linux - 为什么这段代码在 bash 中不起作用？

Question

我正在使用 tail -f 获取日志文件和日志输出此命令输入到 grep 命令并搜索“ERROR”字符串，grep 的输出用于创建/写入临时文件(tmpLog )。

命令:

tail -f logfile |  grep -n -i "ERROR" > tmpLog

日志文件内容

100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.003
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.0033
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.004
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.005
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.006
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.007
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.0023
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.0043
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.004
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.005
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.006
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.007
100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.0043

问题是，我没有在 tmpLog 文件中获取内容，虽然它已创建但内容为空但是当我尝试这个时

tail -f 日志文件 | grep -n -i "错误"*> tmpLog

我在 tmpLog 中有内容

a.sh:21:    tail -f logfile |  grep -n -i "ERROR" > tmpLog &  #> /dev/null
logfile:1:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.003
logfile:2:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.0033
logfile:3:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.004
logfile:4:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.005
logfile:5:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.006
logfile:6:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.007
logfile:7:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.0023
logfile:8:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.0043
logfile:9:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.004
logfile:10:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.005
logfile:11:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.006
logfile:12:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.007
logfile:13:100.66.230.2 - - [03/Apr/2017:11:40:23 +0900] "ERROR /favicon.ico HTTP/1.1" 200 26238 0.0043

但我得到了这一行 a.sh:21: tail -f logfile | grep -n -i "ERROR"> tmpLog & #>/dev/null 在我不想要的日志顶部

Answer 1

您的工作目录中是否有名为a.sh 的文件？

是这个文件的第21行:

tail -f logfile | grep -n -i "ERROR" > tmpLog & #> /dev/null

你似乎得到这个的原因是因为在你的第二个例子中，你有效地运行:

grep -n -i "ERROR" * > tmpLog

它捕获当前目录中包含 ERROR 的所有行。

tail -f 实际上是一个无穷无尽的流，因此不适合以您似乎使用它的方式重定向到文件中。

你可能想看看this SU post关于缓冲如何导致重定向输出的问题。