Searches input for a match to a pattern defined by a regular expression.

Processes 1 line at a time.

Expects text based data, lines terminated in new-line character

Input most commonly a file or list of files but can also be provided using the pipe delimiter or input redirection.

Output is to standard out, so output redirection commonly used to preserve found match.

Output consists of whole lines containing the matched string.

• -i, --ignore-case - ignore case.
  This is a global action.
  grep -i "\<dog\>" - will match Dog, dog, dOg, DOG, doG, etc.
  To look for a word that is upper lower case, use regular expression :
  grep "\<[Dd]og\>" - will match Dog or dog
  
  
• -f FILE, --file=FILE
  Applies multiple regular expressions defined in specified file.
  First true match outputs line. No duplicate lines.
  Helpful if using a lot of wild-cards or searching for quotes.

• -e regex - state expression[s] on command line.
  By default, grep takes a single expression.
  Multiple -e's can be used to state multiple expressions.
  Treated as OR condition, first true match outputs line. No duplicate lines.

• -v, --invert-match - NOT, output line if not true.
  Usually requires anchors to get good results.

• -w, --word-regexp - match on whole word.
  Saves users the trouble of using the word anchors, \< and \>

• -c, --count - Count matches.
  Doesn't show match, but give count of lines which contain match (or with -v, lines that don't match.)

• -l, --files-with-matches - lists files.
  Doesn't show matched lines, just lists file with match in them.

  Usually used on a list of files.

  grep -l "50GB hard drives" $HOME/Mail/*

• -L, --files-without-match - List files that don't contain the string described.

• -q (Posix), --quiet, --silent - Don't show output, but exit with success (rc=0) as soon as a match is located.

• -s, --no-messages - suppress error messages but show any matches found and set return code.

  * -q may cause portability problems.
  A safer way of coding is to redirect output and errors to /dev/null.
  grep "regex" file > /dev/null # no output.
  grep "regex" file 2> /dev/null # no errors.
  grep "regex" file > /dev/null 2>&1 # no output or errors.

• -n, --line-number - print line number of line with match.

local ask read -p "Continue : " ask if echo $ask | grep -e "^[Yy]\(es\)\?$" > /dev/null 2>&1 then echo "It's a yes" elif echo $ask | grep -e "^[Nn]o\?$" > /dev/null 2>&1 then echo "It's a no" else echo "Make up your mind" fi

Then try the following :

ask yes

ask no

ask maybe

ask Y

ask No