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Why does the generated string contain a 'n' – CentOS

I have created this mre:

import subprocess

m = 'rsync'
url = 'mirror.nsc.liu.se/CentOS/7.8.2003/'
flags = '-avHP --delete '
destf = ''
url_prefix = 'rsync://'
root = '/tmp'
dest = 'test'

cmd = m + ' ' + flags + ' ' + url_prefix+url + ' ' +destf + ' ' + root+'/'+dest
print(cmd)
print(cmd.split(' '))
proc=subprocess.run(cmd.split(' '), capture_output=True)
print(proc.stderr)

Which gives me this output:

$ python3 test.py 
rsync -avHP --delete rsync://mirror.nsc.liu.se/CentOS/7.8.2003/  /tmp/test
['rsync', '-avHP', '--delete', '--bwlimit=800', 'rsync://mirror.nsc.liu.se/CentOS/7.8.2003/', '', '/tmp/test']
b'Unexpected local arg: nIf arg is a remote file/dir, prefix it with a colon (:).nrsync error: syntax or usage error (code 1) at main.c(1368) [Receiver=3.1.3]n'

I really don’t understand what’s wrong here. There are no newlines in the string as far as I know.

If I change the creation of the command line to this (remove destf), the problem still remains:

cmd = '' + m + ' ' + flags + ' ' + url_prefix+url + ' ' + ' ' + root+'/'+dest

But with this (also remove one space) it works:

cmd = '' + m + ' ' + flags + ' ' + url_prefix+url + ' ' + root+'/'+dest

If I replace ' ' with ' ' it stops working again. But when I enter the string directly in a shell, it works even with double spaces.

So what is actually happening here? I could use my findings above to shoehorn in a quick fix, but it feels like that will bite me later. So what is it I’m actually missing here? The reason I want the destf parameter is that it is set to -P when I’m using wget and could possibly be something else for another command.

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2

Answers


  1. Chosen as BEST ANSWER
    0

    Thanks to Barmar for the answer that solved it. While I was out on lunch I realized what the problem was.

    When a program is executed, it looks in a variable traditionally called argv which is basically an array of strings. The shell is usually responsible for converting a string into a string array, so if you invoke this command:

    $ foo a   b c    eee

    then, the shell will parse the string into:

    argv[0] = "foo"
argv[1] = "a"
argv[2] = "b"
argv[3] = "c"
argv[4] = "eee"

    But with the approach I used, it instead would end up in:

    argv[0] = "foo"
argv[1] = "a"
argv[2] = ""
argv[3] = ""
argv[4] = "b"
argv[5] = "c"
argv[6] = ""
argv[7] = ""
argv[8] = ""
argv[9] = "eee"

    So there was nothing wrong with the string. The problem was bypassing the conversion done by the shell. But using .split() instead of .split(' ') solved it.


  2. 0

    It’s not complaining about a newline in the string. The error message has multiple lines, and the newline is the separator between the lines.

    The unexpected argument is the empty string '' before '/tmp/test' because destf is empty.

    When you use .split(' '), multiple spaces are not merged into a single delimiter. So when you do

    'foo  bar'.split(' ')

    the result is

    ['foo', '', 'bar']

    rather than 

    ['foo', 'bar']

    If you want multiple spaces to be coalesced into a single delimiter, use .split() instead of .split(' ')

    proc=subprocess.run(cmd.split(), capture_output=True)
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