Bash initialization

When you first login, bash reads these initialization files in order (if they exist):

/etc/profile -- systemwide profile applies to all users

Then, it looks for these files and executes the FIRST one it finds:

~/.bash_profile
~/.bash_login
~/.profile

For interactive non-login shells, it executes:

~/.bashrc

At logout, it looks for this file to execute:

~/.bash_logout

The amazing technicolor multiline bash prompt

Set the PS1 environment variable in bash to customize the prompt. This is the prompt I use. It works best with a black background. If you want to keep it, add it to one of your bash startup files (like .bashrc).
PS1='\[\e[32;1m\]\u@\h \[\e[33;1m\][\w]\n\[\e[36;1m\]\t\[\e[0m\] $ '

Built-in shell variables:

$# number of command line arguments
$? exit value of last command
$$ process ID of current process
$! process ID of last background process $0 command name
$n where n=1-9 are the 1st thru 9th command line arguments
$* all command line arguments
$@ all command line arguments, individually quoted ($1 $2 ...)

If statement

if condition ; then
  commands
elif condition ; then
  commands
else
  commands
fi

Test the return status of the previous command:

if [ $? == 0 ] ; then
  commands
fi

Loops

while condition; do
  commands
done

for var in list; do
  commands
done

for (( expr1; expr2; expr3 )); do
  commands
done

Case statements

The case statement can be used in place of a complex if statement:
case expression in
pattern)
  commands
   ;;
pattern)
  commands
   ;;
*
  commands
esac

Traps

Bash scripts can trap signals to handle error processing better or unexpected events (like the user killing the script).

This traps signal(s) and executes "command" instead:
trap "command" signal [signal ...]

You can list active traps with:
trap -p

You can reset traps with:
trap - signal [signal ...]