How to use a command block to welcome new players on a server in Minecraft?

Solution 1:

if you have them spawn 3 blocks above a hidden command block (powered with a simple clock), you could have that command block do something along the lines of /testfor @p ~0 ~0 ~3 keep in mind that i'm not entirely certain if that's the specific thing to test for to check for players... then hook up a comparator from the first command block to a second one running the message command to @p ~0 ~3 ~3 or ~3 ~0 ~3 depending on which direction the comparator faces (and negative alterations would also be used as necessary)

Solution 2:

My best way to check first login independent on spawn position:

SIMPLE (one title):

1. /scoreboard teams add notFirstLogin
2. build simple clock f.e. 1 sec cyclus with block /testfor @a[team=]
3. result from testfor comparator goes to the welcome message (/title @a[team=]
4. after message put repeater with some delay and then command_block with: /scoreboard teams join notFirstLogin @a[team=]

logic of this proccess:

• player without team is first time on the server
• all players have some team except new guys
• if you have more complicated welcome (more than one title @a[team=] title ...) is necessary put there some waiting values:

COMPLICATED (more titles)

1. /scoreboard teams add notFirstLogin

   /scoreboard objectives add W8ing4Welcome dummy

   /scoreboard objectives add ShowWelcome dummy

2. build simple clock f.e. 1 sec cyclus with two cmd_blocks:

3. /scoreboard players set W8ing4Welcome @a[team=] 1

   3.x: ... others things that are necessary call at first login
   
   3.before_end: 
       small repeater delay 
   
   3.end:
       /scoreboard teams join notFirstLogin @a[team=]
   

4. /testfor @a[score_W8ing4Welcome_min=1] ... here connect all welcomes in series:

   4.1. logical circuit AND within /testfor @a[score_ShowWelcome_min=1] 
   ... checking that here is NOBODY with ShowWelcome=1 ... this is important to prevent covering of messages
   4.2. /scoreboard playes set @a[score_W8ing4Welcome_min=1] ShowWelcome 1
   4.3.  small repeater delay
   4.4. /scoreboard playes set  @a[score_ShowWelcome_min=1] W8ing4Welcome 0
   4.5.  small repeater delay
   4.6. /title @a[score_ShowWelcome_min=1] title ...
   4.7.  small repeater delay
   4.8. /title @a[score_ShowWelcome_min=1] title ...
   4.x  ....
   4.end. /scoreboard playes set @a[score_ShowWelcome_min=1] ShowWelcome 0