Format a Go string without printing?

Sprintf is what you are looking for.

Example

fmt.Sprintf("foo: %s", bar)

You can also see it in use in the Errors example as part of "A Tour of Go."

return fmt.Sprintf("at %v, %s", e.When, e.What)

1. Simple strings

For "simple" strings (typically what fits into a line) the simplest solution is using fmt.Sprintf() and friends (fmt.Sprint(), fmt.Sprintln()). These are analogous to the functions without the starter S letter, but these Sxxx() variants return the result as a string instead of printing them to the standard output.

For example:

s := fmt.Sprintf("Hi, my name is %s and I'm %d years old.", "Bob", 23)

The variable s will be initialized with the value:

Hi, my name is Bob and I'm 23 years old.

Tip: If you just want to concatenate values of different types, you may not automatically need to use Sprintf() (which requires a format string) as Sprint() does exactly this. See this example:

i := 23
s := fmt.Sprint("[age:", i, "]") // s will be "[age:23]"

For concatenating only strings, you may also use strings.Join() where you can specify a custom separator string (to be placed between the strings to join).

Try these on the Go Playground.

2. Complex strings (documents)

If the string you're trying to create is more complex (e.g. a multi-line email message), fmt.Sprintf() becomes less readable and less efficient (especially if you have to do this many times).

For this the standard library provides the packages text/template and html/template. These packages implement data-driven templates for generating textual output. html/template is for generating HTML output safe against code injection. It provides the same interface as package text/template and should be used instead of text/template whenever the output is HTML.

Using the template packages basically requires you to provide a static template in the form of a string value (which may be originating from a file in which case you only provide the file name) which may contain static text, and actions which are processed and executed when the engine processes the template and generates the output.

You may provide parameters which are included/substituted in the static template and which may control the output generation process. Typical form of such parameters are structs and map values which may be nested.

Example:

For example let's say you want to generate email messages that look like this:

Hi [name]!

Your account is ready, your user name is: [user-name]

You have the following roles assigned:
[role#1], [role#2], ... [role#n]

To generate email message bodies like this, you could use the following static template:

const emailTmpl = `Hi {{.Name}}!

Your account is ready, your user name is: {{.UserName}}

You have the following roles assigned:
{{range $i, $r := .Roles}}{{if $i}}, {{end}}{{.}}{{end}}
`

And provide data like this for executing it:

data := map[string]interface{}{
    "Name":     "Bob",
    "UserName": "bob92",
    "Roles":    []string{"dbteam", "uiteam", "tester"},
}

Normally output of templates are written to an io.Writer, so if you want the result as a string, create and write to a bytes.Buffer (which implements io.Writer). Executing the template and getting the result as string:

t := template.Must(template.New("email").Parse(emailTmpl))
buf := &bytes.Buffer{}
if err := t.Execute(buf, data); err != nil {
    panic(err)
}
s := buf.String()

This will result in the expected output:

Hi Bob!

Your account is ready, your user name is: bob92

You have the following roles assigned:
dbteam, uiteam, tester

Try it on the Go Playground.

Also note that since Go 1.10, a newer, faster, more specialized alternative is available to bytes.Buffer which is: strings.Builder. Usage is very similar:

builder := &strings.Builder{}
if err := t.Execute(builder, data); err != nil {
    panic(err)
}
s := builder.String()

Try this one on the Go Playground.

Note: you may also display the result of a template execution if you provide os.Stdout as the target (which also implements io.Writer):

t := template.Must(template.New("email").Parse(emailTmpl))
if err := t.Execute(os.Stdout, data); err != nil {
    panic(err)
}

This will write the result directly to os.Stdout. Try this on the Go Playground.