（global search regular expression(RE) and print out the line）
常用参数：
-w (word regexp)匹配整词
-x (line regexp)匹配整行
-i（ignore case） 不区分大小写
-E 使用正则表达式
-r 递归调用
-c (count)只输出匹配行数量
-n (line number) 显示匹配行及行号
-H 每一个匹配项显示文件名
-m[num] (max num) 最大匹配次数
-o (only match) 只输出匹配项
-l 只打印包含搜索项的文件名
-A[num] 打印匹配内容前num行
-B[num] 打印匹配内容后num行
-C[num] 打印匹配内容前后num行
测试文本内容，两个文件(a.txt,b.html)内容一致
I am sailing, I am sailing
home again 'cross the sea.
I am sailing stormy waters,
to be near you, to be free.
I am flying, I am flying
like a bird 'cross the sky.
I am flying
passing high clouds,
to be near you,
to be free.
Can you hear me,
can you hear me,
through the dark night far away?
I am dying,
forever crying, to be with you;
who can say?
Can you hear me,
#默认搜索 > grep "flying" a.txt I am flying, I am flying I am flying #查看匹次数 > grep -c "am" a.txt 5 #打印匹配行号 > grep -n "am" a.txt 1:I am sailing, I am sailing 3:I am sailing stormy waters, 5:I am flying, I am flying 7:I am flying 14:I am dying, #完全匹配单词，无输出 > grep -nw "sail" a.txt #完全匹配单词 > grep -nw "sailing" a.txt 1:I am sailing, I am sailing 3:I am sailing stormy waters, #匹配整行 > grep -nx "sailing" a.txt > grep -nx "I am flying" a.txt 7:I am flying #-i不区分大小写 > grep -n "AM" a.txt > grep -ni "AM" a.txt 1:I am sailing, I am sailing 3:I am sailing stormy waters, 5:I am flying, I am flying 7:I am flying 14:I am dying, #显示匹配文件名 > grep -H "am" a.txt a.txt:I am sailing, I am sailing a.txt:I am sailing stormy waters, a.txt:I am flying, I am flying a.txt:I am flying a.txt:I am dying #最大匹配2次结束 > grep -H -m2 "am" a.txt a.txt:I am sailing, I am sailing a.txt:I am sailing stormy waters, #使用正则 > grep -E "fly+" a.txt I am flying, I am flying I am flying #-o只显示匹配内容 > grep -E "fly\S+" -o a.txt flying, flying flying #显示匹配内容前后行控制 > grep -n "high" a.txt 8:passing high clouds, #显示匹配内容的前三行 > grep -n -A3 "high" a.txt 8:passing high clouds, 9-to be near you, 10-to be free. 11-Can you hear me, #显示匹配内容的后三行 > grep -n -B3 "high" a.txt 5-I am flying, I am flying 6-like a bird 'cross the sky. 7-I am flying 8:passing high clouds, #显示匹配内容的前后三行 > grep -n -C3 "high" a.txt 5-I am flying, I am flying 6-like a bird 'cross the sky. 7-I am flying 8:passing high clouds, 9-to be near you, 10-to be free. 11-Can you hear me, #-r递归查询 > grep -H "high" -r . --include *.txt ./a.txt:passing high clouds, #查询多个匹配项 > grep -e "high" -e "near" a.txt to be near you, to be free. passing high clouds, to be near you, #显示匹配文件名 > grep -l "high" * a.txt b.html