grep 'string' file                 # lines containing string
grep -i 'string' file              # ignore case
grep -n 'string' file              # show line numbers
grep -v 'string' file              # invert: lines NOT matching
grep -c 'string' file              # count matching lines
grep -w 'run' file                 # whole-word match
grep -F 'a.b(c)' file              # fixed string, no regex
grep -E 'foo|bar' file             # extended regex (alternation, +, ?)
grep -o '[0-9]\+' file             # print only the matching part
grep -l 'string' *                 # only names of files that match
grep -L 'string' *                 # names of files that do not
grep -A 4 'string' file            # 4 lines of trailing context
grep -B 4 'string' file            # 4 lines of leading context
grep -C 4 'string' file            # both

grep -rn 'TODO' src/                          # recursive with line numbers
grep -riI 'string' .                          # ignore case, skip binary files
grep -r --include='*.py' 'import os' .        # only some files
grep -r --exclude-dir=.git --exclude='*.min.js' 'apiKey' .
grep -rl '^#!/.*bash' .                       # find all bash scripts

grep -v -e '^#' -e '^$' file.conf
grep -Ev '^\s*(#|$)' file.conf            # also skips indented comments and whitespace-only lines

grep '^string' file                 # at the start of a line
grep 'string$' file                 # at the end
grep '^string$' file                # lines consisting only of "string"
grep '^$' file                      # blank lines
grep '[Ss]tring' file               # String or string
grep 'B[oO][bB]' file               # BOB, Bob, BOb, BoB
grep '[0-9][0-9]' file              # two digits in a row
grep '[^a-zA-Z0-9]' file            # lines with anything other than letters/digits
grep '[0-9]\{3\}-[0-9]\{4\}' file   # 555-1234 style
grep -E '[0-9]{3}-[0-9]{4}' file    # same, extended syntax
grep '^.$' file                     # lines with exactly one character
grep '"string"' file                # with double quotes (single-quote the pattern)
grep '"*string"*' file              # with or without quotes
grep '^\.' file                     # lines starting with a dot
grep '^\.[a-z][a-z]' file           # a dot then two lowercase letters
grep '\<50\>' file                 # the number 50 only (not 150, 502, ...)
grep '\<the\>' file                # the word "the" only (not other, othello)
grep 'run[- ]time' *.txt            # "run time" or "run-time"
grep -E 'sec_|ice_' /var/log/secure # either string