有的时候，需要在满足查询条件的情况，查询符合条件文本的前后几行。之前一直以为需要用python自行解决，没想到grep里有参数可以简单处理。
比如，在log记录，有的错误日志仅提供了错误内容，正在执行脚本的相关参数需要从上下文中进行推理，这时就可以采用此方法进行处理。
# 显示匹配行的前后3行
grep -3 'ERROR:' weibo.log
grep -C 3 'ERROR:' weibo.log
# 显示匹配行的后4行
grep -A 4 'ERROR:' weibo.log
# 显示匹配行的前2行
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