Gawk is the GNU version of the commonly available UNIX awk program, another popular stream editor. Since the awk program is often just a link to gawk, we will refer to it as awk.
The basic function of awk is to search files for lines or other text units containing one or more patterns. When a line matches one of the patterns, special actions are performed on that line.
Programs in awk are different from programs in most other languages, because awk programs are "data-driven": you describe the data you want 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 read and write.
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, loops, conditions and other programming constructs, advanced features that we will ignore for now.) Each rule specifies one pattern to search for and one action to perform upon finding the pattern.
There are several ways to run awk. If the program is short, it is easiest to run it on the command line:
awk PROGRAM inputfile(s)
If multiple changes have to be made, possibly regularly and on multiple files, it is easier to put the awk commands in a script. This is read like this:
awk -f PROGRAM-FILE inputfile(s)
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