vars.txt

# variable substitutions                 Substrings
${FOO%suffix}   Remove suffix            ${FOO:0:3}       substring (position, length)
${FOO#prefix}   Remove prefix            ${FOO:(-3):3}    substring from the right
${FOO%%suffix}  Remove long suffix       Arguments:
${FOO##prefix}  Remove long prefix       $#               number of arguments
${FOO/from/to}  Replace first match      $*               all positional arguments as a single word
${FOO//from/to} Replace all              $@               all positional arguments as separate strings
${FOO/%from/to} Replace suffix           $1, $2 etc       first, second, .. argument
${FOO/#from/to} Replace prefix           $_               last argument of previous command
# mnemonic: suffix % comes after the $ (variable) on the keyboard,
#   the prefix # before it

# Case
${FOO,}         lowercase first character
${FOO,,}        lowercase whole value
${FOO^}         uppercase first character
${FOO^^}        uppercase whole value

${FOO:-val}     $FOO, or val if unset (or null)
${FOO:=val}     Set $FOO to val if unset (or null)
${FOO:+val}     val if $FOO is set (and not null)
${FOO:?message} Show error message and exit if $FOO is unset (or null)
# Omitting the : removes the (non)nullity checks, e.g. ${FOO-val} expands to val
#   if unset otherwise $FOO.