mjhjmtu

In awk, when is a function definition executed?

Is a function definition executed even before BEGIN ... is executed? Thanks.

This is easily determined; I got curious and decided to test the "worst-case" scenario first, by defining the function at the end of the awk scriptfile:

$ cat go.awk
BEGIN 
 print "The BEGIN block is executing"
 print reverse("The BEGIN block is executing")



 print reverse($0)


function reverse(str) 
 trs=""
 for(i=length(str); i > 0; i--) 
 trs=trs substr(str, i, 1);
 
 return trs

And with:

$ cat input
line one
line two
line three

... the result is:

$ awk -f go.awk < input
The BEGIN block is executing
gnitucexe si kcolb NIGEB ehT
eno enil
owt enil
eerht enil

You can see that the function was defined (the function "definition" was "executed") before the BEGIN block was entered, since the BEGIN block successfully called my user-defined reverse function.

To see that the function is not executed while it's being defined, simply insert a print statement inside the function and observe that there is no output from that statement until the function is called.

The POSIX specification for awk says, most pertinently:

A function can be referred to anywhere in an awk program; in particular, its use can precede its definition. The scope of a function is global.

The gawk man page implies something similar:

... Gawk reads the program text as if all the program-files and com‐
mand line source texts had been concatenated together. This is useful for
building libraries of AWK functions, without having to include them in each
new AWK program that uses them. It also provides the ability to mix library
functions with command line programs.

...

Gawk executes AWK programs in the following order. First, all variable
assignments specified via the -v option are performed. Next, gawk compiles
the program into an internal form. Then, gawk executes the code in the BEGIN
rule(s) (if any), and then proceeds ...