1，假设有文本数据txt内容如下：

          1 2 3

          4 5 6

          7 8 9

     问题1： 现要在 最后一行最后一个数字 9 后面添加一个字符 ’]' ，得到如下内容

                  1 2 3

                  4 5 6

                  7 8 9]

      命令一：awk 'NR>1{print a}{a=$0}END{print gensub(/$/,"]",$0)}' txt

      命令二：awk 'BEGIN{getline a}{print a;a=$0}END{$0=gensub(/$/,"]",1,$0);print} ' txt

      命令三：awk '{if(!getline line){print gensub(/$/,"]",1,$0)}else{print line?$0 RS line:$0}}' txt

     问题2：现要在最后一行最后一个数字 9 后面添加一个字符 ’ ]' ，得到如下内容

                  1 2 3

                  4 5 6

                  7 8 9 ]

      命令一：awk 'NR>1{print a}{a=$0}END{print gensub(/$/," ]",$0)}' txt     

      命令二：awk 'BEGIN{getline a}{print a;a=$0}END{$0=gensub(/$/," ]",1,$0);print} ' txt

      命令三：awk '{if(!getline line){print gensub(/$/," ]",1,$0)}else{print line?$0 RS line:$0}}' txt

       (和问题1的命令相比，" ]"多加了个空格)

     问题3：现要将最后一行最后一个数字 9 替换成字符 ’]' ，得到如下内容

                  1 2 3

                  4 5 6

                  7 8 ]

       命令一：awk 'NR>1{print a}{a=$0}END{print gensub(/.$/,"]",$0)}' txt  

       命令二：awk 'BEGIN{getline a}{print a;a=$0}END{$0=gensub(/.$/,"]",1,$0);print}' txt

       命令三：awk '{if(!getline line){print gensub(/.$/,"]",1,$0)}else{print line?$0 RS line:$0}}' txt

       (和问题1的命令相比，/.$/ 多了个点)

      问题4：现要将最后一行最后一个数字 9 替换成字符 ’]' ，得到如下内容

                  1 2 3

                  4 5 6

                  7 8]

      命令一：awk 'NR>1{print a}{a=$0}END{print gensub(/..$/,"]",$0)}' txt

      命令二：awk 'BEGIN{getline a}{print a;a=$0}END{$0=gensub(/..$/,"]",1,$0);print}' txt

      命令三：awk '{if(!getline line){print gensub(/..$/,"]",1,$0)}else{print line?$0 RS line:$0}}' txt

       (和问题3的命令相比，/..$/ 又多了个点，可以看出多加一个点，就向前多移1位)