Fix fish support

[kenji-miyake]

Although fish-shell support was implemented in #260, I think some parts are wrong.

1. Probably if-condition for _ARC_DEBUG in shell_integration.py is reversed
   • It seems to output to /dev/null when _ARC_DEBUG is set.
2. Fish Support section in README.rst is different from comments in register-python-argcomplete
   • README
     • register-python-argcomplete --shell fish ~/.config/fish/completions/my-awesome-script.fish
   • register-python-argcomplete
     • register-python-argcomplete --shell fish my-favourite-script.py > ~/.config/fish/my-favourite-script.py.fish

@kislyuk @volkov Could you please take a look? Thank you.

[volkov]

Looks good! There were bugs.

[volkov]

I think there is some difference in bash and fish around this place

When I run echo test 8>&1 9>&2 1>/dev/null 2>&1 in fish I see test in console output.
When I run same command in bash there is no output.

[evanunderscore]

The changes look correct to me. set -q returns the number of queried variables that were undefined, i.e. a return code of 0 means the variable was defined. I'm not able to reproduce the issue @volkov is seeing; echo test 8>&1 9>&2 1>/dev/null 2>&1 correctly outputs nothing for me (fish version 2.7.1).

[evanunderscore]

Thanks for the PR!

[volkov]

I'm not able to reproduce the issue @volkov is seeing; echo test 8>&1 9>&2 1>/dev/null 2>&1 correctly outputs nothing for me (fish version 2.7.1).

Look's like it's only 3.1.2 feature (or bug), also can't reproduce on 2.7.1, report it as fish-shell/fish-shell#7646

[evanunderscore]

Thanks for reporting that issue @volkov. If it turns out more changes are required in argcomplete, feel free to open a new issue and/or PR to address it.

[evanunderscore]

It looks like this issue may not be fixed any time soon in fish, so in the meantime you could choose to use the tempfile-based IPC mechanism we included for Git Bash on Windows: https://github.com/kislyuk/argcomplete/blob/9e2c6712c761d05f64f4d558a381d9be9fa13988/README.rst#git-bash-support