code: async function resolveExecutablePath (cmd) {
    let executablePath;
    try {
      executablePath = await fs.which(cmd);
      if (executablePath && await fs.exists(executablePath)) {
        return executablePath;
      }
    } catch (err) {
      if ((/not found/gi).test(err.message)) {
        log.debug(err);
      } else {
        log.warn(err);
      }
    }
    log.debug(`No executable path of '${cmd}'.`);
    if (executablePath) {
      log.debug(`Does '${executablePath}' exist?`);
    }
    return null;
  }
docs: Return an executable path of cmd
  
  @param {string} cmd Standard output by command
  @return {?string} The full path of cmd. `null` if the cmd is not found.