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Capturing or printing variables in bashrc with shell=True in run command of subprocess module

I am learning concurrency with Python 3 Standard Library, 2nd Edition. Is there a way to get the subprocess module to use variables in my .bashrc when I set shell=True?
I tried adding echo "something" in my .bashrc and ran the following but I couldn't see something in the output but I could see $HOME.
import subprocess
completed = subprocess.run('echo $HOME', shell=True)
print('returncode:', completed.returncode)

There are a few ways to go about this. But first, you need to execute Bash. Right now, you're executing /bin/sh, which could be Bash, but you don't want to assume. To fix that, pass in executable argument:
import subprocess
completed = subprocess.run('echo $HOME',
shell=True,
executable='/usr/bin/bash'
)
print('returncode:', completed.returncode)
Now we have that out of the way, there are two ways to ensure your .bashrc is sourced. Since Bash is not running interactively, you need to either,
(A) source it yourself:
import subprocess
completed = subprocess.run('source ~/.bashrc && echo "$HOME"',
shell=True,
executable='/usr/bin/bash'
)
print('returncode:', completed.returncode)
or,
(B) Use BASH_ENV:
import subprocess
completed = subprocess.run('echo "$HOME"',
env={'BASH_ENV': '~/.bashrc'},
shell=True,
executable='/usr/bin/bash'
)
print('returncode:', completed.returncode)
BASH_ENV is a variable used by Bash. If Bash sees the variable, and after expanding the variable (i.e. tilda expansion), the path is to a file that exists, it sources the variable before executing the rest of the script.

Problems running terminal command via Python

I'm working on a small project where I need to control a console player via python. This example command works perfectly on the Linux terminal:
mplayer -loop 0 -playlist <(find "/mnt/music/soundtrack" -type f | egrep -i '(\.mp3|\.wav|\.flac|\.ogg|\.avi|\.flv|\.mpeg|\.mpg)'| sort)
In Python I'm doing the following:
command = """mplayer -loop 0 -playlist <(find "/mnt/music/soundtrack" -type f | egrep -i '(\.mp3|\.wav|\.flac|\.ogg|\.avi|\.flv|\.mpeg|\.mpg)'| sort)"""
os.system(command)
The problem is when I try it using Python it gives me an error when I run it:
sh: 1: Syntax error: "(" unexpected
I'm really confused here because it is the exact same string. Why doesn't the second method work?
Thanks.

Your default user shell is probably bash. Python's os.system command calls sh by default in linux.
A workaround is to use subprocess.check_call() and pass shell=True as an argument to tell subprocess to execute using your default user shell.
import subprocess
command = """mplayer -loop 0 -playlist <(find "/mnt/music/soundtrack" -type f | egrep -i '(\.mp3|\.wav|\.flac|\.ogg|\.avi|\.flv|\.mpeg|\.mpg)'| sort)"""
subprocess.check_call(command, shell=True)

Your python call 'os.system' is probably just using a different shell than the one you're using on the terminal:
os.system() execute command under which linux shell?
The shell you've spawned with os.system may not support parentheses for substitution.

import subprocess
COMND=subprocess.Popen('command',shell=True,stdin=subprocess.PIPE,stdout=subprocess.PIPE,stderr=subprocess.PIPE)
COMND_bytes = COMND.stdout.read() + COMND.stderr.read()
print(COMND_bytes)

pexpect.run can not run a long command

I am using pexpect.run to execute a command. See below:
cmd = "grep -L killed /dir/dumps/*MAC-66.log"
output = pexpect.run(cmd)
When I run this, output equals to:
grep: /dir/dumps/*MAC-66.log: No such file or directory
But when I run the same command in my shell, it works, everytime. I don't see the problem. Any help is appreciated! Does pexpect.run require the command to be split in some fancy way?

Your shell is interpreting the glob, pexpect is not. You could either use python's glob.glob() function to evaluate the glob yourself, or run it through your shell, for example:
cmd = "bash -c 'grep -L killed /dir/dumps/*MAC-66.log'"
Also, if all you're after is output of this command, you ought to check out the subprocess module.

python 2.3 how to run a piped command

I want to run a command like this
grep -w 1 pattern <(tail -f mylogfile.log)
basically from a python script i want to monitor a log file for a specific string and continue with the python script as soon as i found that.
I am using os.system(), but that is hanging. The same command in bash works good.
I have a very old version of python (v2.3) and so don't have sub-process module.
do we have a way to acheive this

In Python 2.3, you need to use subprocess from SVN
import subprocess
import shlex
subprocess.call(shlex.split("/bin/bash -c 'grep -w 1 pattern <(tail -f mylogfile.log)'"))
To be explicit, you need to install it from the SVN link above.
You need to call this with /bin/bash -c due to the shell redirection you're using
EDIT
If you want to solve this with os.system(), just wrap the command in /bin/bash -c since you're using shell redirection...
os.system("/bin/bash -c 'grep -w 1 pattern <(tail -f mylogfile.log)'")

First of all, the command i think you should be using is grep -w -m 1 'pattern' <(tail -f in)
For executing commands in python, use the Popen constructor from the subprocess module. Read more at
http://docs.python.org/library/subprocess.html

If I understand correctly, you want to send the output to python like this -
tail -f mylogfile.log | grep -w 1 pattern | python yourscript.py
i.e., read all updates to the log file, and send matching lines to your script.
To read from standard input, you can use the file-like object: sys.stdin.
So your script would look like
import sys
for line in sys.stdin.readlines():
#process each line here.

Python Popen grep

I'd like Popen to execute:
grep -i --line-buffered "grave" data/*.txt
When run from the shell, this gives me the wanted result. If I start, in the very same directory where I test grep, a python repl and follow the instruction from the docs, I obtain what should be the proper argument list to feed Popen with:
['grep', '-i', '--line-buffered', 'grave', 'data/*.txt']
The result of p = subprocess.Popen(args) is
grep: data/*.txt: No such file or directory
and if I try p = subprocess.Popen(args, shell=True), I get:
Usage: grep [OPTION]... PATTERN [FILE]...
Try `grep --help' for more information.
Any help on how to perform the wanted process? I'm on MacOS Lion.

If you type * in bash the shell expands it to the files in the given directory before executing the command. Python's Popen does no such thing, so what you're doing when you call Popen like that is telling grep there is a file called *.txt in the data directory, instead of all the .txt files in the data directory. That file doesn't exist and you get the expected error.
To solve this you can tell python to run the command through the shell by passing shell=True to Popen:
subprocess.Popen('grep -i --line-buffered grave data/*.txt', shell=True)
Which gets translated to:
subprocess.Popen(['/bin/sh', '-c', 'grep -i --line-buffered "grave" data/*.txt'])
As explained in the documentation of Popen.
You have to use a string instead of a list here, because you want to execute /bin/sh -c "grep -i --line-buffered "grave" data/*.txt" (N.B. quotes around the command, making it a single argument to sh). If you use a list this command is run: /bin/sh -c grep -i --line-buffered "grave" data/*.txt, which gives you the output of simply running grep.

The problem is that shell makes file globbing for you: data/*.txt
You will need to do it youself, for example, by using glob module.
import glob
cmd_line = ['grep', '-i', '--line-buffered', 'grave'] + glob.glob('data/*.txt')