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

The newer way of doing conditios is double brackets ([[ ... ]]).

if [[ -o interactive && -t 0 ]]; then
	mesg y
fi

Conditional Expression Operators:
-b file            - Tests if file is a block special file.
-c file            - Tests if file is a character special file.
-d file            - Tests if file exists and is a directory.
-e file            - Tests if file exists.
-f file            - Tests if file exists and is an ordinary file.
-g file            - Tests if file exists and has its setgid bit set.
-k file            - Tests if file exists and has its sticky bit set.
-n string          - Tests if string is nonempty.
-o option          - Tests if option is turned on.
-p file            - Tests if file exists and is a named pipe (fifo).
-r file            - Tests if file exists and is readable.
-s file            - Tests if file exists and has a size greater than zero.
-t file descriptor - Tests if file descriptor is open and associated with a terminal device.
-u file            - Tests if file exists and has its setuid bit set.
-w file            - Tests if file exists and is writable.
-x file            - Tests if file exists and is executable.
-z string          - Tests if string is empty (length zero).
-G file            - Tests if file exists and is owned by the current group.
-L file            - Tests if file exists and is a symbolic link.
-O file            - Tests if file exists and is owned by the current user.
-S file            - Tests if file exists and is a socket.
file1 -ef file2    - Tests if the two filenames refer to the same file.
file1 -nt file2    - Tests if file1 is newer than file2.
file1 -ot file2    - Tests if file1 is older than file2.
string == pattern  - Tests if the string matches the pattern.
string != pattern  - Tests if the string doesn’t match the pattern.
string1 > string2  - Compares strings based on their ASCII values.
string1 < string2  - Compares strings based on their ASCII values.
string =~ regex    - Tests if the string matches the regular expression (bash 3 only).

[[ $PWD = $HOME/* ]]
	check if we are under home dir (no expansion is done at [[)

Comparing numbers:
[[ $val -ge 3 ]]
(( val >= 3 ))


case $TERM in
	(aixterm|iris-ansi*)
		bindkey ...
		;;
	(xterm|ddterm)
		bindkey ...
		;;
esac

[while|until] condition; do
	commands
done

for ((i=1;i<5;i++)); do
	a[$i]=$i
done

This is the same as a= ( {1..4} ).

repeat 5; p=${p#*/}

[break/continue] n
	break n inner loops (default is one).


Commenting just out one command using colon (:)

grep -q word file && : grep -q word otherfile && echo files both contain word
	here we commented out grep -q word otherfile

colon could be used for side-effects:
: ${VAR:?error: VAR not set}
: ${VAR:=default value}


Commands grouping:

{
	cmd1
	cmd2
} > logfile

{ sleep 3600; make; } &


Run commands in a subshell:

tar cvf - . | ( cd /somwhere/else; tart xvf - )