Some examples of how to work with variables in bash.
### Vars expansion with ${} ###
## Default value
# If a variable is not set a default value will be used. If it's set,
# its value is used
echo ${NAME:-Pepe}
# -> Pepe
NAME="Juanje"
echo ${NAME:-Pepe}
# -> Juanje
## Assign a default value
# Assign a value to a var, only if it already has one
NAME=""
echo $NAME
# ->
echo ${NAME:=Pepe}
# -> Pepe
echo $NAME
# -> Pepe
echo ${NAME:=Juanje}
# -> Pepe
## Show an error if variable does not exist (some vars exists but are empty)
# you can set a customized message
echo ${X?}
# -> -bash: X: parameter null or not set
echo ${X?La variable X no exite}
# -> -bash: X: La variable X no exite
X=""
echo ${X?La variable X no exite}
# ->
X="Algo"
echo ${X?La variable X no exite}
# -> Algo
## Use an alternative value, if var already exists and has a value
echo ${Y:+Alternative value}
# ->
Y=""
echo ${Y:+Alternative value}
# ->
Y="Some value"
echo ${Y:+Alternative value}
# -> Alternative value
## Substrings
# :{start}:{size}
# If no size specified, a substring from {start}, to end
# first char is 0
TEXT="Un texto de ejemplo"
echo ${TEXT:3}
# -> texto de ejemplo
echo ${TEXT:3:5}
# -> texto
## Substring substracting from beginning
# With one # first ocurrence of what comes after # will be substracted
B="blablabla..."
echo ${B#bla}
# -> blabla...
echo ${B#*bla}
# -> blabla...
# With 2 # longer strings will be substracted
echo ${B##bla}
# -> blabla...
echo ${B##*bla}
# -> ...
# Another example:
D="/srv/chroot/var/chroot/etc/apache"
echo ${D#*chroot}
# -> /var/chroot/etc/apache
echo ${D##*chroot}
# -> /etc/apache
## Substring from end
# With one % first ocurrence of what comes after # will be substracted
B="blablabla...blablabla"
echo ${B%bla}
# -> blablabla...blabla
echo ${B%bla*}
# -> blablabla...blabla
# With 2 % longer strings will be substracted
echo ${B%%bla}
# -> blablabla...blabla
echo ${B%%bla*}
# ->
# Example:
D="/srv/chroot/var/chroot/etc/apache"
echo ${D%chroot*}
# -> /srv/chroot/var/
echo ${D%%chroot*}
# -> /srv/
## Variables names starting with a prefix
echo ${!U*}
# -> UID USER
echo ${!B*}
# -> B BASH BASH_VERSINFO BASH_VERSION
echo ${!BASH*}
# -> BASH BASH_VERSINFO BASH_VERSION
## Number of chars of variable value
X="Un texto cualquiera"
echo ${#X}
# -> 19
N=22435
echo ${#N}
# -> 5
# Sustituir una cadena
a=/etc/kung/foo
echo ${a/foo/fu}
# -> /etc/kung/fu