我在CentOS7下安装Python3之后踩坑：

[root@localhost ~]# pip3 install httpbin
[root@localhost ~]# pip3 install gunicorn
[root@localhost ~]# pip list
Package      Version 
------------ --------
...
gunicorn     19.9.0  
httpbin      0.7.0   
...
[root@localhost ~]# 
[root@localhost ~]# gunicorn
-bash: gunicorn: 未找到命令
[root@localhost ~]# 

解决办法

新建虚拟环境，在虚拟环境里面：

[root@localhost bin]# mkvirtualenv Py3_dev
Using base prefix '/usr/local/python3'
New python executable in /root/.virtualenvs/Py3_dev/bin/python3.7
Also creating executable in /root/.virtualenvs/Py3_dev/bin/python
Installing setuptools, pip, wheel...
done.
virtualenvwrapper.user_scripts creating /root/.virtualenvs/Py3_dev/bin/predeactivate
virtualenvwrapper.user_scripts creating /root/.virtualenvs/Py3_dev/bin/postdeactivate
virtualenvwrapper.user_scripts creating /root/.virtualenvs/Py3_dev/bin/preactivate
virtualenvwrapper.user_scripts creating /root/.virtualenvs/Py3_dev/bin/postactivate
virtualenvwrapper.user_scripts creating /root/.virtualenvs/Py3_dev/bin/get_env_details
(Py3_dev) [root@localhost bin]#
(Py3_dev) [root@localhost ~]# pip3 install httpbin
...
(Py3_dev) [root@localhost ~]# pip3 install gunicorn
...
(Py3_dev) [root@localhost ~]# gunicorn
usage: gunicorn [OPTIONS] [APP_MODULE]
gunicorn: error: No application module specified.
(Py3_dev) [root@localhost ~]# gunicorn -b :80 httpbin:app
[2019-03-24 23:52:27 +0800] [18686] [INFO] Starting gunicorn 19.9.0
[2019-03-24 23:52:27 +0800] [18686] [INFO] Listening at: http://0.0.0.0:80 (18686)
[2019-03-24 23:52:27 +0800] [18686] [INFO] Using worker: sync
[2019-03-24 23:52:27 +0800] [18690] [INFO] Booting worker with pid: 18690
[2019-03-24 23:53:11 +0800] [18686] [CRITICAL] WORKER TIMEOUT (pid:18690)
[2019-03-24 23:53:11 +0800] [18690] [INFO] Worker exiting (pid: 18690)
[2019-03-24 23:53:11 +0800] [18727] [INFO] Booting worker with pid: 18727
^C[2019-03-24 23:53:29 +0800] [18686] [INFO] Handling signal: int
[2019-03-24 23:53:29 +0800] [18727] [INFO] Worker exiting (pid: 18727)
[2019-03-24 23:53:30 +0800] [18686] [INFO] Shutting down: Master
(Py3_dev) [root@localhost ~]# 

后记

我也没搞清楚为啥只是在虚拟环境里面能成功。。。欢迎评论指出原因，感谢。

后记：
可以参考： Mac安装gunicorn成功，却无系统命令 https://blog.csdn.net/cq1113/article/details/78531688

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Mac安装gunicorn成功，却无系统命令 https://blog.csdn.net/cq1113/article/details/78531688