bash 仅在字段不为空时打印
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Print only if field is not empty
提问by teamg
I have a text file that I want to pull host and IP info from only if the IP exists in column 4. For example:
我有一个文本文件，仅当 IP 存在于第 4 列中时，我才想从中提取主机和 IP 信息。例如：
cat hostlist.txt
Server One 255.255.255.255 123.123.123.123
Server Two 255.255.255.255
Server Three 255.255.255.255 123.123.123.123
In this case I would only want to see Server One and Three as Server Two has no data in the fourth column.
在这种情况下，我只想看到服务器一和三，因为服务器二在第四列中没有数据。
回答by Kyle Banerjee
awk '{if () print awk 'NF>=4 && ~/^([0-9]{1,3}\.){3}[0-9]{1,3}$/' hostlist.txt
;}' < hostlist.txt
does the trick. It's functionally equivalent to the earlier solution but is simpler since you only check for existence rather than matching a regex.
诀窍。它在功能上等同于早期的解决方案，但更简单，因为您只检查是否存在而不是匹配正则表达式。
回答by RomanPerekhrest
awkapproach:
awk方法：
Server One 255.255.255.255 123.123.123.123
Server Three 255.255.255.255 123.123.123.123
The output:
输出：
$ awk '' hostlists.txt
Server One 255.255.255.255 123.123.123.123
Server Three 255.255.255.255 123.123.123.123
NF>=4- ensures that a record has at least 4 fields
NF>=4- 确保记录至少有 4 个字段
$4~/^([0-9]{1,3}\.){3}[0-9]{1,3}$/- ensures that the 4th field contains IPv4 address (in most simple form. Real IPv4 validation requires an additional conditions)
$4~/^([0-9]{1,3}\.){3}[0-9]{1,3}$/- 确保第 4 个字段包含 IPv4 地址（以最简单的形式。真正的 IPv4 验证需要附加条件）
回答by Benjamin W.
If you can live with lines where field 4 has value 0not being printed, you can simplify to
如果您可以忍受字段 4 具有0未打印值的行，则可以简化为
This is functionally equivalent to {if ($4) print $0;}, but reduced to just a pattern and using the default action of print $0.
这在功能上等同于{if ($4) print $0;}，但简化为一个模式并使用 的默认操作print $0。
