prefix_a=one
prefix_b=two
echo ${!prefix_*}  # all variables names starting with `prefix_`
# => prefix_a prefix_b

${Var,}

${Var^}

str="HELLO WORLD!"
echo "${str,}"   #=> "hELLO WORLD!" (lowercase 1st letter)
echo "${str,,}"  #=> "hello world!" (all lowercase)

str="hello world!"
echo "${str^}"   #=> "Hello world!" (uppercase 1st letter)
echo "${str^^}"  #=> "HELLO WORLD!" (all uppercase)

${foo:+val}

${foo:-val}

{*.csv *.tsv}

echo ${foo:-'default'}

echo "$variable" # => Some string
echo '$variable' # => $variable
# When you use a variable itself — assign it, export it, or else — you write
# its name without $. If you want to use the variable's value, you should use $.
# Note that ' (single quote) won't expand the variables!
# You can write variable without surrounding quotes but it's not recommended.

echo ''Text with single quote\'. Text with double quote\"''

length=7
echo "${variable:0:length}" # => Some st
# This will return only the first 7 characters of the value
echo "${variable: -5}" # => tring
# This will return the last 5 characters (note the space before -5).
# The space before minus is mandatory here.

file="Some/path/to/file.txt"
echo "Extension: ${file##*.}"
echo "Name sans extension: ${file%.*}"
echo "New extension: ${file%txt}gif"

${FOO/%from/to}

a="Hello"
b="world"

echo $a", "$b"!"
echo "$a, $b!"
echo "${a}, ${b}!"

str=$a
str+=", "
str+=$b
str+="!"
echo $str