`grep` - print lines matching a pattern

grep [options] PATTERN [file]...

Options you will use

-i, --ignore-case: Case-insensitive match.
-v, --invert-match: Select the lines that do NOT match.
-c, --count: Print only the count of matching lines.
-n, --line-number: Prefix each line with its line number.
-r, -R, --recursive: Search directories recursively.
-l, --files-with-matches: Print only the names of matching files.
-E, --extended-regexp: Use ERE: + ? | ( ) without backslashes.
-F, --fixed-strings: Treat the pattern as a literal string.
-P: Perl-compatible regex (\d, \s, lookarounds).
-o, --only-matching: Print only the matched part of the line.
-A N, -B N, -C N: Print N lines after / before / around each match.
-w: Match whole words only.
-q: Quiet: exit 0 on the first match, print nothing. For if-tests.
-m, --max-count NUM: Stop after NUM matching lines: -m1 is "the first match only".
-Z, --null: After a file name print a NUL byte instead of a newline (pairs with xargs -0).
-q, --quiet: Print nothing; exit 0 at the first match. For if grep -q ....
-x, --line-regexp: Match only whole lines.
-F, --fixed-strings: The pattern is plain text, not a regular expression.

$ grep -c ' 502 ' /var/log/nginx/access.log

how many 502s

$ sudo grep -B2 -A2 'upstream timed out' /var/log/nginx/error.log

the lines around each timeout

$ grep -oE '[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+' access.log | sort | uniq -c | sort -rn | head

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