/etc/rc.local
script is executed by the "root" user

if you want to execute commands as another user:
su - pi /do/something/as/oracle/user

make bash script:

!/bin/bash

sleep 20
iceweasel

make it executable:
chmod +x myscript.sh

A; B Run A and then B, regardless of success of A
A && B Run B if A succeeded
A || B Run B if A failed
A & Run A in background.

A; B Run A and then B, regardless of success of A
A && B Run B if A succeeded
A || B Run B if A failed
A & Run A in background.

change for all users:

nano /etc/bash.bashrc
export TIME_STYLE=long-iso

bash.bashrc -- sysem-wide .bashrc file for interactive bash shells