awk中FS的一个问题

yuanfaxiang1008

自己做实验碰到个问题：
[root@ shell]# cat a.log
1abc    2abc
3abc    4abc
5abc    6abc

[root@ shell]# awk 'FS=$2 {print FS}' a.log
2abc
a
知道FS是在a.log的第二行才开始生效，所以可以理解输出的第一行，输出第二行就不理解了。

yinyuemi

回复 1# yuanfaxiang1008

再做个实验，瞅瞅到底是肿么梨？

1. 
   
2. awk '{printf "line"NR":\t$1=\""$1"\"\t$2=\""$2"\"\tOLD_FS=\""FS"\""}FS=$2 {printf "\tNEW_FS=\""FS"\""}{print  !FS?"\tNEW_FS=\""FS"\"":""}'  log
   

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waker

1. 
   
2. Advanced Notes: Changing FS Does Not Affect the Fields
   
3. 
   
4. According to the POSIX standard, awk is supposed to behave as if each record is split into fields at the time it is read. In particular, this means that if you change the value of FS after a record is read, the value of the fields (i.e., how they were split) should reflect the old value of FS, not the new one.
   
5. 
   
6. However, many older implementations of awk do not work this way. Instead, they defer splitting the fields until a field is actually referenced. The fields are split using the current value of FS! (d.c.) This behavior can be difficult to diagnose. The following example illustrates the difference between the two methods. (The sed21 command prints just the first line of /etc/passwd.)
   
7. 
   
8.      sed 1q /etc/passwd | awk '{ FS = ":" ; print $1 }'
   
9. 
   
10. which usually prints:
    
11. 
    
12.      root
    
13. 
    
14. on an incorrect implementation of awk, while gawk prints something like:
    
15. 
    
16.      root:nSijPlPhZZwgE:0:0:Root:/:
    
17. 
    
18. Advanced Notes: FS and IGNORECASE
    
19. 
    
20. The IGNORECASE variable (see User-modified) affects field splitting only when the value of FS is a regexp. It has no effect when FS is a single character, even if that character is a letter. Thus, in the following code:
    
21. 
    
22.      FS = "c"
    
23.      IGNORECASE = 1
    
24.      $0 = "aCa"
    
25.      print $1
    
26. 
    
27. The output is ‘aCa’. If you really want to split fields on an alphabetic character while ignoring case, use a regexp that will do it for you. E.g., ‘FS = "[c]"’. In this case, IGNORECASE will take effect.

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waker

ref:  http://www.gnu.org/s/gawk/manual ... d-Splitting-Summary

justlooks

1. 
   
2. # awk '{print "old="FS"$"}FS=$2{print FS}' tt
   
3. old= $               #原FS
   
4. 2abc                  #赋值
   
5. old=2abc$        #第二行开始 后面的因为模式为0不打印
   
6. old=$                #第三行开始 因为第2行的赋值，所以认到$2为a，打印，结束
   
7. a
   
8. 
   
9. 
   
10. 
    
11. # echo "abc"|awk 'BEGIN{FS="";}{for(i=1;i<=NF;i++){print $i}}'
    
12. a
    
13. b
    
14. c
    

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解释完毕

mpstat

lz的主机名居然叫shell

jason680

回复 1# yuanfaxiang1008

这样有比较容易了解...
$ awk '{FS=$2;printf "Line="NR", FS=|"FS"|, ";for(n=0;n++<NFprintf "$"n"=|"$n"|,";print ""}' a.log
Line=1, FS=|2abc|, $1=|1abc|,$2=|2abc|,
Line=2, FS=||, $1=|3abc 4abc|,
Line=3, FS=|a|, $1=|5|,$2=|a|,$3=|b|,$4=|c|,$5=| |,$6=|6|,$7=|a|,$8=|b|,$9=|c|,

yuanfaxiang1008

回复 6# mpstat


    主机名临时改的，哈哈，谢谢大神们的讲解，豁然开朗啊...