usage: generate [-run regexp] [file.go... | packages]

Generate runs commands described by directives within existing
files. Those commands can run any process but the intent is to
create or update Go source files, for instance by running yacc.

Go generate is never run automatically by go build, go get, go test,
and so on. It must be run explicitly.

Go generate scans the file for directives, which are lines of
the form,

    //go:generate command argument...

(note: no leading spaces and no space in "//go") where command
is the generator to be run, corresponding to an executable file
that can be run locally. It must either be in the shell path
(gofmt), a fully qualified path (/usr/you/bin/mytool), or a
command alias, described below.

Note that go generate does not parse the file, so lines that look
like directives in comments or multiline strings will be treated
as directives.

The arguments to the directive are space-separated tokens or
double-quoted strings passed to the generator as individual
arguments when it is run.

Quoted strings use Go syntax and are evaluated before execution; a
quoted string appears as a single argument to the generator.

Go generate sets several variables when it runs the generator:

    $GOARCH
        The execution architecture (arm, amd64, etc.)
    $GOOS
        The execution operating system (linux, windows, etc.)
    $GOFILE
        The base name of the file.
    $GOLINE
        The line number of the directive in the source file.
    $GOPACKAGE
        The name of the package of the file containing the directive.
    $DOLLAR
        A dollar sign.

Other than variable substitution and quoted-string evaluation, no
special processing such as "globbing" is performed on the command
line.

As a last step before running the command, any invocations of any
environment variables with alphanumeric names, such as $GOFILE or
$HOME, are expanded throughout the command line. The syntax for
variable expansion is $NAME on all operating systems.  Due to the
order of evaluation, variables are expanded even inside quoted
strings. If the variable NAME is not set, $NAME expands to the
empty string.

A directive of the form,

    //go:generate -command xxx args...

specifies, for the remainder of this source file only, that the
string xxx represents the command identified by the arguments. This
can be used to create aliases or to handle multiword generators.
For example,

    //go:generate -command yacc go tool yacc

specifies that the command "yacc" represents the generator
"go tool yacc".

Generate processes packages in the order given on the command line,
one at a time. If the command line lists .go files, they are treated
as a single package. Within a package, generate processes the
source files in a package in file name order, one at a time. Within
a source file, generate runs generators in the order they appear
in the file, one at a time.

If any generator returns an error exit status, "go generate" skips
all further processing for that package.

The generator is run in the package's source directory.

Go generate accepts one specific flag:

    -run=""
        if non-empty, specifies a regular expression to select
        directives whose full original source text (excluding
        any trailing spaces and final newline) matches the
        expression.

It also accepts the standard build flags -v, -n, and -x.
The -v flag prints the names of packages and files as they are
processed.
The -n flag prints commands that would be executed.
The -x flag prints commands as they are executed.

For more about specifying packages, see 'go help packages'.