求教 awk 重组问题

英语盲学linux

当设定了OFS后 想要使用新的分隔符 一般都会使用类似 $1=$1,NF+=0

所以想问下 当awk 看到$1=$1 他是怎么来判断的或者说这个是怎么执行的

waker

Advanced Notes: Changing FS Does Not Affect the Fields

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.

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.

dahaoshanhe

回复 2# waker


    你难到没看到楼主的名么
还在这粘英文

waker

dahaoshanhe 发表于 2012-04-20 13:41
回复 2# waker

俺也是英语盲,understand?

英语盲学linux

回复 2# waker


    先谢了 不过我真看不懂

英语盲学linux

回复 2# waker


    不过大体意思是如果没域内的值改变了 那么OFS就会应用新的吗？

serenemoon

回复 1# 英语盲学linux


    我看manua时候，是说每当一个record读进来的时候, 根据 FS 把每个 $1, $2...$NF 的值确定， 同时根据当时OFS也确定了输出样式。

只读取record 或 field 值是不会对已经确定的东西产生影响，但写就不一样了。

如果改变了某一个 field 的值， 比如 $1=$1，（其实把任何值赋给任何 field 都会这样） $1值的改变促使awk重新生成输出样式，这时新的 OFS 就派上用场了。
同时 改变 $0 的值也是一样，$0的值改变了，通常来讲都会影响到 field值的改变（只是我用于理解的一种方式，$0=$0就不会改变$0），所以awk就会重新生成每个field的值，这时 FS 便派上用场了。

It is a not-uncommon error to try to change the field separators in a record simply by setting FS and OFS, and then expecting a plain ‘print’ or ‘print $0’ to print the modified record.

But this does not work, since nothing was done to change the record itself. Instead, you must force the record to be rebuilt, typically with a statement such as ‘$1 = $1’, as described earlier.

waker

英语盲学linux 发表于 2012-04-20 13:55
回复 2# waker

如果不是特殊需要,一般在BEGIN{}中设置FS/OFS就不会有诸多烦恼了