Python Vs bash string slicing

pythonquotingstring

My file a contains the text,

bcd\\\\.

With bash, i read the file and print characters from 4th to 8th position as,

tmp=$(cat a)
echo "${tmp:3:4}"

It prints,

\\\\

All happy. Now i use python's array slicing to print characters from 4th to 8th position as,

>>> f = open('a')
>>> v=f.read()
>>> v[3:7]

It prints,

'\\\\\\\\'

Why does bash and python behave differently when there are backslashes?

It is a matter of how python displays strings. Observe:

>>> f = open('a')
>>> v=f.read()
>>> v[3:7]
'\\\\\\\\'
>>> print v[3:7]
\\\\

When displaying v[3:7], the backslashes are escaped. When printing, print v[3:7], they are not escaped.

The line in your file should end with a newline character. In that case, observe:

>>> v[-1]
'\n'
>>> print v[-1]


>>>

The newline character is displayed as a backslash-n. It prints as a newline.

The results for tab are similar:

>>> s='a\tb'
>>> s
'a\tb'
>>> print s
a       b

You're not actually running a bash command. What you're doing is running an executable directly and passing it arguments.

Try the following script to see what is happening:

import subprocess
p = subprocess.Popen(["echo", "a", "b", "|", "rev"], stdout=subprocess.PIPE)
print p.communicate()

The output will be:

('a b | rev\n', None)

There's no redirection happening, the "|" is being passed in literally. I.e., it is as if you had typed find ... \| parallel .... Thus the error.

There's two ways to fix it.