awk after the number of field or the value of field changed

# from awk manpage
References to non-existent fields (i.e. fields after $NF) produce the null-string.
However, assigning to a  non-exis‐tent field (e.g., $(NF+2) = 5) increases the value of NF,
creates any intervening fields with the null string as their value, and causes the value
of $0 to be recomputed, with the fields being separated by the value of  OFS.

References to negative numbered fields cause a fatal error.

Decrementing NF causes the values of fields past the new value to be lost,
and the value of $0 to be recomputed, with the fields being separated by the value of OFS.

Assigning a value to an existing field causes the whole record to  be  rebuilt  when  $0  is  referenced.
Similarly, assigning a value to $0 causes the record to be resplit, creating new values for the fields.


意思是:
1. 引用不存在的字段，会使增加NF的值，$0要根据OFS的值重新创建，中间的字段设置为空字符串, 然后根据FS的值重新对$0进行分割.
如:
$ echo 'a b c' | awk '{print $0; $6=60; OFS=":"; print $0}'
a b c
a:b:c:::60
可以清楚的看到, 第4,5字段的值都是空字符串.

2. 引用编号小于零的字段，会引起错误.
如:
$ cal | awk '{print $-1}'
awk: (FILENAME=- FNR=1) fatal: attempt to access field -1

3. 减少NF的值，编号大于NF的字段将丢失，$0根据OFS的值重新创建，然后根据FS的值重新分割记录.
如:
$ echo 'a b c d e f' | awk '{print $0; NF=3; print $0}'
a b c d e f
a b c

4. 给存在的字段赋值，使$0根据OFS的值重新创建，然后根据FS的值重新分割记录.
如:
$ echo 'a b c' | awk '{print $0; $1=$1;OFS=":"; FS=":"; print $0; print $1}'
a b c
a:b:c
a

5. 给$0赋值，$0现在是被赋予的新值，$0根据FS的值重新分割记录.
$ echo 'a b c' | awk '{print $0; $0=$0;OFS=":"; FS=":"; print $0; print $1}'
a b c
a b c
a

echo 'a b c' | awk '{print $0; $0="c d e"; print $0; print $1}'
a b c
c d e
c

note:
  a. 4, 5是ChinaUnix.net Shell 编程大赛的第8题.
  b. 可以发现当字段个数或者字段被修改后，都要根据OFS的值进行$0重建, 然后，$0根据FS的值重新分割.
  c. 只修改$0, 使$0用新值根据FS的值重新分割.