pip install＆＃34;解析错误＆＃34;在bash脚本中运行时

Question

我有一个bash脚本来做pip安装几个包。

#!/bin/bash
packages="pycparser cffi ipaddress enum34 cryptography asn1crypto"
pip install ${packages}

我看到了这个错误：

Invalid requirement: 'pycparser cffi ipaddress enum34 cryptography asn1crypto' Traceback (most recent call last):   File "/usr/local/lib/python2.7/dist-packages/pip/_internal/req/req_install.py", line 252, in from_line
    req = Requirement(req)   File "/usr/local/lib/python2.7/dist-packages/pip/_vendor/packaging/requirements.py", line 97, in __init__
    requirement_string[e.loc:e.loc + 8])) InvalidRequirement: Invalid requirement, parse error at "'cffi ipa'"

当我在终端上运行命令pip install pycparser cffi ipaddress enum34 cryptography asn1crypto时，一切都很好。

有人可以帮忙吗？感谢。