My concern is that this PR could break workflows that were written with an assumption that sample1d does not output trailing text. If this is considered an enhancement, I think it would be better to set a new modifier to trigger the behavior.

I did not think any of the options were suitable for a modifier for this so I added an option -E to attempt to deal with trailing text. Default thus remains the same as before.

Looks good. Extra information is now preserved.

Two other things:

• I may have one or more rows of the same date. This cause sample1d to stop:

$ cat <<eof | gmt sample1d -fT --TIME_UNIT=d -I1 -Fa
2021-01-01 1
2021-01-02 2
2021-01-02 3
eof
sample1d [ERROR]: x-values are not monotonically increasing/decreasing (at zero-based record 2)!
sample1d [ERROR]: Failure in gmt_intpol near row 3!

Why are duplicate rows (on the 0th column) not allowed? Could it be?

• In my case, I don't want to interpolates anything else but the 0th column (dates in my case). But sample1d requires a value that it can interpolate (1st column). Any way to make it only look at the 0th column?

I will look to see if we can get away with no data set to resample. Seems like that should be easy. Right now we allow n columns to be resampled and I think 0 should be allowed.
As for duplicates. Remember we are building a spline f(t) and it cannot handle two values at the same time. Just like with surface or greenspline, one would have to average those points before the interpolation. There is -A+d to skip duplicates but that may be limited to spatial curves (cannot remember).

Now, sample1d is OK with interpolating just the time column if there are no other data columns, so @anbj can test this out. I also updated the documentation. I agree this is a useful feature (resample a set of time nodes without having to give a data column when none is needed.

Super!

cat <<eof | gmt sample1d -fT --TIME_UNIT=d -I1 -Fa -E
2021-01-01 hello
2021-01-10 there
eof
2021-01-01T00:00:00	hello
2021-01-02T00:00:00
2021-01-03T00:00:00
2021-01-04T00:00:00
2021-01-05T00:00:00
2021-01-06T00:00:00
2021-01-07T00:00:00
2021-01-08T00:00:00
2021-01-09T00:00:00
2021-01-10T00:00:00	there

Thank you Paul!