SQL1641N error on linux while running db2start using db2 express c on linux (LUW) but the file system is not mounted using nosuid
The files in /opt/ibm/db2/V10.5/ are not to be used for a specific instance.  When you run the db2icrt command, it copies a series of files and directories into ~db2inst1/sqllib, and sets the permissions accordingly.  (Some of the directories in ~db2inst1/sqllib will be soft links back to directories in /opt/ibm/db2/V10.5).  

For example, you should find that ~db2inst1/sqllib/adm/db2start has permissions set to 6555 (i.e. -r-sr-sr-x), owned by root, and have the group owner set to the instance owner's primary group.  

You should not be executing things from /opt/ibm/db2/V10.5/adm.  That is why the permissions of the files in that directory are set the way they are – /opt/ibm/db2/V10.5/adm/db2start should be 0444.

So, to start your instance, you should be executing ~db2inst1/sqllib/adm/db2start.

Be aware that you also need to initialize the db2inst1 user environment prior to starting the instance, so you should use the -i option for sudo to handle this:

sudo -i -u db2inst1 /home/db2inst1/sqllib/adm/db2start


One last note:  All of this configuration (except perhaps the db2start) can be done using a response file and db2setup.   db2_install has been deprecated, so using response files is the way to handle this going forward.