Question

I have a command that looks like this:

ps v -p 2585 | awk '{if ($9 != "%MEM") {print $9}}'

Now this runs fine in bash, it just takes the memory portion of whatever pid you give it. However I am now trying to implement it in python but I am having some issues. Here is what I have in python

cmd1 = ['ps', 'v', '-p', pid]
cmd2 = ['awk', '\'{if ($9 != "%MEM") {print $9}}\'']

Now to run them…

runcmd1 = subprocess.Popen(cmd1, stdout=subprocess.PIPE)
runcmd2 = subprocess.Popen(cmd2, stdin=runcmd1.stdout, stdout=subprocess.PIPE)

I get this error:

awk: '{if (\$9 != "%MEM") {print \$9}}'
awk: ^ invalid char ''' in expression

I used this to print out what the command looked like together…
sys.stdout.write(‘ ‘.join(cmd1)+’ ‘+’|’+’ ‘+’ ‘.join(cmd2)+’\n’)

And it gave:

ps v -p 1073 | awk '{if ($9 != "%MEM") {print $9}}'

I see no difference between this and the actual bash command that works. Can anyone help?

Answer 1

You don’t need to protect the awk commands from the shell when you’re running it via popen (the arguments are already split into a list, so your whitespace is left alone).

cmd2 = ['awk', '{if ($9 != "%MEM") {print $9}}']

will work fine.

---

Note for future reference

Python has some nice ways of writing strings that avoid escaping like you tried to do here, for situations where you do need it:

'''In this string, I don't need to escape a single ' character,
   or even a new-line, because the string only ends
   when it gets three ' characters in a row like this:'''

"""The same is true of double-quotes like this.
Of course, whitespace and both the ' and " quote characters
are safe in here."""