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2.1.5 Context Line Control

Regardless of how these options are set, grep will never print any given line more than once. If the -o (--only-matching) option is specified, these options have no effect and a warning is given upon their use.

-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 leading and trailing output context.
--group-separator=string
When -A, -B or -C are in use, print string instead of -- between groups of lines.
--no-group-separator
When -A, -B or -C are in use, do not print a separator between groups of lines.

Here are some points about how grep chooses the separator to print between prefix fields and line content: