Bash Variable Offset and String Operators

$ r="this is stuff"
$ echo ${r:3}
$ echo ${r:5:2}

Note, ${varname:offset:length}


${varname:?message}  If varname exist and isn't null return value, else,
print var and message.

$ r="new stuff"
$ echo ${r:? "that's r for you"}
new stuff
$ unset r
$ echo ${r:? "that's r for you"}
bash: r:  that's r for you

${varname:+word}    If varname exist and not null return word. Else, return null.

${varname:-word}    If varname exist and not null return value. Else, return word.

Working with arrays in bash - bash arrays.

$ unset p
$ p=(one two three)

$ echo -e "${p[@]}"
one two three

or

$ echo -e "${p[*]}"
one two three

$ echo -e "${#p[@]}"
3

$ echo -e "${p[0]}"
one

$ echo -e "${p[1]}"
two