Linux用awk和tee写入同一文件:非常奇怪的行为 [英] Linux write into same file with awk and tee: very odd behaviour

问题描述

我试图做一些不寻常的事情，并覆盖

I was trying to do something unusual and overwrite the same file on Unexpected new line when writing out in Unix Shell Script this question just out of curiosity

我发现，在某些尝试下，我可以准备> to_same_file，它就像您在第一次尝试中看到的那样工作，然后在随后的尝试中生成了一个空文件，我的假设是，这必须与处理时间有关.第一次尝试花了更长的时间准备并有时间去I/O，因为在其他尝试中它发生得更快，并且没有机会在读取文件后及时将其写入文件.发生奇怪的行为

I found that on some attempts I could tee > to_same_file and it worked as you can see on the very first attempt and then subsequent attempts produced an empty file, my assumption is, this must be related to processing time.. Meaning on the first attempt it took longer to get to tee and had time to i/o where as it happens faster on the other attempts and has no chance to write to the file in time that it has read it.. just interested to understand why this odd behaviour occured

me@desktop:~/$ cp 2.csv 1.csv
me@desktop:~/$ cat 1.csv
ABCD89A, Admin, shop, Stall Count, 2014-01-06 09:00:00, 0
ABCD89N, Admin, shop, Stall Count, 2014-01-06 09:00:00, 0
me@desktop:~/$ awk  -F"," '{ 
     timestamp=$5;  
     gsub(":"," ",timestamp); 
     gsub("-"," ",timestamp);   
     EPOCH=(mktime(timestamp))
     } 
     {
      print $0","EPOCH
      }' 1.csv  2>&1 | tee > 1.csv
me@desktop:~/$ cat 1.csv
ABCD89A, Admin, shop, Stall Count, 2014-01-06 09:00:00, 0,1388998800
ABCD89N, Admin, shop, Stall Count, 2014-01-06 09:00:00, 0,1388998800
me@desktop:~/$ cp 2.csv 1.csv
me@desktop:~/$ cat 1.csv 
ABCD89A, Admin, shop, Stall Count, 2014-01-06 09:00:00, 0
ABCD89N, Admin, shop, Stall Count, 2014-01-06 09:00:00, 0
me@desktop:~/$ awk  -F"," '{ 
     timestamp=$5;  
     gsub(":"," ",timestamp); 
     gsub("-"," ",timestamp);   
     EPOCH=(mktime(timestamp))
     } 
     {
      print $0","EPOCH
      }' 1.csv  2>&1 | tee > 1.csv
me@desktop:~/$ cat 1.csv 
me@desktop:~/$ cp 2.csv 1.csv
me@desktop:~/$ awk  -F"," '{ 
     timestamp=$5;  
     gsub(":"," ",timestamp); 
     gsub("-"," ",timestamp);   
     EPOCH=(mktime(timestamp))
     } 
     {
      print $0","EPOCH
      }' 1.csv  2>&1 | tee > 1.csv
me@desktop:~/$ cat 1.csv 
me@desktop:~/$ cp 2.csv 1.csv
me@desktop:~/$ cat 1.csv 
ABCD89A, Admin, shop, Stall Count, 2014-01-06 09:00:00, 0
ABCD89N, Admin, shop, Stall Count, 2014-01-06 09:00:00, 0
me@desktop:~/$ awk  -F"," '{ 
     timestamp=$5;  
     gsub(":"," ",timestamp); 
     gsub("-"," ",timestamp);   
     EPOCH=(mktime(timestamp))
     } 
     {
      print $0","EPOCH
      }' 1.csv  2>&1 | tee -a > 1.csv
me@desktop:~/$ cat 1.csv 
me@desktop:~/$ 

推荐答案

一个具有相同问题的小型独立测试用例是这样的:

A small, self contained test case with the same problem is this:

cat file | tee > file

此管道包括两个并行运行的部分.

This pipeline consists of two parts that runs in parallel.

cat file尝试打开并读取文件.

cat file tries to open and read from the file.

tee > file试图截断文件.

根据文件是首先(部分)读取还是被截断，您将获得部分或全部数据，或者只是一个空文件.

Depending on whether the file is (partially) read or truncated first, you'll get either parts or all of your data, or just an empty file.

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