=subprocess.PIPE
, stderr=subprocess.PIPE
)
(stdin,stderr) = p.communicate()
except (subprocess.CalledProcessError, OSError ) as err:
exit(1)
if p.returncode != 0:
print("Failure in returncode of command:")

Anybody suffering from this pain. It also works with params enclosed with quotation marks.
params = ["ls", "-la"]
subprocess.check_output(" ".join(params), shell=True)