$ tail *.txt
==> foo.txt <==
foo
FOO

==> foobar.txt <==
foo
bar
foobar

$ grep foo foo.txt
foo

$ grep -i foo foo.txt  # -i, --ignore-case
foo
FOO

$ grep -n foo foobar.txt  # -n, --line-number
1:foo
3:foobar

$ grep -o foo foobar.txt  # -o, --only-matching
foo
foo

$ grep -v foo foobar.txt  # -v, --invert-match
bar

$ grep -l foo *  # -l, --files-with-matches
foo.txt
foobar.txt

$ grep -H foo *  # -H, Always print filename headers
foo.txt:foo
foobar.txt:foo
foobar.txt:foobar

$ grep -r foo .  # -R, -r, --recursive, Prints filename headers by default
./foo.txt:foo
./foobar.txt:foo
./foobar.txt:foobar

$ grep -rh foo .  # -h, --no-filename
foo
foo
foobar

$ grep -w foo foobar.txt  # -w, --word-regexp
foo

$ grep -C2 bar foobar.txt  # -C[num, --context=num], num=2 by default, See also -A(after), -B(before)
foo
bar
foobar

$ grep -F foo.* foobar.txt  # -F, --fixed-strings, literal match

$ grep -q foo foo.txt; echo $?  # -q, --quiet, --silent
0
$ grep -q bar foo.txt; echo $?
1

$ grep -e '--foo' foo.txt  # -e [pattern], --regexp=pattern, Indicate following argument as regexp explicitly
# Useful when [pattern] contains '--' which would cause ambiguity as above.
# Consider `grep --foo` or something.

$ grep -Z foo foobar.tar.gz; echo $?  # -Z, -z, --decompress, Behave as zgrep
Binary file foobar.tar.gz matches
0
$ grep -Z baz foobar.tar.gz; echo $?
1