I kept getting this error when installing pow on macOS. There are some old threads about it, with fixes for previous verisons of OSX prior to El Capitan, but for the most part it seems to be a solved bug.

philihp@sterling ~$ curl get.pow.cx | sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  9039  100  9039    0     0   8763      0  0:00:01  0:00:01 --:--:--  8767
*** Installing Pow 0.5.0...
*** Installing local configuration files...
*** Installing system configuration files as root...
Password: hunter2

/Users/philihp/Library/Application Support/Pow/Versions/0.5.0/lib/command.js:20
throw err;
^
Error: ENOENT, open '/tmp/pow.98645.1466285627293.3185'

If you're getting this error too, perhaps the solution will be similar to mine?

It turned out that I had at some point changed the default shell of my root user to /usr/local/bin/fish, then uninstalled fish, and never bothered to set it back. I guess this isn't really a problem most of the time because not much should (hopefully) ever run as root, but it finally surfaced here when trying to install Pow.

My solution was the following:

  1. Enable the root user with dsenableroot
  2. If you try to sudo su, you will get something like su: /usr/local/bin/fish: No such file or directory
  3. Change root's shell with sudo chsh -s /bin/sh
  4. Disable the root user with dsenableroot -d

Once I did this, installing worked as normal.

philihp@sterling ~$ curl get.pow.cx | sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  9039  100  9039    0     0  17142      0 --:--:-- --:--:-- --:--:-- 17151
*** Installing Pow 0.5.0...
*** Installing local configuration files...
*** Starting the Pow server...
*** Performing self-test...
*** Installed

For troubleshooting instructions, please see the Pow wiki:
https://github.com/basecamp/pow/wiki/Troubleshooting

To uninstall Pow, `curl get.pow.cx/uninstall.sh | sh`