Grep a file, but show several surrounding lines

Use the shell globbing syntax:

grep pattern -r --include=\*.cpp --include=\*.h rootdir

The syntax for --exclude is identical.

Note that the star is escaped with a backslash to prevent it from being expanded by the shell (quoting it, such as --include="*.cpp", would work just as well). Otherwise, if you had any files in the current working directory that matched the pattern, the command line would expand to something like grep pattern -r --include=foo.cpp --include=bar.cpp rootdir, which would only search files named foo.cpp and bar.cpp, which is quite likely not what you wanted.

Update 2021-03-04

I've edited the original answer to remove the use of brace expansion, which is a feature provided by several shells such as Bash and zsh to simplify patterns like this; but note that brace expansion is not POSIX shell-compliant.

The original example was:

grep pattern -r --include=\*.{cpp,h} rootdir

to search through all .cpp and .h files rooted in the directory rootdir.

Here are some ways to do it:

grep --color 'pattern\|$' file
grep --color -E 'pattern|$' file
egrep --color 'pattern|$' file

The | symbol is the OR operator. Either escape it using \ or tell grep that the search text has to be interpreted as regular expressions by adding -E or using the egrep command instead of grep.

The search text "pattern|$" is actually a trick, it will match lines that have pattern OR lines that have an end. Because all lines have an end, all lines are matched, but the end of a line isn't actually any characters, so it won't be colored.

To also pass the colored parts through pipes, e.g. towards less, provide the always parameter to --color:

grep --color=always 'pattern\|$' file | less -r
grep --color=always -E 'pattern|$' file | less -r
egrep --color=always 'pattern|$' file | less -r