我如何在 Python 中执行 comm Linux 命令

Question

I want to extract lines from File1 which are not present in File2

File1

a  
b  
c  

File2

a  
c  

so the output should be:

b  

One possible command in bash is:

comm -23 <(sort File1) <(sort File2) > File  

And it works perfectly well in bash, but I don't know how correctly to implement in Python.

I've tried with

import os  
os.system("comm -23 <(sort File1) <(sort File2) > File")  

And is not working. Any hint?

Answer 1

How about a pure python solution?

with open('file1', 'r') as f:
    lines1 = set(f.read().splitlines())

with open('file2', 'r') as f:
    lines2 = set(f.read().splitlines())

print(lines1.difference(lines2))

Or with less memory overhead:

with open('file1') as f, open('file2') as f2:
    lines1 = set(map(str.rstrip, f))
    print(lines1.difference(map(str.rstrip, f2)))

Answer 2

If you must use a shell, do it safely:

subprocess.call(['bash', '-c',
    'comm -23 <(sort "$1") <(sort "$2") >"$3"', '_',
    infile1_name, infile2_name, outfile_name])

That is to say: Instead of passing the filenames in as part of your code, pass them as out-of-band variables such that their names can't be interpreted by the shell.

Answer 3

It doesn't work because you need use bash run the command:

os.system("bash -c 'comm -23 <(sort File1) <(sort File2) > File'")

Normally, os.system() use sh run the command. But however, there's little different between bash and sh .

So in this case, I used bash -c 'command' call bash run the command. Then it could works.

From the manual of bash :

-c If the -c option is present, then commands are read from the first non-option argument command_string .

If there are arguments after the command_string , they are assigned to the positional parameters, starting with $0 .