How to Store the Result of an Executed Shell Command in a Variable in Python

How to store the result of an executed shell command in a variable in python?

Use the subprocess module instead:

import subprocess
output = subprocess.check_output("cat syscall_list.txt | grep f89e7000 | awk '{print $2}'", shell=True)

Edit: this is new in Python 2.7. In earlier versions this should work (with the command rewritten as shown below):

import subprocess
output = subprocess.Popen(['awk', '/f89e7000/ {print $2}', 'syscall_list.txt'], stdout=subprocess.PIPE).communicate()[0]

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As a side note, you can rewrite

cat syscall_list.txt | grep f89e7000

To

grep f89e7000 syscall_list.txt

And you can even replace the entire statement with a single awk script:

awk '/f89e7000/ {print $2}' syscall_list.txt

Leading to:

import subprocess
output = subprocess.check_output(['awk', '/f89e7000/ {print $2}', 'syscall_list.txt'])

How would I store the shell command output to a variable in python?

By using module subprocess. It is included in Python's standard library and aims to be the substitute of os.system. (Note that the parameter capture_output of subprocess.run was introduced in Python 3.7)

>>> import subprocess
>>> subprocess.run(['cat', '/etc/hostname'], capture_output=True)
CompletedProcess(args=['cat', '/etc/hostname'], returncode=0, stdout='example.com\n', stderr=b'')
>>> subprocess.run(['cat', '/etc/hostname'], capture_output=True).stdout.decode()
'example.com\n'

In your case, just:

import subprocess

v = subprocess.run(['cat', '/etc/redhat-release'], capture_output=True).stdout.decode()

Update: you can split the shell command easily with shlex.split provided by the standard library.

>>> import shlex
>>> shlex.split('cat /etc/redhat-release')
['cat', '/etc/redhat-release']
>>> subprocess.run(shlex.split('cat /etc/hostname'), capture_output=True).stdout.decode()
'example.com\n'

Update 2: os.popen mentioned by @Matthias

However, is is impossible for this function to separate stdout and stderr.

import os

v = os.popen('cat /etc/redhat-release').read()

Running shell command and capturing the output

In all officially maintained versions of Python, the simplest approach is to use the subprocess.check_output function:

>>> subprocess.check_output(['ls', '-l'])
b'total 0\n-rw-r--r--  1 memyself  staff  0 Mar 14 11:04 files\n'

check_output runs a single program that takes only arguments as input.¹ It returns the result exactly as printed to stdout. If you need to write input to stdin, skip ahead to the run or Popen sections. If you want to execute complex shell commands, see the note on shell=True at the end of this answer.

The check_output function works in all officially maintained versions of Python. But for more recent versions, a more flexible approach is available.

Modern versions of Python (3.5 or higher): run

If you're using Python 3.5+, and do not need backwards compatibility, the new run function is recommended by the official documentation for most tasks. It provides a very general, high-level API for the subprocess module. To capture the output of a program, pass the subprocess.PIPE flag to the stdout keyword argument. Then access the stdout attribute of the returned CompletedProcess object:

>>> import subprocess
>>> result = subprocess.run(['ls', '-l'], stdout=subprocess.PIPE)
>>> result.stdout
b'total 0\n-rw-r--r--  1 memyself  staff  0 Mar 14 11:04 files\n'

The return value is a bytes object, so if you want a proper string, you'll need to decode it. Assuming the called process returns a UTF-8-encoded string:

>>> result.stdout.decode('utf-8')
'total 0\n-rw-r--r--  1 memyself  staff  0 Mar 14 11:04 files\n'

This can all be compressed to a one-liner if desired:

>>> subprocess.run(['ls', '-l'], stdout=subprocess.PIPE).stdout.decode('utf-8')
'total 0\n-rw-r--r--  1 memyself  staff  0 Mar 14 11:04 files\n'

If you want to pass input to the process's stdin, you can pass a bytes object to the input keyword argument:

>>> cmd = ['awk', 'length($0) > 5']
>>> ip = 'foo\nfoofoo\n'.encode('utf-8')
>>> result = subprocess.run(cmd, stdout=subprocess.PIPE, input=ip)
>>> result.stdout.decode('utf-8')
'foofoo\n'

You can capture errors by passing stderr=subprocess.PIPE (capture to result.stderr) or stderr=subprocess.STDOUT (capture to result.stdout along with regular output). If you want run to throw an exception when the process returns a nonzero exit code, you can pass check=True. (Or you can check the returncode attribute of result above.) When security is not a concern, you can also run more complex shell commands by passing shell=True as described at the end of this answer.

Later versions of Python streamline the above further. In Python 3.7+, the above one-liner can be spelled like this:

>>> subprocess.run(['ls', '-l'], capture_output=True, text=True).stdout
'total 0\n-rw-r--r--  1 memyself  staff  0 Mar 14 11:04 files\n'

Using run this way adds just a bit of complexity, compared to the old way of doing things. But now you can do almost anything you need to do with the run function alone.

Older versions of Python (3-3.4): more about check_output

If you are using an older version of Python, or need modest backwards compatibility, you can use the check_output function as briefly described above. It has been available since Python 2.7.

subprocess.check_output(*popenargs, **kwargs)  

It takes takes the same arguments as Popen (see below), and returns a string containing the program's output. The beginning of this answer has a more detailed usage example. In Python 3.5+, check_output is equivalent to executing run with check=True and stdout=PIPE, and returning just the stdout attribute.

You can pass stderr=subprocess.STDOUT to ensure that error messages are included in the returned output. When security is not a concern, you can also run more complex shell commands by passing shell=True as described at the end of this answer.

If you need to pipe from stderr or pass input to the process, check_output won't be up to the task. See the Popen examples below in that case.

Complex applications and legacy versions of Python (2.6 and below): Popen

If you need deep backwards compatibility, or if you need more sophisticated functionality than check_output or run provide, you'll have to work directly with Popen objects, which encapsulate the low-level API for subprocesses.

The Popen constructor accepts either a single command without arguments, or a list containing a command as its first item, followed by any number of arguments, each as a separate item in the list. shlex.split can help parse strings into appropriately formatted lists. Popen objects also accept a host of different arguments for process IO management and low-level configuration.

To send input and capture output, communicate is almost always the preferred method. As in:

output = subprocess.Popen(["mycmd", "myarg"], 
                          stdout=subprocess.PIPE).communicate()[0]

Or

>>> import subprocess
>>> p = subprocess.Popen(['ls', '-a'], stdout=subprocess.PIPE, 
...                                    stderr=subprocess.PIPE)
>>> out, err = p.communicate()
>>> print out
.
..
foo

If you set stdin=PIPE, communicate also allows you to pass data to the process via stdin:

>>> cmd = ['awk', 'length($0) > 5']
>>> p = subprocess.Popen(cmd, stdout=subprocess.PIPE,
...                           stderr=subprocess.PIPE,
...                           stdin=subprocess.PIPE)
>>> out, err = p.communicate('foo\nfoofoo\n')
>>> print out
foofoo

Note Aaron Hall's answer, which indicates that on some systems, you may need to set stdout, stderr, and stdin all to PIPE (or DEVNULL) to get communicate to work at all.

In some rare cases, you may need complex, real-time output capturing. Vartec's answer suggests a way forward, but methods other than communicate are prone to deadlocks if not used carefully.

As with all the above functions, when security is not a concern, you can run more complex shell commands by passing shell=True.

Notes

1. Running shell commands: the shell=True argument

Normally, each call to run, check_output, or the Popen constructor executes a single program. That means no fancy bash-style pipes. If you want to run complex shell commands, you can pass shell=True, which all three functions support. For example:

>>> subprocess.check_output('cat books/* | wc', shell=True, text=True)
' 1299377 17005208 101299376\n'

However, doing this raises security concerns. If you're doing anything more than light scripting, you might be better off calling each process separately, and passing the output from each as an input to the next, via

run(cmd, [stdout=etc...], input=other_output)

Or

Popen(cmd, [stdout=etc...]).communicate(other_output)

The temptation to directly connect pipes is strong; resist it. Otherwise, you'll likely see deadlocks or have to do hacky things like this.

Get output of shell script and store to variable

You can do this with the subprocess module in the Python Standard Library.

import subprocess

command = "unzip -p'" +filepath+"' docProps/app.xml | grep -oP '(?<=\<Pages\>).*(?=\</Pages\>)'"
completed_process = subprocess.run(command, shell=True, text=True, capture_output=True)
output = completed_process.stdout

Please note that if you use shell=True in subprocess.run() and the filepath input is not trusted (e.g. user-provided), you have a security problem, i.e., arbitrary code execution. The same goes for os.system().
See the official documentation for a possible solution using shlex.quote().

Python executing commands and storing result in a variable

Use subprocess.check_output to capture the output.

from subprocess import check_output
t = check_output(['ls', '-a'])
print t

Note that this will raise a CalledProcessError exception if ls were to return a non-zero exit-code.

As for the other part of your question, the subprocess module is the preferred (and best, IMO) way to run sub-processes in Python. You should favor it over os.popen.

python command saved in a variable

You can use the eval function:

command = 'print("Hello")'
eval(command)

How to store executed command (of cmd) into a variable?

Try (for Python3.7+):

import subprocess
data = subprocess.run(["tree", "D://"], capture_output=True)

For Python<3.7:

import subprocess
data = subprocess.run(["tree", "D://"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)

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