如果安裝db2後忘記實例，那就是默認的用戶名，用戶名也忘記了，到/home/下查找，
新安裝db2後，如果客戶端連接報錯：ERRROCODE=-4499，SQLSTATE=08001
官方三個原因：
DB2 is stopped.
The wrong DB2 database listening port number is specified.
The wrong database name is specified
執行db2start命令可以啓動服務
db2與其他的數據庫不同，需要執行命令創建數據庫後才能執行客戶端連接
查詢db2的端口號 db2 get dbm dfg
如果查詢實例下的數據庫信息，切換到該實例用戶執行：db2 list db directory
其他相關報錯（轉載）：
Error 2: DB2 return code is -4222
For this problem, the host name is probably specified incorrectly.
Error 3: DB2 return code is -4214
Three problems might be the cause of this error.
The user name is invalid
The password is invalid.
Both the user name and the password are invalid.
Resolving the problem
The following procedures might help you resolve the connection failure problem:
Resolving Error 1: DB2 return code is -4499
Check whether DB2 is started.
Check whether the database exists. Use the command
db2 list db directory to list the databases.
Check the DB2 listening port. The DB2 instance is listening on Port 50000 by default, but the port number might have been modified.
Restart DB2 or restart the computer on which DB2 is running, because there might be too many connections already active for DB2 to respond to the new request.
Resolving Error 2: DB2 return code is -4222
If the wrong host name is specified, or the DNS server cannot parse the host name, then you can use the command ping hostname, where hostname is the host name, to test the connection. You can also use the IP address directly in this command.
Check the privileges of folder /tmp/CM/db2cmv8. The DB2 Content Manager installer performs the su command to DB2 Content Manager accounts to test the connection. For example, if you have an icmadmin account as the library server administrator, the installer
uses this account, creates a shell script under the /tmp/CM/db2cmv8 folder, and runs it to see the result. If the folder does not have the write and execute privileges for icmadmin, the connection test fails.
Resolving Error 3: DB2 return code is -4214
Check that the user name and password are specified correctly.
Check that the password does not contain special characters, such as the dollar sign ($).