How to insert custom string in the result of Linux paste command

Maybe there's a way to do this with paste, but it can certainly be done easily with awk:

paste c1 c2 | awk '{ printf("var %s = some_function(%s);\n", $1, $2); }'

linux shell paste

process your file through with a separate command and pipe it to the paste command

your_command c1 | paste c1 - c2

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