To my surprise there apparently hasn't been support for bash command
line completion for the gem command. I've cobbled together a file that
add just that. See

http://schuerig.de/michael/blog/index.php/2007/02/24/gem-bash-complet...

I've only tested it on Debian/Linux, on other Linux/Unix variants things
might work somewhat differently. Also, I don't even try to do
completion for gem names; doing that for remote gems would be much to
slow and I haven't looked into getting that information from the local
source_cache.

Michael

  set gemcommands=(build cert check cleanup repository contents \
      dependency environment help install list outdated pristine \
      query rdoc search sources specification uninstall unpack \
      update repository)
  complete gem 'p/1/$gemcommands/' 'n/help/$gemcommands/' \
               'p%*%D:/usr/local/lib/ruby/gems/1.8/gems/% %' 'n/*/f:*/'

I haven't bothered with option completion, but of course that can be
added with a similar effort to yours.

Remote gems should indeed be doable; it would mostly need a fast command
to dump a list of gem names, or have something write them to a text file
beforehand.  If the full list is too big for that to be reasonable, the
plaintext index could be split by prefix.

Thanks for the script.

I've had a go at hacking in gem name completion for install and
uninstall.  For uninstall it uses installed names, for install it uses
the names in the source cache.  Only tested with gem 0.9.1.

Regards,
George.

----------------------------------------------------------------------

# Michael Schuerig, mich...@schuerig.de, 2007-02-24
# George Ogata, george.og...@gmail.com, 2007-03-01
# Free for all uses.

GEM_CACHE_FILE=`ruby -rubygems -rrubygems/source_info_cache \
                 -e 'print Gem::SourceInfoCache.new.cache_file'`
GEM_AVAILABLE_FILE=`dirname $GEM_CACHE_FILE`/source_cache_names

#
# Output all available gem names.
#
function available_gems {
    if [ ! -e $GEM_AVAILABLE_FILE -o \
         $GEM_CACHE_FILE -nt $GEM_AVAILABLE_FILE ]; then
        ruby <<EOF > $GEM_AVAILABLE_FILE
            require 'rubygems/source_info_cache_entry'
            text = File.read('$GEM_CACHE_FILE')
            names = Marshal.load(text).values.map do |entry|
              entry.source_index.latest_specs.keys
            end.flatten
            puts names.uniq
EOF
    fi

    cat $GEM_AVAILABLE_FILE

    COMPREPLY=()
    cur=${COMP_WORDS[COMP_CWORD]}
    prev=${COMP_WORDS[COMP_CWORD-1]}

    COMMANDS='build cert check cleanup contents dependency\
      environment help install list query rdoc search specification\
      uninstall unpack update'

    COMMON_OPTIONS='\
      --source
      -p --http-proxy --no-http-proxy\
      -h --help\
      --config-file\
      --backtrace\
      --debug'

    CERT_OPTIONS='\
      -a -add\
      -l --list\
      -r --remove\
      -b --build\
      -C --certificate\
      -K --private-key\
      -s --sign'

    CHECK_OPTIONS='\
      -v --verify\
      -a --alien\
      -t --test\
      -V --version'

    CLEANUP_OPTIONS='\
      -d --dry-run'

    CONTENTS_OPTIONS='\
      -l --list\
      -V --version\
      -s --spec-dir\
      -v --verbose'

    DEPENDENCY_OPTIONS='\
      -v --version\
      -r --reverse-dependencies --no-reverse-dependencies\
      -p --pipe'

    HELP_OPTIONS=$COMMANDS

    INSTALL_OPTIONS='\
      -v --version\
      -l --local\
      -r --remote\
      -b --both\
      -i --install-dir\
      -B --build-root\
      -d --rdoc --no-rdoc\
      --ri --no-ri\
      -f --force --no-force\
      -t --test --no-test\
      -w --wrappers --no-wrappers\
      -P --trust-policy\
      --ignore-dependencies\
      -y --include-dependencies'

    LIST_OPTIONS='\
      -d --details --no-details\
      -l --local\
      -r --remote\
      -b --both'

    QUERY_OPTIONS='\
      -n --name-matches\
      -d --details --no-details\
      -l --local\
      -r --remote\
      -b --both'

    RDOC_OPTIONS='\
      --all\
      --rdoc --no-rdoc\
      --ri --no-ri\
      -v --version'

    SEARCH_OPTIONS='\
      -d --details --no-details\
      -l --local\
      -r --remote\
      -b --both'

    SPECIFICATION_OPTIONS='\
      -v --version\
      -l --local\
      -r --remote\
      -b --both\
      -all'

    UNINSTALL_OPTIONS='\
      -a --all --no-all\
      -i --ignore-dependencies --no-ignore-dependencies\
      -x --executables --no-executables\
      -v --version'

    UNPACK_OPTIONS='\
      -v --version'

    UPDATE_OPTIONS='\
      -i --install-dir\
      -B --build-root\
      -d --rdoc --no-rdoc\
      -ri --no-ri\
      -f --force --no-force\
      -t --test --no-test\
      -w --wrappers --no-wrappers\
      -P --trust-policy\
      --ignore-dependencies\
      -y --include-dependencies\
      --system'

    case "${prev}" in
      build)
        completions="$COMMON_OPTIONS $BUILD_OPTIONS"
        ;;
      cert)
        completions="$COMMON_OPTIONS $CERT_OPTIONS"
        ;;
      check)
        completions="$COMMON_OPTIONS $CHECK_OPTIONS"
        ;;
      cleanup)
        completions="$COMMON_OPTIONS $CLEANUP_OPTIONS"
        ;;
      contents)
        completions="$COMMON_OPTIONS $CONTENTS_OPTIONS"
        ;;
      dependency)
        completions="$COMMON_OPTIONS $DEPENDENCY_OPTIONS"
        ;;
      environment)
        completions="$COMMON_OPTIONS $ENVIRONMENT_OPTIONS"
        ;;
      help)
        completions="$COMMON_OPTIONS $HELP_OPTIONS"
        ;;
      install)
        completions="$COMMON_OPTIONS $INSTALL_OPTIONS `available_gems`"
        ;;
      list)
        completions="$COMMON_OPTIONS $LIST_OPTIONS"
        ;;
      query)
        completions="$COMMON_OPTIONS $QUERY_OPTIONS"
        ;;
      rdoc)
        completions="$COMMON_OPTIONS $RDOC_OPTIONS"
        ;;
      search)
        completions="$COMMON_OPTIONS $SEARCH_OPTIONS"
        ;;
      specification)
        completions="$COMMON_OPTIONS $SPECIFICATION_OPTIONS"
        ;;
      uninstall)
        completions="$COMMON_OPTIONS $UNINSTALL_OPTIONS `installed_gems`"
        ;;
      unpack)
        completions="$COMMON_OPTIONS $UNPACK_OPTIONS"
        ;;
      update)
        completions="$COMMON_OPTIONS $UPDATE_OPTIONS"
        ;;
      *)
        completions="$COMMANDS $COMMON_OPTIONS"
        ;;
    esac

    COMPREPLY=( $( compgen -W "$completions" -- $cur ))
    return 0