grep Context line control Print lines before and/or after matching pattern

Example

Usually grep prints only matching lines. In the example below seq 9 generates a list of numbers from 1 to 9, one per line, and grep prints a single matching line:

seq 9 | grep 5
# 5

The -C n option (or --context=n in long form) prints n lines before and after each matching line, in addition to the matching line itself:

seq 9 | grep -C 2 '5'
# 3
# 4
# 5
# 6
# 7

Naturally, fewer than n lines will be printed if end-of-file or beginning-of-file is reached.

If we want to print lines only before or only after, but not both, we can use -B n (--before-context=n) or -A n (--after-context=n):

seq 9 | grep -B 2 '5'
# 3
# 4
# 5

seq 9 | grep -A 2 '5'
# 5
# 6
# 7

Note these options are not available in POSIX (see the POSIX specifications for grep).

If the contexts of two or more matching lines overlap, then all the lines are printed together as one large context. In the example below, 5 is part of the context of both 3 and 7:

seq 9 | grep -E --context=2 '3|7'
# 1
# 2
# 3
# 4
# 5
# 6
# 7
# 8
# 9

However, if the contexts do not overlap, they are printed out with a group separator line. By default this is double hyphen (--):

seq 9 | grep -E --context=2 '2|8'
# 1
# 2
# 3
# 4
# --
# 6
# 7
# 8
# 9

We can set a different group separator line using the --group-separator=SEP option, or suppress this line entirely by using the --no-group-separator option:

seq 9 | grep -E --context=0 --group-separator='****' '2|8'
# 2
# ****
# 8

seq 9 | grep -E --context=0 --group-separator='' '2|8'
# 2
# 
# 8

seq 9 | grep -E --context=0 --no-group-separator '2|8'
# 2
# 8

Finally, if we choose the -v option to print non-matching lines, then context is provided around those lines instead:

seq 9 | grep -E -v '1|3|4|5|6|7|9'
# 2
# --
# 8

seq 9 | grep -E -v -C 1 '1|3|4|5|6|7|9'
# 1
# 2
# 3
# --
# 7
# 8
# 9