If-else and error handing in command prompt - shell

Question

I am trying to detect if certain ruby gem is not installed, install it first and then proceed.
For instance:
gem which rails
returns path to gem's driver file (.rb) on stdout, if gem exists, if not it checks throws error on stderr.
How can I solve it?
Pseudo code:
if throws "gem which example1" ( gem install example1 )
if throws "gem which example2" ( gem install example2 )
if throws "gem which example3" ( gem install example3 )
:: install other stuff
Either cmd or powershell would work.
I need to add these in AppVeyor CI yaml configuration file. AppVeyor CI has this wonderful feature of caching the directory for DRY'ing installations to improve build performance. So I have cached the gems directory and it restores fine on running the build, but then gem install reinstalls the gem anyway!

Answer 1

By using powershell to run the command you can use error redirection to check if the command was successful:
$gemPath = gem which example3 2>$null
if ($gemPath)
{
# We got a path for the gem driver
}
else
{
# Failed, gem might not be installed
}
Explanation: We're telling powershell to store the result of "gem which example3" in the variable $gemPath, however if there is an error we want to redirect the error to the variable $null by using 2>$null
You can also replace 2 with the number corresponding to the stream you'd like to capture (Success, Error, Warning etc)
* All output
1 Success output
2 Errors
3 Warning messages
4 Verbose output
5 Debug messages
More on redirection here About_Redirection

Answer 2

Try, in .cmd script:
gem which rails 1>nul 2>&1
IF ERRORLEVEL 1 (gem install rails)
I'm assuming here you want the outputs to stdout and stderr stifled for purposes of this batch script - otherwise, drop the 1>nul 2>&1.