NR和FNR的典型应用

[root@21 opt]#cat a
张三|000001
李四|000002

[root@21 opt]#cat b
000001|10
000001|20
000002|30
000002|15

#################################
想要得到的结果是将用户名，帐号和金额在同一行打印出来,如下:
张三|000001|10
张三|000001|20
李四|000002|30
李四|000002|15
#############################################
用如下语句处理后，得到以上结果：
awk -F'|' 'NR==FNR{a[$2]=$0;next}{print a[$1] FS $2}' a b



解释：
awk -F'|' 'NR==FNR{a[$2]=$0;next}{print a[$1] FS $2}' a b

There is no BEGIN block, and FS="|" by -F'|' argument

start to first file 'a' 
1. read file a line 1 and get data 张三|000001
  A: $0=张三|000001
  B: $1=张三
  C: $2=000001

  NR and FNR are the same equal to 1, and run NR=FNR block
  NR==FNR{a[$2]=$0;next}
    A: a[$2]=$0
      a[000001]=张三|000001
    B: next
      next cycle and get next line data 
  
2. read file a line 2 and get data 李四|000002
  A: $0=李四|000002
  B: $1=李四
  C: $2=000002

  NR and FNR are the same equal to 2, and run NR=FNR block
  NR==FNR{a[$2]=$0;next}
  A: a[$2]=$0
    a[000002]=李四|000002
  B: next
    next cycle and get next line data 

end of the file a, and get next file b data

3. read file b line 1, and get data 000001|10
  A: $0=000001|10
  B: $1=000001
  C: $2=10

  now, NR is 3 and FNR is 1, they are not eqaul
  and didn't run NR=FNR block, 
  and run next block {print a[$1] FS $2} 
    a[$1] => a[000001] => 张三|000001
    FS => |
    $2 => 10
    you will see the output
      张三|000001|10

4. read file b line 2, and get data 000001|20
  A: $0=000001|20
  B: $1=000001
  C: $2=20

  NR is 4 and FNR is 2, they are not eqaul
  and didn't run NR=FNR block, 
  and run next block {print a[$1] FS $2} 
    a[$1] => a[000001] => 张三|000001
    FS => |
    $2 => 20
    you will see the output
    张三|000001|20

cycle to read the file b  
5. read file b line 3, and get data 000002|30
  ...
  output==> 李四|000002|30

6. read file b line 4, and get data 000002|15
  ...
  output==> 李四|000002|15