Ajax异步问题(Ajax asynchronous issue)

 要在倒数第二行更改Sweet to Green，但只有该行包含Sweet pepper ： 
 
$ sed 'x; ${/Sweet pepper/s/Sweet/Green/;p;x}; 1d' file.txt
cabbage
spinach
collard greens
corn salad
Green pepper
kale
 
 要将整个倒数第二行替换为胡萝卜，不管它包含什么， 
 
$ sed 'x; ${s/.*/carrots/;p;x}; 1d' file.txt
cabbage
spinach
collard greens
corn salad
carrots
kale
 
 怎么运行的 
 
 让我们采取这个命令并一次一步地检查它： 
 
sed 'x; ${s/.*/carrots/;p;x}; 1d'
 
 
 
 x 
 
 这交换了模式空间（保存最近读取的行）和保持空间。 
 完成后，保持空间将包含最近读取的行，并且模式空间将包含上一行。 （当我们刚读完第一行时就是例外，在这种情况下，保留空间会有第一行，模式空间将是空的。） 
 
 
 ${s/.*/carrots/;p;x} 
 
 当我们在最后一行，由$表示时，模式空间保持倒数第二行，我们可以执行任何我们喜欢的替换或其他命令。 当我们完成后，我们用p打印倒数第二行。 最后，我们交换模式并用x再次保留空间，以使模式空间再次包含最后一行。 sed将打印这个，因为默认情况下，在命令结束时， sed打印模式空间中的任何内容。 
 
 
 1d 
 
 当我们在第一行时，用1表示，模式空间是空的（因为没有前一行），我们删除它（ d ）。 
 
 
 一个更简单的方法 
 
 这种方法很容易理解，代价是执行速度较慢： 
 
$ tac file.txt | sed '2 {/Sweet pepper/s/Sweet/Green/}' | tac
cabbage
spinach
collard greens
corn salad
Green pepper
kale
 
 而且，对于胡萝卜来说： 
 
$ tac file.txt | sed '2 s/.*/carrots/' | tac
cabbage
spinach
collard greens
corn salad
carrots
kale
 
 工作原理：在这里，我们使用tac来改变行的顺序。 注意： 
 
$ tac file.txt
kale
Sweet pepper
corn salad
collard greens
spinach
cabbage
 
 这样，倒数第二行成为第二行。因此，我们只是简单地告诉sed在第二行上操作。之后，我们再次使用tac来按照正确的顺序放置行。 

To change Sweet to Green on the second to last line but only if that line contains Sweet pepper:
 
$ sed 'x; ${/Sweet pepper/s/Sweet/Green/;p;x}; 1d' file.txt
cabbage
spinach
collard greens
corn salad
Green pepper
kale
 
To replace the whole of the second to last line, regardless of what it contains, to carrots:
 
$ sed 'x; ${s/.*/carrots/;p;x}; 1d' file.txt
cabbage
spinach
collard greens
corn salad
carrots
kale
 
How it works
 
Let's take this command and examine it one step at a time:
 
sed 'x; ${s/.*/carrots/;p;x}; 1d'
 
 
 
x
 
This exchanges the pattern space (which holds the most recently read line) and the hold space.
 
When this is done, the hold space will contain the most recently read line and the pattern space will contain the previous line. (The exception is when we have just read the first line. In that case, the hold space will have the first line and the pattern space will be empty.)
 
 
${s/.*/carrots/;p;x}
 
When we are on the last line, indicated by the $, the pattern space holds the second to last line and we can perform whatever substitutions or other commands that we like. When we are done, we print the second to last line with p. Lastly, we swap pattern and hold space again with x so that the pattern space will again contain the last line. sed will print this because, by default, at the end of the commands, sed prints whatever is in the pattern space.
 
 
1d
 
When we are on the first line, indicated by the 1, the patten space is empty (because there was no previous line) and we delete it (d).
 
 
A still simpler method
 
This method is easy to understand at the cost of slower execution speed:
 
$ tac file.txt | sed '2 {/Sweet pepper/s/Sweet/Green/}' | tac
cabbage
spinach
collard greens
corn salad
Green pepper
kale
 
And, for carrots:
 
$ tac file.txt | sed '2 s/.*/carrots/' | tac
cabbage
spinach
collard greens
corn salad
carrots
kale
 
How it works: Here, we use tac to reverse the order of the lines. Observe:
 
$ tac file.txt
kale
Sweet pepper
corn salad
collard greens
spinach
cabbage
 
In this way, the second-to-last line becomes line number 2. Thus, we just simply tell sed to operate on line number 2. Afterward, we use tac again to put the lines but in correct order.