Metacharacters and Wildcard

• Metacharacters
  • Special characters that possess special meaning to the shell.
  • Used in pattern matching (a.k.a. filename expansion or file globbing) and regular expressions. dollar sign ($)
• mark the end of a line
• Used in regular expressions

caret (^)

• mark the beginning of a line
• Used in regular expressions

period (.)

• Match a single position
• Used in regular expressions

asterisk (*)

• used in pattern matching
• wildcard character
• Matches zero to an unlimited number of characters except for the leading period (.) in a hidden filename.
• Used in regular expressions

question mark (?)

• Used in pattern matching
• Wildcard character
• Matches exactly one character except for the leading period in a hidden filename.
• Used in regular expressions

pipe (|)

• Send the output of one command as input to the next.
• Also used to define alternations in regular expressions.
• Can use as many times in a command as you need. (pipeline) - Can be used as an OR operator (alternation)
  • this|that|other

angle brackets (< >)

• Redirections

curly brackets ({})

• Used in regular expressions
• Match an element a specific number of times

square brackets ([])

• Used in pattern matching
• Wildcard character
• Match either a set of characters or a range of characters for a single character position.
• Order in which they are listed has no importance.
• Range of characters must be specified in a proper sequence such as [a-z] or [0-9].
• Used in regular expressions

parentheses (())

• Create a sub shell

plus (+)

• Match a character one or more time

exclamation mark (!)

• inverse matches

semicolon (;)

• Run a second command after the ;

Quoting Mechanisms

• Disable special meaning of metacharacters

Backslash (\)

• Escape character
• Cancel out a special character’s meaning.

single quotation (‘’)

• Mask the meaning of all encapsulated special characters.

double quotation (“”)

• Mask the meaning of all but the backslash (), dollar sign ($), and single quotes (‘’).

Regular expressions (regexp or regex)

• Text pattern or an expression that is matched against a string of characters in a file or supplied input in a search operation.
• Pattern may include a single character, multiple random characters, a range of characters, word, phrase, or an entire sentence.
• Any pattern containing one or more white spaces must be surrounded by quotation marks.

grep command

• Searches the contents of one or more text files or input supplied for a match.

Flags -i

• case insensitive search

-n

• number the lines

-v

• exclude these lines

-w

• find an exact match for a word

-E

• match one or the other

-e

• -Use patterns for matching