The shell is a wonderful friend. If you have read the rest of the book up to this point, you may well be dizzy with the possibilities it presents. But the shell is still tremendously limited compared to many languages. We'll give you just a taste of other tools and languages you can explore.
Two classic tools, AWK and Sed, are commonly invoked from the shell. Each operates on input one line at a time. You can think of them as assembly lines on which workers load a file line by line. Each line is processed in order. These are classic examples of filters, an idea closely associated with GNU/Linux. Filters are strung together in pipes with the | character, with the output of each command becoming the input for the next.
The next section introduces regular expressions. They're a language all their own, but one where you can do a lot by learning a few simple features. They turn up in very similar forms all over: in text editors such as vi, in commands such as
grep, in AWK and Sed, and in all the languages that follow.
Scripting languages were invented to make programming easy and allow people to create applications quickly. Unlike AWK and Sed, they usually run by themselves, not as part of pipes or other contexts where they just produce a line of output for each line of input. In contrast to the shell, these languages offer such advantages as:
This book has short sections on the three most popular scripting languages in the free software world today: Perl, Python, and Ruby. You will encounter many tools and products that provide customization through those languages.
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