爲什麼grep會返回子進程stdin中的所有輸出？

Question

ubuntu@ubuntu:/home/ubuntuUser$ cat test.txt 
This is a test file 
used to validate file handling programs 
#pyName: test.txt 
this is the last line 
ubuntu@ubuntu:/home/ubuntuUser$ cat test.txt | grep "#pyName" 
#pyName: test.txt 
ubuntu@ubuntu:/home/ubuntuUser$ " 

#1 >>> a = subprocess.Popen(['cat test.txt'], 
          stdin=subprocess.PIPE, 
          stdout=subprocess.PIPE, 
          stderr=subprocess.PIPE, 
          shell=True) 
#2 >>> o, e = a.communicate(input='grep #pyName') 
#3 >>> print o 
#3 This is a test file 
#3 used to validate file handling programs 
#3 #pyName: test.txt 
#3 this is the last line 
#3 
#3 >>> 

問題：

Q1：在文件打印只匹配行 外殼grep命令，同時通過子打印grep的整個文件。怎麼了？

Q2： 如何通過communications（）發送的輸入添加到 初始命令（'cat test.txt'）？ #2之後，初始命令是否會在「|」之後通過輸入字符串進行通信，和shell命令獲取像cat test.txt | grep #pyName？

Answer 1

@prasath，如果你正在尋找一個例子來使用通信（）

[root@ichristo_dev]# cat process.py -- A program that reads stdin for input 
#! /usr/bin/python 

inp = 0 
while(int(inp) != 10): 
    print "Enter a value: " 
    inp = raw_input() 
    print "Got", inp 


[root@ichristo_dev]# cat communicate.py 
#! /usr/bin/python 

from subprocess import Popen, PIPE 

p = Popen("./process.py", stdin=PIPE, stdout=PIPE, stderr=PIPE, shell=True) 
o, e = p.communicate("10") 
print o 


[root@ichristo_dev]#./communicate.py 
Enter a value: 
Got 10 

Answer 2

也許將grep傳遞給communicate()-函數不會如您所設想的那樣工作。您可以通過直接從文件grepping簡化流程如下：

In [14]: a = subprocess.Popen(['grep "#pyName" test.txt'], stdin = subprocess.PIPE, stdout = subprocess.PIPE, stderr=subprocess.PIPE, shell = True) 

In [15]: a.communicate() 
Out[15]: ('#pyName: test.txt\n', '') 

這可能是一個聰明很多，做你想做的蟒蛇做什麼。如果它在文件中，以下內容將打印您的行。

In [1]: with open("test.txt") as f: 
    ...:  for line in f: 
    ...:   if "#pyName" in line: 
    ...:   print line 
    ...:   break 
    ...:   
#pyName: test.txt 

Answer 3

你在這裏做什麼實質上是cat test.txt < grep ...這顯然不是你想要的。要建立一個管道，你需要啓動兩個進程，而第一個的標準輸出連接到第二的標準輸入：

cat = subprocess.Popen(['cat', 'text.txt'], stdout=subprocess.PIPE) 
grep = subprocess.Popen(['grep', '#pyName'], stdin=cat.stdout, stdout=subprocess.PIPE) 
out, _ = grep.communicate() 
print out