Shell Script: Assignment-Or Operator (a= b || c)

Question

As the title describes, what's the proper way to do an or-assignment (eg a= b || c) in shell scripting, specifically csh vs bash? I cannot test this, so perhaps the example above works.

I thought this was pretty common in scripting languages, but for those that don't quite understand, the variable a will retain the value of b if truthy, otherwise the value of c. In the example b and c are expressions.

The usecase is typically to set to a to some kind of value if supplied, otherwise to use a default value (eg a= $1 || "Foo").

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I'm not sure what the reason is for the close votes as this question is not:

• broad
• vague
• incomplete

Please comment if you require some further explanation and need some amendment.

Answer 1

For bash you want

a=${b:-$c}

http://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion

${parameter:-word}

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.