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What does this bash syntax means?

Does this mean that ETL_PORT variable will be ETL_PORT if ETL_PORT is set , and otherwise defaults to 6090 if not set?

ETL_PORT="${ETL_PORT:-6090}"
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The construct ${var:-val} will use a default value of val if the variable var is unset at the time of that invocation. It's but one of many handy parameter expansions. I have a script that will generate this helpful cheat sheet:

${V}             Base string                     |reallyextremelylongfilename.ext
  --- Default substitutions ---
${nullvar}       Provided example case           |
${#nullvar-def}  Default value if unset or null  |def
${#nullvar:-def} Default value if unset          |def
  --- Default assignments ---
${1}             Provided example case           |
${$1=def}        Default value if unset or null  |def
${$1:=def}       Default value if unset          |def
  --- String metadata ---
${#V}            String length                   |31
  --- Substring extraction ---
${V:6}           Substring from position         |extremelylongfilename.ext
${V:6:9}         Substring with length from pos. |extremely
  --- Substring deletion ---
${V#*a}          Delete shortest prefix match    |llyextremelylongfilename.ext
${V##*a}         Delete longest prefix match     |me.ext
${V%e*}          Delete shortest suffix match    |reallyextremelylongfilename.
${V%%e*}         Delete longest suffix match     |r
  --- Substring replacement ---
${V/long/short}  Replace first match             |reallyextremelyshortfilename.ext
${V/#r*a/REA}    Replace prefix match            |REAme.ext
${V/%.e*/.dat}   Replace suffix match            |reallyextremelylongfilename.dat
${V//e/II}       Replace all matches             |rIIallyIIxtrIImIIlylongfilIInamII.IIxt
  --- Other handy things ---
${V?Message}     exit 1 with 'Message' output if V is not set or is null
${V:?Message}    exit 1 with 'Message' output if V is not set
${V+Value}       If V is set, use 'Value', otherwise null

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