How to subset filenames in bash `ls` output - bash

Question

So I have a list of files that start with a date (yyyymmdd) and then they have different endings. I would like to filter all dates and the find the uniq ones and count them. So something like this pseudocode:
ls | grep 'file[0:8]' | unique | wc -l
But this obviuosly doesn't work. So is there any way on how to do this more or less easily?
The data I have looks something like this:
20160124_vv_iw2.slc 20170118_vv_iw2.slc.tops_par 20190120_vv_iw2.slc.par
20160124_vv_iw2.slc.par 20170915_vv_iw2.slc 20190120_vv_iw2.slc.tops_par
20160124_vv_iw2.slc.tops_par 20170915_vv_iw2.slc.par 20200911_vv_iw2.slc
20160827_vv_iw2.slc 20170915_vv_iw2.slc.tops_par 20200911_vv_iw2.slc.par
20160827_vv_iw2.slc.par 20180113_vv_iw2.slc 20200911_vv_iw2.slc.tops_par
20160827_vv_iw2.slc.tops_par 20180113_vv_iw2.slc.par 20200923_vv_iw2.slc
20170118_vv_iw2.slc 20180113_vv_iw2.slc.tops_par 20200923_vv_iw2.slc.par
20170118_vv_iw2.slc.par 20190120_vv_iw2.slc 20200923_vv_iw2.slc.tops_par

Answer 1

Don't use ls in scripts.
printf "%-8.8s\n" * | uniq | wc -l
More generally, you could do something like
for file in *; do
echo "${file:0:8}"
done | uniq | wc -l
Like any line-oriented approach, this will break if you have file names with newlines in them.
If you just want to split at the first underscore, "${file%%_*}" does that.

Answer 2

ls -1 | sed -E 's/^[^_]+_//' | sort -u | wc -l
or
ls -1 | sed -E 's/^[[:digit:]]+_//' | sort -u | wc -l

Answer 3

Use this Perl one-liner, combined with uniq | wc -l
perl -le 'print sort /^(\d+)/ for glob "*";' | uniq | wc -l
8
The Perl one-liner uses these command line flags:
-e : Tells Perl to look for code in-line, instead of in a file.
-l : Strip the input line separator ("\n" on *NIX by default) before executing the code in-line, and append it when printing.
glob "*" returns the list of all files and directories.
/^(\d+)/ returns the regex matches, here, the stretches of digits at the beginning of the file names. Use something like /^(\d{8})/ if you need the exact number of digits.
SEE ALSO:
perldoc perlrun: how to execute the Perl interpreter: command line switches
perldoc perlrequick: Perl regular expressions quick start