1. 需求

[root@e1zone awk_join]# cat a.txt
id num
1 100
2 20
3 25
4 35
5 10
6 0
[root@e1zone awk_join]# cat b.txt
id type
1 a3
2 a3l
3 a4l
4 a6l
5 a8l

id type  num
1 a3 100
2 a3l 20
3 a4l 25
4 a6l 35
5 a8l 10
6 unknown 0

2. awk实现

#核心是先构造数组，按数组匹配，else输出不匹配的
[root@e1zone awk_join]#awk 'NR==FNR{a[$1]=$0;next}{if($1 in a)print a[$1],$2;else print $1,"unknown",$NF}' b.txt a.txt
id type  num
1 a3 100
2 a3l 20
3 a4l 25
4 a6l 35
5 a8l 10
6 unknown 0