/usr/share/bash-completion/completions/ninja is in ninja-build 1.8.2-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 | # Copyright 2011 Google Inc. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# Add the following to your .bashrc to tab-complete ninja targets
#   . path/to/ninja/misc/bash-completion
_ninja_target() {
    local cur prev targets dir line targets_command OPTIND
    # When available, use bash_completion to:
    #   1) Complete words when the cursor is in the middle of the word
    #   2) Complete paths with files or directories, as appropriate
    if _get_comp_words_by_ref cur prev &>/dev/null ; then
        case $prev in
            -f)
                _filedir
                return 0
                ;;
            -C)
                _filedir -d
                return 0
                ;;
        esac
    else
        cur="${COMP_WORDS[COMP_CWORD]}"
    fi
    if [[ "$cur" == "--"* ]]; then
        # there is currently only one argument that takes --
	COMPREPLY=($(compgen -P '--' -W 'version' -- "${cur:2}"))
    else
	dir="."
	line=$(echo ${COMP_LINE} | cut -d" " -f 2-)
        # filter out all non relevant arguments but keep C for dirs
	while getopts :C:f:j:l:k:nvd:t: opt $line; do
	    case $opt in
                # eval for tilde expansion
		C) eval dir="$OPTARG" ;;
	    esac
	done;
	targets_command="eval ninja -C \"${dir}\" -t targets all 2>/dev/null | cut -d: -f1"
	COMPREPLY=($(compgen -W '`${targets_command}`' -- "$cur"))
    fi
    return
}
complete -F _ninja_target ninja
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