Tuesday, 30 August 2011

Checking and setting shell environment variables to default value

There are two possibilities here. The example above, namely using:

${MyVariable:=SomeDefault}

for the first time you need to refer to $MyVariable. This takes the env. var MyVariable and, if it is currently not set, assigns the value of SomeDefault to the variable for later use.

You also have the possibility of:

${MyVariable:-SomeDefault}
 
which just substitutes SomeDefault for the variable where you are using this construct. It doesn't assign the value SomeDefault to the variable, and the value of MyVariable will still be null after this statement is encountered.

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