First of all install steam, or the Half Life Dedicated Server (a.k.a. hldsupdatetool, see This site)
chmod +x if necessary. It will put steam in the current directory.
Check the tool is at the last version with
./steam (multiple times if necessary, until it displays the help message).
Then the install a game just do well with
./steam -command update -game "left4dead2" -dir .
(It will install l4d2 in the current directory, creating
Hint: the game names can be changed to another game. See the list with
./steam -command list
Bonus: run your server in screen (within the good directory)
screen -d -m -S l4d2-server ./srcds_run -console -game left4dead2 -log on -hostip 88.xxx.x.x -hostport 27016 -clientport 27006 -nohltv +exec server.cfg +map c5m1_waterfront -autoupdate