How can I just extract one underbar-separated field from a filename?

问题

I have a list of file names like this:

REG_2016120200hourly_d01_20161202_00_00_00.nc

Of this name I would like to extract and put in a variable:

1)date 20161202

    for file in /path/*;
    do
     filename=$(basename -- "$file")
     date=${filename:4:8}
     echo $date
    done

and this is working, the script give me 20161202 and I don't know why

2)timestep 00

I need to take the firsts two zero 00 and I'm trying with

timestep=${filename:34:36} but this doesn't work.

I'm a little bit surprised because I used the same method in other scripts and I have never had problems.

Thank you

回答1:

timestep="${filename:34:2}"

2 is length.

---

From man bash:

${parameter:offset:length}: Substring Expansion. Expands to up to length characters of the value of parameter starting at the character specified by offset. [...]

来源：https://stackoverflow.com/questions/60346648/how-can-i-just-extract-one-underbar-separated-field-from-a-filename