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Not sure how I'd missed this, but we need ${foo//pattern/string} to
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Bash completion for the Nix and NixOS command line tools.

The aim is full completion support for every argument, option and option argument, as long as everything that's needed is available locally. For instance when accessing a nixpkgs repo through an url, and it has been previously downloaded, completion should be offered using the local copy. Any behavior which doesn't agree with the actual execution of the command is considered a bug. Issues are very welcome as I primarily use zsh and therefor won't catch that many bugs through daily usage.

A thank you goes out to Spencer Whitt who started nix-zsh-completions, as a lot of the boilerplate (options etc.) is taken from there.


For quick testing just source the _nix file: . _nix, and start tabbing.

Some arguments support several types of input, but due to bash's limited completion system only exposes one type at a time. For instance nix eval <tab> will give you the default completion which is attribute paths, but nix eval ./<tab> will give you file completion (as store paths are valid input). If you aren't getting file completion on an option or argument which support it when starting off with ./, ~/ or / please report it in an issue and it should be fixed promptly. Another example is nix run --file channel:<tab> which will complete channel names instead of files.

Completing attribute paths to packages

The preferred way to reference a package in Nix is by attribute path, not by name. Attribute paths look like this nixos.mplayer or nixos.gnome.gedit, where nixos is a collection of all packages. If <tab> results in eg. something like nixos you'll need to manually add a . to access the packages available in nixos.

When using nix-env it's best to always add the --attr or -A flag as nix-env defaults to looking up packages by name which aren't completed fully (this is a legacy problem, the new nix command only works on attribute paths by default).

Completion of attribute paths is context aware, so eg. nix-env -i -f some/path/ -A <tab> complete paths in ./some/path/default.nix. It will also pick up default.nix or shell.nix in the current directory for nix-build -A and nix-shell -A. Things like nix-shell -I nixpkgs=. -p <tab> is also supported.

Installation and dependencies

NixOS 18.09 or newer

Completion should work out of the box from 18.09 and onwards. If it's not working make sure that programs.bash.enableCompletion isn't set to false in /etc/nixos/configuration.nix.

Other distros

You need bash-completion setup correctly. Installing the bash-completion package with the native package manager should probably do the trick.

Then you can install nix-bash-completions from the cloned git repo with nix-env -i -f default.nix, or from nixpkgs eg. nix-env -f '<nixpkgs> -iA nix-bash-completions'.

Make sure that $XDG_DATA_DIRS includes ~/.nix-profile/share, which will tell bash-completion where to find the script when completion is done. Be careful though: make sure that $XDG_DATA_DIRS also includes your distribution's defaults (like /usr/local/share/:/usr/share/), or you may not be able to launch some applications from the console. Adding this to your .bashrc should work in general:

export XDG_DATA_DIRS="$HOME/.nix-profile/share:${XDG_DATA_DIRS:-/usr/local/share:/usr/share}"


In addition to setting up bash-completion and installing nix-bash-completions through nix-env, you'll need a newer version of Bash (version 4 or greater) as Apple refuses to ship GPL v3 licensed software.

You might also need to copy or link the installed files in ~/.nix-profile/share/bash-completion/completions/ to ~/.local/share/bash-completion/completions/ so bash-completion will know where to look for the completion script. Though I don't know if it's necessary or that it will work, as I don't have a mac. If someone is using this successfully on macOS and would like to share the necessary steps then I'll happily add it to the readme.


The script runs on top of _parse which is a bare bones implementation of zsh's _arguments with some minor modifications to the syntax, and a bunch of stuff not implemented. A brief description of the _parse syntax follows, for anyone interested in reading the code, or using _parse for other completion scripts.

_parse syntax

_parse takes as arguments specifications of options and normal arguments.

The spec looks like this at the moment ([] denotes optional syntax):

Argument spec:

  • :action

Argument specs tell _parse how to handle normal arguments (arguments not required by options). The first argument will be handled by the first :action passed to _parse, the second by the second argument spec passed, and so on. Actions starting with a * will handle all further arguments. _parse puts all the encountered normal arguments in the array $line for easy lookup when handling further completion.

There's two types of actions:

  • [*]->string
  • [*]_function

When completing an argument with a ->string action _parse will set $state to string, return 1 and hand over control to the calling function which then need to handle the actual completion.

The [*]_function_ spec causes _function to be called, making it handle completion. At the moment this isn't that useful as passing options to _function isn't supported. ->string actions together with a case statement is much more flexible.

An example using the different actions:

local state
local -a line
local -A opt_args opts
_parse ':_known_hosts' ':*->FILE' && return
case "$state" in
        COMPREPLY=($(compgen -f $cur))

Here the function _known_hosts will handle completion for the first argument and then all further arguments will complete files through the $state case.

The options spec have a few more pieces on top of actions:

  • [(pattern|pattern ...)][*]--option[:action[:action2] ... ]

The simplest case just being:

  • --option

Which tells _parse to add --option when completing options.

If the option is present on the command line further option completion will exclude options matching any of the patterns. By default --option will exclude itself. If we want an option to be repeatable we add the * prefix to the option.

Adding actions to an option spec tells _parse that the option takes arguments. These actions are specified in the exact same way as the argument spec. Option arguments can later be looked up in the $opt_args associative array using the option as a key. The presence of any option can be checked with ${opts[option]}.

A reduced version of nix-shell spec:

local state
local -a line
local -A opt_args opts
_parse ':*->FILE' '(--attr|-A)'{--attr,-A}':->ATTR_PATH' \
       '(--packages|-p|shell)'{--packages,-p}'' '':*->FILE-OR-PACKAGE') \
       && return
case "$state"
      if [[ "${opts[--packages]}" || "${opts[-p]}" ]];
        #Complete packages
        COMPREPLY=($(compgen -f $cur))

At the moment _parse only understands long options, eg. --option, and stackable short options, eg. -f. Syntax like -some-option is not supported.


  • Only the first short option is completed, but eg. nix-env -iA is recognized, and -i won't be offered on any new option completion. Completing stacked options is probably not easily doable in bash.
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