brew/Library/Contributions/brew_bash_completion.sh

# Bash completion script for brew(1)
#
# To use, edit your .bashrc and add:
#   source `brew --prefix`/Library/Contributions/brew_bash_completion.sh

_brew_to_completion()
{
    local cur="${COMP_WORDS[COMP_CWORD]}"

    # Subcommand list
    [[ ${COMP_CWORD} -eq 1 ]] && {
        local actions="--cache --cellar --config --env --prefix --repository audit cat cleanup
            configure create deps diy doctor edit fetch help home info install link list log options
            outdated prune remove search test uninstall unlink update upgrade uses versions"
        local ext=$(\ls $(brew --repository)/Library/Contributions/examples 2> /dev/null |
                    sed -e "s/\.rb//g" -e "s/brew-//g")
        COMPREPLY=( $(compgen -W "${actions} ${ext}" -- ${cur}) )
        return
    }

    # some flags take arguments
    # kind of pointless to use a case statement here, but it's cleaner
    # than a bunch of string comparisons and leaves room for future
    # expansion
    case "${COMP_WORDS[1]}" in
    # flags that take a formula
    --cache|--cellar|--prefix)
        local ff=$(\ls $(brew --repository)/Library/Formula 2> /dev/null | sed "s/\.rb//g")
        local af=$(\ls $(brew --repository)/Library/Aliases 2> /dev/null | sed "s/\.rb//g")
        COMPREPLY=( $(compgen -W "${ff} ${af}" -- ${cur}) )
        return
        ;;
    esac

    # Find the previous non-switch word
    local prev_index=$((COMP_CWORD - 1))
    local prev="${COMP_WORDS[prev_index]}"
    while [[ $prev == -* ]]; do
        prev_index=$((--prev_index))
        prev="${COMP_WORDS[prev_index]}"
    done

    # handle subcommand options
    if [[  "$cur" == --* ]]; then
        case "${COMP_WORDS[1]}" in
        audit)
            local opts=$([[ "${COMP_WORDS[*]}" =~ "--strict" ]] || echo "--strict")
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        cleanup)
            local opts=$([[ "${COMP_WORDS[*]}" =~ "--force" ]] || echo "--force")
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        create)
            local opts=$(
                local opts=(--autotools --cmake --no-fetch)
                for o in ${opts[*]}; do
                    [[ "${COMP_WORDS[*]}" =~ "$o" ]] || echo "$o"
                done
            )
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        deps)
            local opts=$(
                local opts=(--1 --all)
                for o in ${opts[*]}; do
                    [[ "${COMP_WORDS[*]}" =~ "$o" ]] || echo "$o"
                done
            )
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        diy|configure)
            local opts=$(
                local opts=(--set-version --set-name)
                for o in ${opts[*]}; do
                    [[ "${COMP_WORDS[*]}" =~ "$o" ]] || echo "$o"
                done
            )
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        fetch)
            local opts=$(
                local opts=(--deps --force --HEAD)
                for o in ${opts[*]}; do
                    [[ "${COMP_WORDS[*]}" =~ "$o" ]] || echo "$o"
                done
            )
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        info|abv)
            local opts=$(
                local opts=(--all --github)
                for o in ${opts[*]}; do
                    [[ "${COMP_WORDS[*]}" =~ "$o" ]] || echo "$o"
                done
            )
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        install)
            local opts=$(
                local opts=(--force --verbose --debug --use-clang --use-gcc
                    --use-llvm --ignore-dependencies --build-from-source --HEAD
                    --interactive $(brew options --compact "$prev"))

                # options that make sense with '--interactive'
                if [[ "${COMP_WORDS[*]}" =~ "--interactive" ]]; then
                    opts=(--force --git --use-clang --use-gcc --use-llvm --HEAD)
                fi

                for o in ${opts[*]}; do
                    [[ "${COMP_WORDS[*]}" =~ "$o" ]] || echo "$o"
                done
            )
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        list|ls)
            local opts=$(
                local opts=(--unbrewed --verbose --versions)

                # the three options for list are mutually exclusive
                for o in ${opts[*]}; do
                    if [[ "${COMP_WORDS[*]}" =~ "$o" ]]; then
                        opts=()
                    else
                        echo "$o"
                    fi
                done
            )
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        options)
            local opts=$(
                local opts=(--all --compact --installed)
                for o in ${opts[*]}; do
                    [[ "${COMP_WORDS[*]}" =~ "$o" ]] || echo "$o"
                done
            )
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        outdated)
            local opts=$([[ "${COMP_WORDS[*]}" =~ "--quiet" ]] || echo "--quiet")
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        search)
            local opts=$(
                local opts=(--fink --macports)
                for o in ${opts[*]}; do
                    [[ "${COMP_WORDS[*]}" =~ "$o" ]] || echo "$o"
                done
            )
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        uninstall|remove|rm)
            local opts=$([[ "${COMP_WORDS[*]}" =~ "--force" ]] || echo "--force")
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        update)
            local opts=$(
                local opts=(--rebase --verbose)
                for o in ${opts[*]}; do
                    [[ "${COMP_WORDS[*]}" =~ "$o" ]] || echo "$o"
                done
            )
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        uses)
            local opts=$([[ "${COMP_WORDS[*]}" =~ "--installed" ]] || echo "--installed")
            COMPREPLY=( $(compgen -W "$opts" -- ${cur}) )
            return
            ;;
        esac
    fi

    case "$prev" in
    # Commands that take a formula
    cat|deps|edit|fetch|home|homepage|info|install|log|missing|options|uses|versions)
        local ff=$(\ls $(brew --repository)/Library/Formula 2> /dev/null | sed "s/\.rb//g")
        local af=$(\ls $(brew --repository)/Library/Aliases 2> /dev/null | sed "s/\.rb//g")
        COMPREPLY=( $(compgen -W "${ff} ${af}" -- ${cur}) )
        return
        ;;
    # Commands that take an existing brew
    abv|cleanup|link|list|ln|ls|remove|rm|test|uninstall|unlink)
        COMPREPLY=( $(compgen -W "$(\ls $(brew --cellar))" -- ${cur}) )
        return
        ;;
    # Commands that take an outdated brew
    upgrade)
        COMPREPLY=( $(compgen -W "$(brew outdated --quiet)" -- ${cur}) )
        return
        ;;
    esac
}

complete -o bashdefault -o default -F _brew_to_completion brew