Checking if a ruby gem is installed from bash script

I need to, from a bash script, check to see if certain Ruby gems are installed .

I thought I could do something like

if ! gem list <name>; then do_stuff; fi

but testing on the command line using echo $? shows that gem list <name> returns 0 regardless of if name is actually found.

Does this mean I have to use grep to filter the output of gem list, or is there a better way I can check to see if a gem is installed?

gem list <name> -i will return the string true if the gem is installed and false otherwise. Also, the return codes are what you would expect.

For more informations, see gem help list.

Edit: @Riateche correctly observed that this might give false positives if you search for a gem name that is a substring of an otherwise installed gem. To avoid this, use a regex syntax:

gem list '^<name>$' -i

(Example: gem list '^mini$' -i).

It looks as if the gem spec command will fail with an error if the named gem is not installed. So:

if ! gem spec <name> > /dev/null 2>&1; then
  echo "Gem <name> is not installed!"
fi

I don't know if this is the canonical way of solving this, but it works.

You can also use the query sub-command to the gem command.

$ gem query -i -n bundler 
true

$ gem help query
Usage: gem query [options]

  Options:
    -i, --[no-]installed             Check for installed gem
    -I                               Equivalent to --no-installed
    -v, --version VERSION            Specify version of gem to query
                                     for use with --installed
    -n, --name-matches REGEXP        Name of gem(s) to query on matches the
                                     provided REGEXP

Will also look for specific versions as well, using the -v switch.

$ gem query -i -n bundler -v 1.6.2.1
false