substring expansion

${1:0:1}: This is a substring expansion. It extracts a substring from $1, starting from the 0th character (the first character) and with a length of 1 character.

param expansion

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Note: we can use : (colon, bash built-in command) to do nothing and be a placeholder to allow the expansion of the variable in the next command:

: "${POSTGRES_HOST_AUTH_METHOD:=}"

shell substitution example

${x:=y} (Assign if unset):

• If the variable x is unset or empty, this substitution assigns the value y to x and expands to the value y.
• If x is already set (even to an empty string), it expands to the current value of x.

${x:-y} (Default if unset):

• If the variable x is unset or empty, this substitution simply expands to the value y.
• It does not modify the value of x in any way.
• If x is already set (even to an empty string), it expands to the current value of x.

Here's a table summarizing the behavior:

Scenario	${x:=y}	${x:-y}
x is unset or empty	Assigns y to x, expands to y	Expands to y
x is set (even to "")	Expands to current value of x (might overwrite with y for :=)	Expands to current value of x

Choosing the right substitution:

• Use ${x:=y} when you want to both set a default value and use it in the expansion. This is useful for initializing variables before using them.
• Use ${x:-y} when you only need a default value for the expansion and don't want to modify the variable itself.