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Programs like sort, tail, wc, grep, uniq read
some input, perform some simple transformation on it and write some output.
Such programs are called filters. Here we will briefly discuss some
other well known filters: tr for character transliteration, comm
for comparing files. The two most used
filters are sed, which stands for stream editor and awk,
named after its three authors. Both of these are generalizations of grep.
Most of this material is borrowed from ``Programming in the UNIX Environment''
by Kernighan and Pike .