UNIX/LINUX grep command

Grep command in Unix/Linux is the short form of 'global search for the regular expression'.

The grep command is a filter that is used to search for lines matching a specified pattern and print the matching lines to standard output.

Syntax:

grep [options] [pattern] [file]

The pattern is specified as a regular expression. A regular expression is a string of characters that is used to specify a pattern matching rule. Special characters are used to define the matching rules and positions.

1. Anchor Characters: '^' and '$' at the beginning and end of the pattern are used to anchor the pattern to the start of the line, and to the end of the line respectively.

   Example : "^Name" matches all lines that start with the string "Name".

2. Wildcard Character: '.' Is used to match any character.

   Example : "^.$" will match all lines with any single character.

3. Escaped Characters: Any of the special characters can be matched as a regular character by escaping them with a '\'.

   Example: "$*" will match the lines that contain the string "$*".

4. Character Range: A set of characters enclosed in a '[' and ']' pair specify a range of characters to be matched.

   Example: "[aeiou]" will match all lines that contain a vowel. A hyphen can be used while specifying a range to shorten a set of consecutive characters. E.g. "[0-9]" will match all lines that contain a digit. A carat can be used at the beginning of the range to specify a negative range. E.g. "[^xyz]" will match all lines that do not contain x, y or z.

5. Repetition Modifier: A '*' after a character or group of characters is used to allow matching zero or more instances of the preceding pattern.

The grep command supports a number of options for additional controls on the matching:

• -i: performs a case-insensitive search.
• -n: displays the lines containing the pattern along with the line numbers.
• -v: displays the lines not containing the specified pattern.
• -c: displays the count of the matching patterns.
• -h: Display the matched lines, but do not display the filenames.
• -l: Displays list of a filenames only.
• -e exp: Specifies expression with this option. Can use multiple times.
• -f file: Takes patterns from file, one per line.
• -E: Treats pattern as an extended regular expression (ERE).
• -w: Match whole word
• -o: Print only the matched parts of a matching line, with each such part on a separate output line.

Examples:

• Match all lines that start with 'hello'. E.g: "hello world"

  $ grep "^hello" file1
  

• Match all lines that end with 'done'. E.g: "well done"

  $ grep "done$" file1
  

• Match all lines that contain any of the letters 'a', 'b', 'c', 'd' or 'e'.

  $ grep "[a-e]" file1
  

• Match all lines that do not contain a vowel

  $ grep "[^aeiou]" file1
  

• Match all lines that contain the word hello in upper-case or lower-case

  $ grep -i "hello"
  

• Inverting the pattern match : You can display the lines that are not matched with the specified search sting pattern using the -v option.

  $ grep -v "unix" file1

• Specifies expression with -e option. Can use multiple times :

  $ grep –e "Agarwal" –e "Aggarwal" –e "Agrawal" file1