Pipes实现LeetCode(195.第十行)

[LeetCode] 195.Tenth Line 第十行

How would you print just the 10th line of a file?

For example, assume that file.txt has the following content:

Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9
Line 10

Your script should output the tenth line, which is:

Line 10

[show hint]

Hint:
1. If the file contains less than 10 lines, what should you output?
2. There's at least three different solutions. Try to explore all possibilities.

这道题让我们用Bash脚本来打印一个txt文件的第十行，可以用很多种方法来实现，我们先来看一种是用awk来实现的方法，awk是强大的文本分析工具，具有流控制、数学运算、进程控制、内置的变量和函数、循环和判断的功能。其中NR表示行数，$0表示当前记录，所以我们可以用if来判断行数为第十行时，将内容打印出来即可：

解法一：

awk '{if(NR == 10) print $0}' file.txt

我们也可以用更简洁的写法来打印出第十行如下：

解法二：

awk 'NR == 10' file.txt

我们也可以使用流编辑工具sed来做。-n默认表示打印所有行，p限定了具体打印的行数：

解法三：

sed -n 10p file.txt

我们也可以使用tail和head关键字来打印。其中head表示从头开始打印，tail表示从结尾开始打印，-你表示根据文件行数进行打印，一些区别与联系请见下列例子：

tail -n 3 file.txt: 打印file文件的最后三行内容　　　　　　

tail -n +3 file.txt： 从file文件第三行开始打印所有内容

head -n 3 file.txt： 打印file文件的前三行

head -n -3 file.txt： 打印file文件除了最后三行的所有内容

至于竖杠|为管道命令，用法: command 1 | command 2 他的功能是把第一个命令command1执行的结果作为command 2的输入传给command 2。了解了这些知识，那么下面一行命令就很好理解了，首先输入file文件从第十行开始的所有内容，然后将输出内容的第一行打印出来即为第十行：

解法四：

tail -n +10 file.txt | head -n 1

下面这种方法跟上面刚好相反，先输出file文件的前十行，然后从输出的第十行开始打印，那么也能正好打印第十行的内容：

解法五：

head -n 10 file.txt | tail -n +10

参考资料：

https://leetcode.com/discuss/29526/super-simple-solution

https://leetcode.com/discuss/29591/simple-solution-using-awk

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