I assist with a server running RHEL v4. It has runlevel 3 set as default in the inittab.

On occasion, I like to switch to runlevel 5 to run gui tools. I do this by running 'init 5' . When I am done, I run 'init 3'. I see processes being started and/or killed.

My question is this: does the runlevel change via init cause any interruption to the application this server is running? It seems that there is no interruption to any service.

For a very brief period of time (way less then .5 seconds) yes

You should know that typing 'startx' minus the single quotemarks is a better way to do the samething as 'init 5' / 'init 3' bit.

It should not. It will only affect services that are scheduled to change status (ie. those that weren't started in the current runlevel, that should be in the new runlevel and vice versa).

What happens when you change runlevel is that init calls the /etc/rc.d/rc script* (with the new runlevel as the argument) which looks in the appropriate rc.d directories to determine which scripts have already been run for the current runlevel, and which scripts need to be started / stopped in the new runlevel. It then runs the appropriate start / stop scripts.

* This is the name of the script in RedHat / Fedora (different distribution may use different names).

Unlike in some unix operating systems only those in the new runlevel will be run. Some UNIX operating systems will also run scripts that are between (e.g. moving from 3 to 5 will first go to 4 and then 5). This is not the case for Linux.

If a service does not change status when you change runlevel then it won't be affected.

I'd second Lenard's suggestion to use `startx` instead which is guaranteed to work in any distro.