Execute Shell Script from Python with multiple pipes

Question

I want to execute the following Shell Command in a python script:

dom=myserver    
cat /etc/xen/$myserver.cfg | grep limited | cut -d= -f2 | tr -d \"

I have this:

dom = myserver

limit = subprocess.call(["cat /etc/xen/%s.cfg | grep limited | cut -d= -f2", str(dom)])
subprocess.call(['/root/bin/xen-limit', str(dom), str(limit)])

It does not work, but I don't know why..

Update:

c1 = ['cat /etc/xen/%s.cfg']
p1 = subprocess.Popen(c1, stdout=subprocess.PIPE)

c2 = ['grep limited']
p2 = subprocess.Popen(c2, stdin=p1.stdout, stdout=subprocess.PIPE)

c3 = ['cut -d= -f2']
p3 = subprocess.Popen(c3, stdin=p2.stdout, stdout=subprocess.PIPE)

c4 = ['tr -d \"']
p4 = subprocess.Popen(c4, stdin=p3.stdout, stdout=subprocess.PIPE)

result = p4.stdout.read()

limit = subprocess.call([result])
subprocess.call(['/root/bin/xen-limit', str(dom), str(limit)])

The whole pipeline could be replaced with the single command awk -F= '/limited/ {print gsub("\"", "", $2)}' /etc/xen/$myserver.cfg (with the necessary replacement for $myserver applied). — chepner
– chepner, Commented Oct 11, 2013 at 14:11

Aidan Kane · Accepted Answer · 2013-10-11 13:06:18Z

7

You can glue several subprocesses together:

c1 = ['ls']
p1 = subprocess.Popen(c1, stdout=subprocess.PIPE)

c2 = ['wc']
p2 = subprocess.Popen(c2, stdin=p1.stdout,
                      stdout=subprocess.PIPE)

result = p2.stdout.read()

Notice how we've set the stdin of p2 to be the stdout of p1.

EDIT: simplified the example

answered Oct 11, 2013 at 13:06

Aidan Kane

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1 Comment

Vince Over a year ago

Thanks for you answer! I have edited my post above. Something like that?

Vince · Accepted Answer · 2013-10-11 14:05:00Z

1

Made it! :D Thanks

dom = myserver    
c1 = ['/bin/cat', '/etc/xen/%s.cfg' % (str(dom))]
p1 = subprocess.Popen(c1, stdout=subprocess.PIPE)

c2 = ['grep', 'limited']
p2 = subprocess.Popen(c2, stdin=p1.stdout,
                  stdout=subprocess.PIPE)

c3 = ['cut', '-d=', '-f2']
p3 = subprocess.Popen(c3, stdin=p2.stdout,
                  stdout=subprocess.PIPE)

c4 = ['tr', '-d', '\"']
p4 = subprocess.Popen(c4, stdin=p3.stdout,
                  stdout=subprocess.PIPE)

result = p4.stdout.read()
subprocess.call(['/root/bin/xen-limit', str(dom), str(result)])

answered Oct 11, 2013 at 14:05

Vince

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