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What is the difference when assigning a bash variable value with this syntax?

Sorry if this is something really simple or has already been asked, yet due to the nature of the question I cannot think of any search terms to put on search engines.

Lately I have seen some bash scripts that they assign variable values like this:

$ MY_BASH_VAR=${MY_BASH_VAR:-myvalue} $ echo "$MY_BASH_VAR" myvalue

What is the difference from the most usual way of assigning a value like this:

MY_BASH_VAR=myvalue $ echo "$MY_BASH_VAR" myvalue

You can look at [http://linux.die.net/man/1/bash](http://linux.die.net/man/1/bash)

${parameter:-word} Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

This provides a default value : MY_BASH_VAR keeps its value if defined otherwise, it takes the default “myvalue”

bruce@lorien:~$ A=42 bruce@lorien:~$ A=${A:-5} bruce@lorien:~$ echo $A 42