Using Parameter Expansion in POSIX Shell

In POSIX shell scripts, we can use parameter expansion to reference a variable

message="hello"
printf '%s\n' "${message}"

The usefulness of this syntax is in its variations that handle variables when null/not null:

Form	Meaning
`${var:-word}`	use word if var unset/null
`${var:=word}`	use word and set var to word if var unset/null
`${var:+word}`	use word if var set and not null
`${var:?word}`	exit with error and print word if var unset/null

Another useful set of variations involve prefix/suffix deletion of a variable's value:

Form	Meaning
`${var#pat}`	remove the smallest prefix of var matching pattern pat
`${var##pat}`	remove the largest prefix of var matching pattern pat
`${var%pat}`	remove the smallest suffix of var matching pattern pat
`${var%%pat}`	remove the largest suffix of var matching pattern pat

Examples:

#!/bin/sh
#
# baz.sh

var=

printf 'var: "%s"\n' "$var"
printf 'using fallback value "%s"\n' "${var:-"some_fallback_value"}"
printf 'var: "%s"\n' "$var"
printf 'setting var to value "%s"\n' "${var:="some_value"}"
printf 'var: "%s"\n' "$var"
printf 'using "%s" because var is set\n\n' "${var:+"another_value"}"

var='/duck/duck/duck/goose/goose/goose'

printf 'original var:            %s\n' "$var"
printf 'smallest prefix removed: %s\n' "${var#*/duck}"
printf 'largest prefix removed:  %s\n' "${var##*/duck}"
printf 'smallest suffix removed: %s\n' "${var%/goose*}"
printf 'largest suffix removed:  %s\n\n' "${var%%/goose*}"

var='/home/user/.local/bin/baz.sh'

printf 'filepath: %s\n' "$var"
printf 'dirname:  %s\n' "${var%/*}"
printf 'basename: %s\n\n' "${var##*/}"

var=

printf 'this should not print: "%s"\n' "${var:?"var is unset or null"}"

Output:

$ ./baz.sh
var: ""
using fallback value "some_fallback_value"
var: ""
setting var to value "some_value"
var: "some_value"
using "another_value" because var is set

original var:            /duck/duck/duck/goose/goose/goose
smallest prefix removed: /duck/duck/goose/goose/goose
largest prefix removed:  /goose/goose/goose
smallest suffix removed: /duck/duck/duck/goose/goose
largest suffix removed:  /duck/duck/duck

filepath: /home/user/.local/bin/baz.sh
dirname:  /home/user/.local/bin
basename: baz.sh

./baz.sh: 30: var: var is unset or null
$ printf 'exit code: %s\n' $?
exit code: 2

The Open Group Base Specifications Issue 7 (2.6.2 Parameter Expansion)