os.Getenv and os.LookupEnv don't return the value of $HISTFILE, $HISTSIZE, and $SAVEHIST

I'm using zsh, and I'm trying to access these environment variables:

$ echo $HISTFILE
/home/amir/.zsh_history
$ echo $HISTSIZE
50000
$ echo $SAVEHIST
10000

But os.LookupEnv returns "", false for all three, and os.Getenv, naturally, returns an empty string:

package main

import (
	"fmt"
	"os"
)

func main() {
	histfile, found := os.LookupEnv("HISTFILE")
	if found {
		fmt.Println(histfile)
	} else {
		fmt.Println("$HISTFILE not found")
	}

	histsize, found := os.LookupEnv("HISTSIZE")
	if found {
		fmt.Println(histsize)
	} else {
		fmt.Println("$HISTSIZE not found")
	}

	savehist, found := os.LookupEnv("SAVEHIST")
	if found {
		fmt.Println(savehist)
	} else {
		fmt.Println("$SAVEHIST not found")
	}
}

$ go run main.go
$HISTFILE not found
$HISTSIZE not found
$SAVEHIST not found

Now, if I export these variables in $HOME/.zshrc:

$ grep -iE "histfile|histsize|savehist" $HOME/.zshrc
export HISTFILE="$HOME/.zsh_history"
export HISTSIZE=1000000
export SAVEHIST=1000000

Then it works and correct values are returned:

$ go run hyst.go
/home/amir/.zsh_history
1000000
1000000

When I haven't explicitly exported these variables, the values of $HISTSIZE and $SAVEHIST are different, but they're not empty. So why do these functions return empty strings for these variables?

That's because $HISTFILE, $HISTSIZE, and $SAVEHIST are not, by default, environment variables, but rather simply shell variables set by oh-my-zsh:

## History file configuration
[ -z "$HISTFILE" ] && HISTFILE="$HOME/.zsh_history"
[ "$HISTSIZE" -lt 50000 ] && HISTSIZE=50000
[ "$SAVEHIST" -lt 10000 ] && SAVEHIST=10000

These are the values you see when you use echo before using export in $HOME/.zshrc.

It's important to make a distinction between environment and shell variables:

• Environment variables are accessible in child processes, but shell variables are not. You can verify this statement by simply creating a child process:

  $ # Child processes don't inherit shell variables
  $ key=value; sh -c 'echo "key=$key"'
  key=
  
  $ # They do, however, inherit environment variables
  $ export key=value; sh -c 'echo "key=$key"'
  key=value
  

• To be able to access a shell variable in the child process, you can export it, just as you did in $HOME/.zshrc:

  $ key=value; export key; sh -c 'echo "key=$key"'
  key=value
  

Now, to see if a specific variable is an environment variable or not, instead of using echo, you can directly check the list of environment variables.

$ env | grep -i "shell"; echo $?
SHELL=/usr/bin/zsh
0

So $SHELL is, in fact, an environment variable. Now for the history variables:

$ env | grep -iE "histfile|histsize|savehist"; echo $?
1

The exit code is 1, meaning that it failed to find these names in the list of environment variables.