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Editorial Team
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Asked: 2026-05-25T16:48:57+00:00

I understand the colon operator in bash that acts like a null , and

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I understand the colon operator in bash that acts like a null, and I know it’s used in parameter expansion, as well as being used other ways, but can someone explain this: 

: ${SOMETHING='value'}

From experimentation I know that this sets the environment variable $SOMETHING to 'value' but why?

“Just because it does” is a valid answer but then please point me to the documentation for it (which I can’t seem to find) or a proper name for this usage would be useful. I’m hoping there’s a more enlightening explanation though.

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1 Answer

  1. Editorial Team

    The expression ${SOMETHING='value'} sets SOMETHING to value if it isn’t already set. This is a useful operator to have in many situations. However, it also returns the assigned value, so if you simply executed

    ${SOMETHING='value'}

    then your shell would try to invoke the command value. This might or might not do something unwanted; at the least it would throw a message “value: command not found”.

    To avoid this you can use the no-op :, which evaluates its argument and then throws it away, rather than executing it. This is documented here.

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