设置变量时找不到bash命令

我正在编写一个shell脚本,我设置的变量很少,其值是命令的输出.

我得到的错误是:

$  $tag_name="proddeploy-$(date +"%Y%m%d_%H%M")"
-bash: =proddeploy-20141003_0500: command not found

现在,我确实阅读了其他类似的问题并在此基础上尝试了各种各样的事情:

将命令分成两个调用

$ $deploy_date=date +"%Y%m%d_%H%M"
bash: =date: command not found
$ $tag_name="proddeploy-$deploy_date"
bash: proddeploy- command not found

尝试使用反引号

$ $tag_name=`proddeploy-$(date +"%Y%m%d_%H%M")`
bash: proddeploy-20141003_1734: command not found
bash: =: command not found

试过用 $()

$ $tag_name=$(proddeploy-$(date +"%Y%m%d_%H%M"))
bash: proddeploy-20141003_1735: command not found
bash: =: command not found

但在每种情况下,命令输出都会被执行.如何让它停止执行命令输出并只存储为变量？我需要这个在ZSH和BASH上工作.