hi I am trying to write a script to count the number of files, with slightly different subset name, in a directory

for example, in directory /data, there are a subset of files that are name as follow

 
/data/data_1_(1to however many).txt
/data/data_2_(1 to however many).txt
/data/data_3_(1to however many).txt
etc etc etc

Something like this?

cd /data
ls | awk -F_ '{A[$1 FS $2]++} END{for(i in A) print i, A[i]}'

would you minding explaining the script so I could adapt it accordingly and test?

Sure:

awk -F_ '                     # set field separator to underscore
  {
    A[$1 FS $2]++             # count the number of times $1 FS $2 occurs (field 1 and field2 separated by an underscore (for example "data_1")
  } 
END{
  for(i in A) print i, A[i]   # At the end print the results
}
'

Thanks for your reply. What is

A[$1 FS $2]++

That is an element in the associative array A with index $1 FS $2 (FS="_" , so this means $1_$2 ) that gets incremented by 1 ( ++ )

The code has worked impressively, many thanks for that.

I want to write a line to determine that, if the number of files with any prefix is more than 5, then print out the prefix names in one line (separated by a single space), such as

data_1 data_2

I wrote this line

ls | awk -F_ '{A[$1 FS $2]++} END {for (j in A) {if (A[j] > 5) {printf j, " "}}}'

data_1data_2