Colon at the beginning of line in docker entrypoint bash script [duplicate]

Question

I was looking through an entrypoint script for a docker image and they had the following lines (53-54)

: ${POSTGRES_USER:=postgres}
: ${POSTGRES_DB:=$POSTGRES_USER}

I saw this thread What is the purpose of the : (colon) GNU Bash builtin?

and was able to figure out : meant true and that the := was used to assign default values, but it doesn't really touch upon what does : at the beginning of the line at least for this specific case.

Answer 1

In the Bourne shell and derivatives like Bash, : is a no-op command: that is, it doesn't do anything, but arguments are evaluated normally. Contrast this with a comment (#), which does nothing at all (everthing following the # is simply ignored).

This syntax:

: ${POSTGRES_USER:=postgres} 

Is commonly used to assign default values to variables. The syntax ${POSTGRES_USER:=postgres} (a) will set the variable POSTGRES_USER to postgres only if it does not already have a value, and will then (b) evaluate to the value of $POSTGRES_USER. If we used a comment:

# ${POSTGRES_USER:=postgres} 

...nothing at all would happen, because the shell simply ignores the entire line. Using : means that the ${...} expression is still evaluated, so POSTGRES_USER is assigned a value if necessary.

Update

If there was no : at the beginning of the line, then this:

${POSTGRES_USER:=postgres} 

would be a shell command! The shell would first evaluate the variable expansion and come up with something like postgres, so you would have a shell script that effectively looked like this:

postgres 

Which would probably give you the error:

bash: postgres: command not found...