Answer by Kusalananda for Count the maximum character length for all the data fields in a simplified csv file and output to txt

Using Miller (mlr) to calculate the max length of each field's values. The input is read as CSV, and the output is produced as an "xtab" file (one key+value pair on each):

$ mlr --c2x stats1 -a maxlen --fr . fileThese_maxlen                                                       10are_maxlen                                                         11the_maxlen                                                         12column_headings_which_may_be_very_long_but_they_don't_count_maxlen 13

The --fr . arguments to the stats1 operation is to calculate the maximum length of all fields with names that match the regular expression . (i.e. every field that is named).

As you can see, Miller retains the field names and adds a _maxlen suffix to each.

To read the CSV file as if its first line was a record rather than the headers, then remove that first line and do the same max calculation:

$ mlr --c2x -N filter -x 'NR == 1' then stats1 -a maxlen --fr . file1_maxlen 102_maxlen 113_maxlen 124_maxlen 13

With an additional rename operation, we can remove the _maxlen suffix from the names of all fields:

$ mlr --c2x -N filter -x 'NR == 1' then stats1 -a maxlen --fr . then rename -r '(.*)_maxlen$,\1' file1 102 113 124 13