What does PURE mean in startup assigns?

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So, what is the "PURE" flag actually used for? I know that some commands complain they cant be made resident in the shell unless you use this.

Does that mean they arent really supposed to be made resident?

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So what qualifies a pure or impure command? Is it something about the way they were written?

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Piru:

"Pure apps can't use globals/static data."

I thought all C string literals were basically static though? If that is true does it mean you can use global and static data provided it is constant?