为什么我得到一个空字符串？

Question

I want to put the output of the bash command in a variable but I get an empty string.

import subprocess

out = subprocess.check_output("echo hello world", shell=True)
print out + ' ok'

the output is:

hello world
 ok

instead of:

hello world
hello world ok

why does this happen?

Answer 1

The output of echo includes a newline. The result is not written to your terminal, the output is captured by check_output() , but you then print that output including the newline:

>>> import subprocess
>>> out = subprocess.check_output("echo hello world", shell=True)
>>> out
'hello world\n'

giving you 'hello world' and ' ok' on two separate lines when printed.

You could remove the newline afterwards; using str.strip() would remove all whitespace from the start and end of the string, for example:

print out.strip() + ' ok'

On some shells, the echo command takes an -n switch to suppress the newline:

echo -n hello world

Answer 2

One thing that comes to mind is that subprocess commands add a newline . It may be clumsy, but try this:

print out.rstrip('\n') + ' ok'

Answer 3

echo prints the text with a new line character \\n appended to it. If you want to omit that, use printf instead.

>>> import subprocess
>>> out = subprocess.check_output("printf 'hello world'", shell=True)

>>> print out
>>> 'hello world'

Answer 4

You captured the output by invoking check_output . According to the docs https://docs.python.org/2/library/subprocess.html#subprocess.check_output

subprocess.check_output(args, *, stdin=None, stderr=None, shell=False, universal_newlines=False)
    Run command with arguments and return its output as a byte string.