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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 ‘--’ 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.
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