golang rand.Int(). Why every time same values?

I have a code:

package main

import ("fmt"
	"math/rand"
	"strconv"
)

func main() {
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
    fmt.Println(strconv.Itoa(rand.Int()))
}

When I run it (go run code.go), I get every time same values:

5577006791947779410
8674665223082153551
6129484611666145821
4037200794235010051
3916589616287113937
6334824724549167320
605394647632969758
1443635317331776148
894385949183117216
2775422040480279449
4751997750760398084
7504504064263669287
1976235410884491574
3510942875414458836

Second try:

package main

import ("fmt"
        "math/rand"
        "strconv"
)

func main() {
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
    fmt.Println(strconv.Itoa(rand.Intn(100)))
}

Same behaviour. Every time's

81
87
47
59
81
18
25

<s>Is this a joke? Why it happens?
Here is no description about non-random same result.
I can see only pseudo-random term without explanation what that means.

^{Looks like even bash is more logical and stable...}
</s>

This is a C way

You need to seed it. It says right in the docs

> Random numbers are generated by a Source. Top-level functions, such as Float64 and Int, use a default shared Source that produces a deterministic sequence of values each time a program is run. Use the Seed function to initialize the default Source if different behavior is required for each run.

Generally

rand.Seed(time.Now().UnixNano())

You don't appear to have called Seed for math/rand before using the generator.

If Seed is not called, the generator behaves as if seeded by Seed(1). That is not a joke - actually for a PRNG to be deterministic and repeatable is desirable in many cases.

For different numbers, seed with a different value, such as time.Now().UnixNano().

Addition to the other answers, good explanation found about this in medium article go-how-are-random-numbers-generated

> Go implements two packages to generate random numbers:
>
> - a pseudo-random number generator (PRNG) in the package math/rand cryptographic
> - pseudorandom number generator (CPRNG), implemented in crypto/rand
>
> If both generate random numbers, your choice will be based on a
> tradeoff between genuinely random numbers and performance.

As other answers explain, math/rand packages populate random numbers reading from a source. So if you need random numbers, you need to set random seed calling rand.Seed()

In other case you can use crypto/rand package to generate random numbers. It generates random numbers which can not be deterministic. But performance is bit lower than math/rand package.

I have added sample code below for that. you can run and see different out put here.

package main

import (
	"crypto/rand"
	"math/big"
)

func main() {
	for i := 0; i < 4; i++  {
		n, _ := rand.Int(rand.Reader, big.NewInt(100))
		println(n.Int64())
	}
}