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这个 bash 语法是什么意思？ ETL_PORT =“$ {ETL_PORT：-6090}”

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这个 bash 语法是什么意思？ ETL_PORT =“$ {ETL_PORT：-6090}”

这个 bash 语法是什么意思？

这是否意味着如果设置了 ETL_PORT ，则 ETL_PORT 变量将为 ETL_PORT ，否则如果未设置，则默认为 6090 ？

ETL_PORT="${ETL_PORT:-6090}"

答案1

如果该变量在调用时未设置，则该构造${var:-val}将使用默认值。它只是许多方便的参数扩展之一。我有一个脚本可以生成这个有用的备忘单：valvar

${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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