grep是一个在文件中查找匹配字符串很有帮助的命令。指到现在，我才知道如何使用grep查看匹配字串的上下行内容。

让我们看一个例子

ALICE was beginning to get
very tired of sitting by
her sister on the bank
and of having nothing to do:
once or twice she had peeped
into the book her sister was reading,
but it had no pictures or conversations in it,
"and what is the use of a book," thought Alice,
"without pictures or conversations?'

如果我们现在要搜索带"bank”的行，我们应该这样写

# grep bank test.txt

然后我们会得到

her sister on the bank

如果我们现在想要得到这行，和这行前面的一行，我们应该这样写

# grep -B1 bank test.txt

得到的结果是这样的

very tired of sitting by
her sister on the bank

如果我们要得到包含"bank”的行和其后面的两行，我们应该这样写

# grep -A2 bank test.txt

这样得到的结果是这样的

her sister on the bank
and of having nothing to do:
once or twice she had peeped

是不是很方便，当然这两个参数还可以同时使用。至于例子，这里就不写了，留着读者自己去探索吧。

原文可以参考：http://www.jansipke.nl/grep-lines-before-and-after-matched-line

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