让我们用一个例子来理解。我们可以使用带有选项的grep：

-A 5  # this will give you 5 lines after searched string.
-B 5  # this will give you 5 lines before searched string.
-C 5  # this will give you 5 lines before & after searched string

实例File.txt包含6行，以下是操作。

[abc@xyz]~/% cat file.txt # print all file data
this is first line
this is 2nd line
this is 3rd line
this is 4th line
this is 5th line
this is 6th line

[abc@xyz]~% grep "3rd" file.txt # we are searching for keyword '3rd' in the file
this is 3rd line

[abc@xyz]~% grep -A 2 "3rd" file.txt # print 2 lines after finding the searched string
this is 3rd line
this is 4th line
this is 5th line

[abc@xyz]~% grep -B 2 "3rd" file.txt # Print 2 lines before the search string.
this is first line
this is 2nd line
this is 3rd line

[abc@xyz]~% grep -C 2 "3rd" file.txt # print 2 line before and 2 line after the searched string 
this is first line
this is 2nd line
this is 3rd line
this is 4th line
this is 5th line

记住选项的技巧：选项：

-一个 -> 之后的平均值-B级 -> B表示之前。-c类 -> 介于两者之间

grep astring myfile -A 5 -B 5

这将grep“myfile”表示“跨步”，并在每次比赛前后显示5行

对于BSD或GNUgrep，可以使用-B num设置匹配前的行数，使用-A num设置匹配后的行数。

grep -B 3 -A 2 foo README.txt

如果希望前后行数相同，可以使用-C num。

grep -C 3 foo README.txt

这将显示前3行和后3行。

Grep有一个名为Context Line Control的选项，您可以使用--Context，

| grep -C 5

or

| grep -5

应该会成功的

-A和-B将起作用，-C n（对于n行上下文）或-n（对于n个上下文……只要n是1到9）也会起作用。

在/some/file.txt中搜索“17655”，显示前后10行上下文（使用Awk），输出前有行号，后有冒号。当grep不支持-[ACB]选项时，在Solaris上使用此选项。

awk '

/17655/ {
        for (i = (b + 1) % 10; i != b; i = (i + 1) % 10) {
                print before[i]
        }
        print (NR ":" ($0))
        a = 10
}

a-- > 0 {
        print (NR ":" ($0))
}

{
        before[b] = (NR ":" ($0))
        b = (b + 1) % 10
}' /some/file.txt;