grep -- output context line

grep输出匹配的上下文 (Context Line Control):

    -A NUM, --after-context=NUM
      Print  NUM  lines  of trailing context after matching lines.

   -B NUM, --before-context=NUM
      Print  NUM  lines  of leading context before matching lines.

   -C NUM, -NUM, --context=NUM
      Print  NUM  lines  of output context.

Places a line containing a group separator (--) between contiguous groups of matches.

note: With the -o or --only-matching option, these has no effect and a warning is given