[Tutor] SyntaxError: can't assign to literal while using ""blkid -o export %s | grep 'TYPE' | cut -d"=" -f3" % (fs)" using subprocess module in Python

[Steven D'Aprano]

On Wed, Nov 07, 2018 at 10:22:00AM +0100, srinivasan wrote:

> Could you please help me  as it seems to be like grep and cut commands are
> not working in the above line ie., on *cmd = "blkid -o export %s | grep
> \'TYPE\' | cut -d\"=\" -f3" % fs*?

Then get them working first. What happens when you execute that line 
directly on the command line?

But I have to wonder, are you being paid by the hour? Because it seems 
like a waste of time and effort to be using a pipeline of commands for 
this. You seem to be deliberately turning a small, easy job into a huge, 
difficult job.

The *easiest* way to do this is to do the string processing in Python, 
not using the hard-to-use shell commands.

Python has its own grep module:

import grep

but since all you are doing is looking for a plain string with no 
wildcards, you don't even need that. Collect the output of blkid into a 
string, then do your processing in Python:

# Untested
for line in output.splitlines():
    # each line looks something like this
    # "/dev/md0: UUID="3cb91a07-14c9-4ae4-81e1-6eb662eabeee" TYPE="ext3"
    p = line.find('TYPE="')
    if p == -1:
        continue  # skip the line
    result = line[p:]
    if line.endswith('"'):
        result = result[:-1]
    print("Found file type", result)


Much easier to read and write and debug than a pipeline of Unix 
command line tools, and likely to be faster too.



-- 
Steve