tl;dr: put set shell=bash in your .vimrc

I recently had to reinstall OS X after a catastrophic hard drive failure. I had most of my configuration on Github (which is something I highly recommend), luckily, so it was a pretty easy move thanks to that and some well-placed backups.

So, I pulled down my config, loaded the submodules, and threw it all in my ~/.vim directory. Everything went smooth as butter until I tried to run vim-ack. I had originally thought that it was an error with vim-ack, and tried alternate versions of vim-ack. But I still got an error. So I tried five different snapshots of MacVim. I still got the error. Then I tried copying a file with NERDTree, and I got another error. Delete and add worked, but not copy. Mystifying.

vim-ack error:

Searching ...
Error detected while processing function <SNR>21_Ack:
line   23:
E40: Can't open errorfile /var/folders/98/_ymf2wy554n6qrs32ptrwrl40000gn/T/vx96wXC/2
Press ENTER or type command to continue

NERDTree error:

Cannot execute shell /usr/local/bin/bash
Error detected while processing function <SNR>19_showMenu..30..47..NERDTreeCopyNode..54..112:
line    8:
E484: Can't open file /var/folders/98/_ymf2wy554n6qrs32ptrwrl40000gn/T/vx96wXC/3
Error detected while processing function <SNR>19_showMenu..30..47:
line    6:
E171: Missing :endif
Error detected while processing function <SNR>19_showMenu..30:
line   19:
E171: Missing :endif

I tried chowning the temp files; I tried unsetting TEMPDIR. Nothing worked.

After browsing page after page of non-helpful Google results, I came back to a three-year-old conversation that I had already been to twice. I noticed one little line in there:

set shell=bash

And thought "Well, I've tried everything else."

It worked.