Python子进程:如何使用管道三次？

Question

我想在以下行使用子进程:

convert ../loxie-orig.png bmp:- | mkbitmap -f 2 -s 2 -t 0.48 | potrace -t 5 --progress -s -o ../DSC00232.svg

我感谢其他帖子的子流程文档,但在示例中我们只使用了两次管道.

所以,我尝试三个命令中的两个,它的工作原理

p1 = subprocess.Popen(['convert', fileIn, 'bmp:-'], stdout=subprocess.PIPE)
# p2 = subprocess.Popen(['mkbitmap', '-f', '2', '-s', '2', '-t', '0.48'], stdout=subprocess.PIPE)
p3 = subprocess.Popen(['potrace', '-t' , '5', '-s' , '-o', fileOut], stdin=p1.stdout,stdout=subprocess.PIPE)
p1.stdout.close()  # Allow p1 to receive a SIGPIPE if p3 exits.
output = p3.communicate()[0]

你能帮助我完成第三个命令吗？

非常感谢你.

Answer 1

只需按照相同的示例添加第三个命令:

p1 = subprocess.Popen(['convert', fileIn, 'bmp:-'], stdout=subprocess.PIPE)
p2 = subprocess.Popen(['mkbitmap', '-f', '2', '-s', '2', '-t', '0.48'], 
     stdin=p1.stdout, stdout=subprocess.PIPE)
p1.stdout.close()
p3 = subprocess.Popen(['potrace', '-t' , '5', '-s' , '-o', fileOut],        
     stdin=p2.stdout,stdout=subprocess.PIPE)
p2.stdout.close()

output = p3.communicate()[0]

Answer 2

subprocess.Popen()与选项一起使用shell=True,您可以将整个命令作为单个字符串传递给它.

Answer 3

def runPipe(cmds):
try: 
    p1 = subprocess.Popen(cmds[0].split(' '), stdin = None, stdout = subprocess.PIPE, stderr = subprocess.PIPE)
    prev = p1
    for cmd in cmds[1:]:
        p = subprocess.Popen(cmd.split(' '), stdin = prev.stdout, stdout = subprocess.PIPE, stderr = subprocess.PIPE)
        prev = p
    stdout, stderr = p.communicate()
    p.wait()
    returncode = p.returncode
except Exception, e:
    stderr = str(e)
    returncode = -1
if returncode == 0:
    return (True, stdout.strip().split('\n'))
else:
    return (False, stderr)

然后像这样执行：

runPipe(['ls -1','head -n 2', 'head -n 1'])