bash commands from within python

Question

I'm looking for the best way to use bash commands from within python. What ways are there? I know of os.system and subprocess.Popen.

I have tried these:

bootfile = os.system("ls -l /jffs2/a.bin | cut -d '/' -f 4")
print bootfile

This returns a.bin as expected but also it retuns 0 afterwards and so prints:

a.bin
0

with bootfile now being set to 0. The next time I print bootfile it just shows up as 0. Which is the exit value I guess, how do i stop this value interfering?

I have also tried:

bootfile = subprocess.Popen("ls -l /jffs2/a.bin | cut -d '/' -f 4")
print bootfile

but it seems to break the script, as in I get nothing returned at all, have I done that right?

Also which of these is better and why? Are there other ways and what is the preferred way?

What is your goal with your bash commands ? I'm sure you could do this in python without calling bash. — qdelettre
– qdelettre, Commented Feb 28, 2013 at 13:08
Have you considered reading the documentation for subprocess.Popen at all? — Cairnarvon
– Cairnarvon, Commented Feb 28, 2013 at 13:09
I wish to return a symlink, this points to the name of the firmware. So in this case I want to return a.bin which will actually point to the name of the actual bin file — Paul
– Paul, Commented Feb 28, 2013 at 13:09
I am reading it now, I was using an example online and it seems it was terribly wrong. — Paul
– Paul, Commented Feb 28, 2013 at 13:10

Diego Herranz · Accepted Answer · 2013-02-28 14:16:28Z

3

Using os.readlink (proposed by @kojiro) and os.path.basename for getting only the namefile:

os.path.basename(os.readlink('/jffs2/a.bin'))

answered Feb 28, 2013 at 14:16

Diego Herranz

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1 Comment

Paul Over a year ago

That's perfect thanks, now I want to know why popen returns none and I am satisfied.

anishsane · Accepted Answer · 2013-03-01 14:16:06Z

2

kojiro's comment about os.readlink is probably what you want. I am explaining what you were trying to implement.

os.system would return you exit status of the command run.

subprocess.Popen will create a pipe, so that you can capture the output of the command run.
Below line will capture output of the command run:

bootfile = subprocess.Popen(["bash","-c","ls -l /jffs2/a.bin | cut -d '/' -f 4"], stdout=subprocess.PIPE).communicate()[0]

More details at http://docs.python.org/library/subprocess.html

edited Mar 1, 2013 at 14:16

answered Feb 28, 2013 at 13:25

anishsane

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3 Comments

user9876 Over a year ago

That subprocess.Popen command won't work. See the subprocess.Popen docs (in particular the "shell" parameter to Popen()).

Paul Over a year ago

thanks that's great, I jsut have to adjust it because instead of returning a.bin, it returns jffs2/a.bin and then none, setting the variable to none then.

Paul Over a year ago

I got it to return the binfile now, however it still returns "none" afterwards. You had changed the cut command for some reason :)

user9876 · Accepted Answer · 2013-03-01 16:21:52Z

1

The right answer, as @kojiro says, is:

os.readlink('/jffs2/a.bin')

But if you really wanted to do this the complicated way, then in Python 2.7:

cmd = "ls -l /jffs2/a.bin | cut -d '/' -f 4"
bootfile = subprocess.check_output(cmd, shell=True)

Or on older Pythons:

cmd = "ls -l /jffs2/a.bin | cut -d '/' -f 4"
p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE)
bootfile = p.communicate()[0]
if p.returncode != 0:
    raise Exception('It failed')

edited Mar 1, 2013 at 16:21

answered Mar 1, 2013 at 11:11

user9876

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3 Comments

Paul Over a year ago

thanks for that, good to know different ways. My problem with popen is bootfile = p.communicate[0] returns both the correct answer and then None after it.

user9876 Over a year ago

I just fixed a typo... try p.communicate()[0]. (p.communicate() returns a tuple, the [0] bit takes the first element from that tuple).

Paul Over a year ago

Hi, I had fixed your typo myself thanks, I should have mentioned that. The thing is, when I am accessing index 0 it is giving me the correct value and none also, then marking the variable as none. Which makes no sense to me. I did it another way but not knowing why bothers me :)

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