Python Popen shell script but fail

Question

I want to execute bash command

 '/bin/echo </verbosegc> >> /tmp/jruby.log'

in python using Popen. The code does not raise any exception, but none change is made on the jruby.log after execution. The python code is shown below.

>>> command='/bin/echo </verbosegc> >> '+fullpath >>> command '/bin/echo </verbosegc> >> /tmp/jruby.log' >>process = subprocess.Popen(command.split(), stdout=subprocess.PIPE, stderr=subprocess.PIPE, close_fds=True) >>> output= process.communicate()[0] >>> output '</verbosegc> >> /tmp/jruby.log\n

I also print out the process.pid and then check the pid using ps -ef | grep pid. The result shows that the process pid has been finished.

Using split() this way is a code smell. Don't do it.

Charles Duffy
– Charles Duffy

2015-12-08 18:05:29 +00:00
Commented Dec 8, 2015 at 18:05 — Charles Duffy
– Charles Duffy, Commented Dec 8, 2015 at 18:05

Padraic Cunningham · Accepted Answer · 2015-12-08 18:14:23Z

2

Just use pass file object if you want to append the output to a file, you cannot redirect to a file unless you set shell=True:

command = ['/bin/echo', '</verbosegc>'] with open('/tmp/jruby.log',"a") as f: subprocess.check_call(command, stdout=f,stderr=subprocess.STDOUT)

edited Dec 8, 2015 at 18:14

answered Dec 8, 2015 at 18:08

Padraic Cunningham

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3 Comments

Padraic Cunningham Over a year ago

@CharlesDuffy, that is what I have done in my answer, I meant there is no way to use >> without using shell=True

Charles Duffy Over a year ago

Quite right, and as edited the commentary is no longer misleading; +1.

Padraic Cunningham Over a year ago

@shijiexu, I used check_call as I don't see the logic in using Popen when you want to redirect the output to a file.

Tim Pierce · Accepted Answer · 2015-12-08 18:00:09Z

The first argument to subprocess.Popen is the array ['/bin/echo', '</verbosegc>', '>>', '/tmp/jruby.log']. When the first argument to subprocess.Popen is an array, it does not launch a shell to run the command, and the shell is what's responsible for interpreting >> /tmp/jruby.log to mean "write output to jruby.log".

In order to make the >> redirection work in this command, you'll need to pass command directly to subprocess.Popen() without splitting it into a list. You'll also need to quote the first argument (or else the shell will interpret the "<" and ">" characters in ways you don't want):

command = '/bin/echo "</verbosegc>" >> /tmp/jruby.log' process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, close_fds=True)

This still fails when executing Popen. >>> process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, close_fds=True) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/homes/sxu3/tools/python_install/lib/python2.7/subprocess.py", line 710, in init errread, errwrite) File "/homes/sxu3/tools/python_install/lib/python2.7/subprocess.py", line 1335, in _execute_child raise child_exception OSError: [Errno 2] No such file or directory
You're right: you need to add the shell=True argument to make Popen invoke a shell. I was careless. So: subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, close_fds=True, shell=True)
Arrays are usable both with and without shell=True, though their semantics vary wildly between the cases -- see my answer for an example of the former. :)

Charles Duffy · Accepted Answer · 2015-12-08 18:14:38Z

Consider the following:

command = [ 'printf "%s\n" "$1" >>"$2"', # shell script to execute '', # $0 in shell '</verbosegc>', # $1 '/tmp/jruby.log' ] # $2 subprocess.Popen(command, shell=True)

The first argument is a shell script referring to $1 and $2, which are in turn passed as separate arguments. Keeping data separate from code, rather than trying to substitute the former into the latter, is a precaution against shell injection (think of this as an analog to SQL injection).

Of course, don't actually do anything like this in Python -- the native primitives for file IO are far more appropriate.

beroe · Accepted Answer · 2015-12-09 20:57:15Z

Have you tried without splitting the command and using shell=True? My usual format is:

 process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, shell=True) output = process.stdout.read() # or .readlines()

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