grep(1) -C 2 -F '[error]' web.log
print lines matching a pattern
-C NUM, -NUM, --context=NUM
       Print NUM lines of output context.  Places a  line  containing  a  group  separator  (--)  between
       contiguous  groups  of  matches.   With the -o or --only-matching option, this has no effect and a
       warning is given.
-F, --fixed-strings
       Interpret  PATTERN  as  a  list  of  fixed  strings,  separated by newlines, any of which is to be
       matched.  (-F is specified by POSIX.)
grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus
(-) is given as file name) for lines containing a match to the given PATTERN.  By  default,  grep  prints
the matching lines.
source manpages: grep