If Service Exists Condition

You can't do this in DOS, as DOS is not Windows and doesn't even have the concept of a "service".

In a Windows batch file you can use the sc command to find services:

sc query | findstr SERVICE_NAME

This will enumerate all services and yield their respective names.

You can search for a specific service with

sc query | findstr /C:"SERVICE_NAME: myservice"

Remember that this search is case-sensitive. You can add the /I switch to findstr to avoid that.

Off the top of my head, you can check if a specific service is running, as mentioned by bmargulies, using the "net" command, piping the result into "find". Something like the following would check if a service was running, and if so stop it. You can then start it without worrying about if it was already running or not:

net start | find "SomeService"
if ERRORLEVEL 1 net stop "SomeService"
net start "SomeService"

If you're using findstr to do a search, as some of the other answers have suggested, then you would check for ERRORLEVEL equal to 0 (zero)... if it is then you have found the string you're looking for:

net start | findstr "SomeService"
if ERRORLEVEL 0 net stop "SomeService"
net start "SomeService"

Essentially most DOS commands will set ERRORLEVEL, allowing you to check if something like a find has succeeded.