Bash Parameter Expansions

=====

Basic

$ NAME="Matrix"

$ echo $NAME
Matrix

$ echo "$NAME"
Matrix

$ echo "${NAME}"
Matrix

Indirection

$ food="Cake"

$ Cake="Cup cake"

$ echo "${!food}"
Cup cake

Variable Name Expansion

This expands to a list of all set variable names beginning with the string PREFIX. The elements of the list are separated by the first character in the IFS-variable ( by default).

$ test1="Matrix1"

$ test2="Matrix2"

$ test3="Matrix3"

$ echo "${!test@}"
test1 test2 test3

$ echo "${!test*}"
test1 test2 test3

NOTE: When @ is used and the expansion appears within double quotes, each variable name expands to a separate word.

$ cat <<EOF > test.sh
echo $1
EOF

$ ./test.sh "${!test@}"
test1

$ ./test.sh "${!test*}"
test1 test2 test3

Default Value / Alternative Value

Use a default value

${var:-DEFAULT}
${var-DEFAULT}
var is unset (never was defined) or null (empty), this one expands to DEFAULT, otherwise it expands to the value of var, as if it just was ${var}. If you omit the : (colon), like shown in the second form, the default value is only used when the var was unset, not when it was empty.

$ unset food

$ echo ${food-Cake}
Cake

$ echo ${food:-Cake}
Cake

$ food=""

$ echo ${food-Cake}


$ echo ${food:-Cake}
Cake

$ food="Bread"

$ echo ${food-Cake}
Bread

$ echo ${food:-Cake}
Bread

Assign a default value

${var:=DEFAULT}
${var=DEFAULT}
This one works like the using default values, but the default text you give is not only expanded, but also assigned to the var, if it was unset or null. Equivalent to using a default value, when you omit the :(colon), as shown in the second form, the default value will only be assigned when the var was unset.

$ unset food

$ echo ${food=Cake}
Cake

$ echo ${food}
Cake

$ unset food

$ food=""

$ echo ${food=Cake}


$ echo ${food:=Cake}
Cake

$ echo ${food}
Cake

Use an alternate value

${var:+WORD}
${var+WORD}
This form expands to nothing if the var is unset or empty. If it is set, it does not expand to the var’s value, but to WORD. For the second form, expand to WORD only when var is empty.

$ unset foo

$ echo "${foo:+bread}"


$ echo "${foo+bread}"


$ unset foo; foo="Cake"

$ echo "${foo+bread}"
bread

$ echo "${foo:+bread}"
bread

$ unset foo; foo=""

$ echo "${foo+bread}"
bread

$ echo "${foo:+bread}"

Display error if null or unset

${var:?WORD}
${var?WORD}
If var is unset or empty, the expansion of WORD will be used as appendix for an error message. The second form is only apply to unset.

$ unset foo

$ echo "${foo?not set}"
-bash: foo: not set

$ echo "${foo}"


$ echo "${foo:?not set}"
-bash: foo: not set

$ foo=""

$ echo "${foo:?not set}"
-bash: foo: not set

$ echo "${foo?not set}"

Parameters Expansion Matrix