CS 497C

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CS 497C Introduction to UNIXLecture 28 -
Filters Using Regular Expressions grep and sed

• Chin-Chih Changchang_at_cs.twsu.edu

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grep Searching for a Pattern

• The sample database shown on Page 434 can be
  found in http//www.mhhe.com/engcs/compsci/das/dat
  a.mhtml
• grep is a filter used to search a file for a
  pattern.
• It scans a file for the occurrence of a pattern
  and, depending on the options used, displays
• Lines containing the selected pattern.

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grep Searching for a Pattern

• Lines not containing the selected pattern. (-v)
• Line numbers where the pattern occurs. (-n)
• Number of lines containing the pattern. (-c)
• Filenames where the pattern occurs. (-l)
• Its syntax treats the first argument as the
  pattern and the rest as filenames
• grep options pattern filename(s)
• grep sales emp.lst
• When you use multiword strings as the pattern,
  you must quote the pattern.

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grep Options

• grep neil obryan emp.lst
• The -c (count) option counts the occurrences.
• grep -c directory emp.lst
• The -n (number) option can be used to display the
  line numbers containing the pattern.
• grep -n marketing emp.lst

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grep Options

• The -l (list) option displays only the names of
  files where a pattern has been found.
• grep -l manager .lst
• The -i (ignore) option makes the match
  case-insensitive.
• To look for a pattern that begins with a hyphen,
  use the -e option.
• grep e -mtime /var/spool/cron/crontabs/

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grep Options

• In Linux, you can use the -e option to match
  multiple patterns.
• grep -e woodhouse -e wood emp.lst
• A regular expression is an ambiguous expression
  formed with some special and ordinary characters,
  which is expanded by a command to match more than
  one string.
• grep uses a regular expression to match a group
  of similar patterns.

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Regular Expressions

• A regular expression uses a character class that
  encloses a group of characters within a pair of
  rectangular brackets .
• grep wooddehouse emp.lst
• Regular expressions use the (caret) to negate
  the character class, while the shell use !
  (bang).
• A single nonalphabetic string is represented by
  a-zA-Z.

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Regular Expressions

• The (asterisk) matches the zero or more
  occurrences of the preceding character.
• grep wilcocxks emp.lst
• A . (dot) matches a single character. The shell
  use the ? character to indicate that.
• The dot along with the (.) signifies any
  number of characters, or none.
• grep p.woodhouse emp.lst

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Regular Expressions

• A pattern can be matched at the beginning of a
  line with a , and at the end with a .
• Because it is the command that interprets these
  characters, a regular expression should be quoted
  to prevent the shell from interfering.
• grep 2 emp.lst
• The . and lose their meanings when placed
  inside the character class. Then you need to
  escape these characters.

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egrep and fgrep The Other Members

• egrep extends greps capabilities. It uses to
  delimit multiple patterns.
• egrep woodhousewoodcock emp.lst
• fgrep accepts only fixed strings and is faster
  than the other two.
• Both commands support the -f (file) option to
  take such patterns from the file.
• fgrep f pat.lst emp.lst