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 -- around disjoint groups of lines.

--no-group-separator

When -A, -B or -C are in use, print disjoint groups of lines adjacent to each other.

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

• Matching lines normally use ‘:’ as a separator between prefix fields and actual line content.
• Context (i.e., non-matching) lines use ‘-’ instead.
• When no context is specified, matching lines are simply output one right after another.
• When nonzero context is specified, lines that are adjacent in the input form a group and are output one right after another, while a separator appears by default between disjoint groups on a line of its own and without any prefix.
• The default separator is ‘--’, however whether to include it and its appearance can be changed with the options above.
• Each group may contain several matching lines when they are close enough to each other that two otherwise adjacent but divided groups connect and can just merge into a single contiguous one.