awk
The basic function of awk is to search files for lines (or other
units of text) that contain certain patterns. When a line matches one
of the patterns, awk performs specified actions on that line.
awk keeps processing input lines in this way until the end of the
input files are reached.
Programs in awk are different from programs in most other languages,
because awk programs are data-driven; that is, you describe
the data you wish to work with, and then what to do when you find it.
Most other languages are procedural; you have to describe, in great
detail, every step the program is to take. When working with procedural
languages, it is usually much
harder to clearly describe the data your program will process.
For this reason, awk programs are often refreshingly easy to both
write and read.
When you run awk, you specify an awk program that
tells awk what to do. The program consists of a series of
rules. (It may also contain function definitions,
an advanced feature which we will ignore for now.
See section User-defined Functions.) Each rule specifies one
pattern to search for, and one action to perform when that pattern is found.
Syntactically, a rule consists of a pattern followed by an action. The
action is enclosed in curly braces to separate it from the pattern.
Rules are usually separated by newlines. Therefore, an awk
program looks like this:
pattern { action }
pattern { action }
...
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