在 bash 脚本中运行 tcpdump
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Running tcpdump inside bash script
提问by user238021
I am trying to get some numbers from tcpdump inside a shell script and print that number.
我试图从 shell 脚本中的 tcpdump 获取一些数字并打印该数字。
Here is my script
这是我的脚本
while true
do
{
b=`tcpdump -n -i eth1 | awk -F'[, ]' '{print }'`
echo $b
}
done
When I execute this script, I get this
当我执行这个脚本时，我得到了这个
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
Is there anything special I need to do to capture tcpdump o/p inside shell script ?
我需要做什么特别的事情来捕获 shell 脚本中的 tcpdump o/p 吗？
回答by Gordon Davisson
By default, tcpdumpruns forever (or until it's interrupted by Control-C or something similar). The
默认情况下，tcpdump永远运行（或直到它被 Control-C 或类似的东西中断）。这
b=`tcpdump ...`
construct runs until tcpdumpexits... which is never ... and thenputs its output into $b. If you want to capture the output from a single packet, you can use tcpdump -c1 ...(or -c5to capture groups of 5, or similar). Alternately, you could let it run forever but capture its output one line at a time with a while readloop (although you need to use tcpdump -lto prevent excessive buffering):
构造运行直到tcpdump退出...永远不会...然后将其输出放入$ b。如果要捕获单个数据包的输出，可以使用tcpdump -c1 ...（或-c5捕获 5 个或类似的组）。或者，您可以让它永远运行，但使用while read循环一次捕获一行输出（尽管您需要使用它tcpdump -l来防止过度缓冲）：
tcpdump -l -n -i eth1 | awk -F'[, ]' '{print }' | while read b; do
echo $b
done
I'm not entirely sure what your script is supposed to do, but I see some other issues. First, unless your version of tcpdumpis much more consistent than mine, printing the 10th comma-delimited field of each packet will notget you anything meaningful. Here's some sample output from my computer:
我不完全确定您的脚本应该做什么，但我看到了一些其他问题。首先，除非您的版本tcpdump比我的更一致，否则打印每个数据包的第 10 个逗号分隔字段不会让您获得任何有意义的信息。这是我的计算机的一些示例输出：
00:05:02:ac:54:1e
1282820004:1282820094
90
73487384:73487474
1187212630:1187212720
90
90
host
2120673524
Second, what's the point of capturing the output into a variable, then printing it? Why not just run the command and let it output directly? Finally, echo $bmay garble the output due to word splitting and wildcard expansion (for example, if $b happened to be "*", it would print a list of files in the current directory). For this reason, you should double-quote variables when you use them (in this case, echo "$b").
其次，将输出捕获到变量中然后打印它有什么意义？为什么不直接运行命令，让它直接输出呢？最后，echo $b可能由于分词和通配符扩展导致输出乱码（例如，如果 $b 恰好是“*”，它会打印当前目录中的文件列表）。出于这个原因，您应该在使用变量时用双引号引用它们（在本例中为echo "$b"）。
