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CSC 4630

CSC 4630. Meeting 2 January 22, 2007. Filters. Definition: A filter is a program that takes a text file as an input and produces a text file as an output. UNIX context Write filters to use stdin as the input file and stdout as the output file.

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CSC 4630

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  1. CSC 4630 Meeting 2 January 22, 2007

  2. Filters • Definition: A filter is a program that takes a text file as an input and produces a text file as an output. • UNIX context • Write filters to use stdin as the input file and stdout as the output file. • Use pipe to connect filters. Notation is the vertical bar |

  3. Filter History • Originally conceived by M. D. McIlroy in the early 1970’s • The UNIX notation for pipeline, the vertical bar, was introduced by K. L. Thompson. • “The Unix time-sharing system,” Comm. ACM, July 1974. • “The Unix programming environment,” Software Practice and Experience, January 1979.

  4. Filter Examples • detab -- replaces tab characters in a text file with the appropriate number of space characters • entab -- replaces long strings of space characters with the appropriate number of tab characters • compress – replaces long strings of the same character with a coding for the string and its length

  5. Filter Examples (2) • expand -- reverses the action of the compress filter • translit – in simplest form, takes two argument strings of equal length and changes all occurrences of elements of first string into corresponding elements of second string. • Example: translit abc xyz changes all a’s to x’s , b’s to y’s and c’s to z’s.

  6. Simple Filters: head and tail • Use head to look at the first few lines of a file • Keeps the first n lines of input and discards the rest head [-n] [file] • Use tail to look at the last few lines of a file • Keeps the last n lines of input and discards the rest tail [-n] [file] • Discards the first n-1 lines of input and keeps the rest tail [+n] [file]

  7. tr Command • trtranslates characters • Two arguments, given as strings • Three options, c d s • Often used for letter case conversion • Useful for “cleaning up” formatted output

  8. tr Examples • tr A-Z a-z Converts all upper case letters to lower case • tr –s <tab><space> <space><space> • tr –s ‘\033\010’ ‘\010\010’ • tr –d <CR>^Z • tr –d ‘\015\032’ • tr –cs A-Za-z ‘\012’

  9. tr Examples (2) • tr a-b-d abcd • tr a-c xyz • tr -ac xyz • tr \\ • tr –c \ x • tr \c x

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