STATA: Executing code over multiple dataset with different variables

Many times panel datasets comes as yearly data files, which may have variables that are dropped or added over time. This is especially true as the panel gets longer. There are times when you just want to automatically go over each yearly file and execute some code, but if you try to execute code on a variable that doesn't exist STATA will halt the execution of your do file. You can use a bunch of if statements, but an easy solution:

capture confirm variable varname
if _rc == 0 {
code block
}

The capture command allows a command to be executed even if it creates an error, in this case the command is: confirm variable varname). If there is an error capture puts a code in the scalar _rc, but allows the do file to keep running.

So in this case if the variable didn't exist the scalar _rc would be 111, if the variable does exist _rc is set to 0. So the only time that the code block is executed is if the variable actually exists, if it doesn't then the do file skips that step and then keeps going.