CS 497C Introduction to UNIX Lecture 29: Filters Using Regular Expressions grep and sed

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CS 497C Introduction to UNIXLecture 29 -
Filters Using Regular Expressions grep and sed

• Chin-Chih Changchang_at_cs.twsu.edu

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Regular Expressions

• egreps extended set includes two special
  characters - and ?. They are often used in
  place of to restrict the matching scope.
• - matches one or more occurrences of the
  previous character.
• ? matches zero or one occurrence of the
  previous character.
• egrep true?man emp.lst

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Regular Expressions

• The , ( and ) can be used to search for multiple
  patterns.
• egrep wood(housecock) emp.lst
• sed is a multipurpose too which combines the work
  of several filters.
• Designed by Lee McMahon, it is derived from the
  ed line editor.
• sed is used to perform noniteractive operations.

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sed The Stream Editor

• sed has numerous features almost bordering on a
  programming language but its functions have been
  taken over by perl.
• Everything in sed is an instruction. An
  instruction combines an address for selecting
  lines with an action to be taken on them
• sed options address action file(s)
• The address and action are enclosed within single
  quotes.

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sed The Stream Editor

• The components of a sed instruction are shown as
  below
• sed 1, s/bold/BOLD/g foo
• address action
• You can have multiple instructions in a single
  sed command, each with its own address and action
  components.
• Addressing in sed is done in two ways
• By line number (like 3,7p).
• By specifying a pattern (like /From/p).

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Line Addressing

• In the first form, the address specifies either a
  single line or a set of two (3,7) to select a
  group of contiguous lines.
• The second one uses one or two patterns.
• In either case, the action (p, the print command)
  is appended to this address.
• You can simulate head -3 by the 3q instruction in
  which 3 is the address and q is the quit action.

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Line Addressing

• sed 3q emp.lst
• sed uses the p (print) command to print the
  output.
• sed 1,2p emp.lst
• By default, sed prints all lines on the standard
  output in addition to the lines affected by the
  action. So the addressed lines are printed twice.