Purpose: to describe awk, an early Unix tiny language named after the initials of its authors, Aho, Weinberger, Kernighan.
See http://cm.bell-labs.com/cm/cs/awkbook/index.html which is a page for an official book by the creators of the language. See also news:comp.lang.awk . Another good resource for awk documentation is http://www.gnu.org/manual/gawk-3.1.0/gawk.html .
Programmers often come to the Tcl newsgroups asking how can I do this awk like operation in Tcl or how to invoke awk from exec. This is because Awk's ability to scan through a file and manipulate the contents pre-dates Perl's functionality to do this, and frankly awk's abilities, while cruder in many ways, are also simpler (simpler even than Tcl!).
The BOOK Mastering Regular Expressions covers regular expressions in perl, awk, and tcl.
For a Tcl variation on awk functionality, see owh - a fileless tclsh.