fix(doesCommandExist): fix to exec.Command

based on [this lovely example](https://siongui.github.io/2018/03/16/go-check-if-command-exists/), you cannot directly call a built-in command in some systems, so you much call it in a round-about way.

This commit also changes the `TestDoesCommandExist`: it checks if the runtime os is darwin or linux before attempting to use the commands

cmd/print.go

@@ -81,7 +81,7 @@ func printToPager(w io.Writer, buf *bytes.Buffer) (err error) {
  }
  }
  }
- pager := exec.Command(envPager, "-R")
+ pager := exec.Command("/bin/sh", "-c", envPager, "-R")
  pager.Stdin = buf
  pager.Stdout = w
  err = pager.Run()
@@ -239,7 +239,7 @@ func doesCommandExist(cmdName string) bool {
  if cmdName == "" {
  return false
  }
- cmd := exec.Command("command", "-v", cmdName)
+ cmd := exec.Command("/bin/sh", "-c", "command -v "+cmdName)
  if err := cmd.Run(); err != nil {
  return false
  }

cmd/print_test.go

@@ -1,6 +1,7 @@
 package cmd
 
 import (
+ "runtime"
  "testing"
 )
 
@@ -29,10 +30,12 @@ func TestPrintByteInfo(t *testing.T) {
 }
 
 func TestDoesCommandExist(t *testing.T) {
- if doesCommandExist("ls") == false {
- t.Error("ls command does not exist!")
- }
- if doesCommandExist("ls111") == true {
- t.Error("ls111 command should not exist!")
+ if runtime.GOOS == "darwin" || runtime.GOOS == "linux" {
+ if doesCommandExist("ls") == false {
+ t.Error("ls command does not exist!")
+ }
+ if doesCommandExist("ls111") == true {
+ t.Error("ls111 command should not exist!")
+ }
  }
 }