how to make data.table omit extra col

I am trying to load a very large tab file by data.table::fread, my input data may have a extra tab before last col,like:

#The entire table is tab split, we just show finally tab as \t
col1    col2    col3\t # a tab here, I have many cols in fact (>3000)
1    2    9\t
1    3    3\t
3    9    6\t

I noted fread will infer colname if have extra col, but its not correctly

# fread infered as 
V1   col1    col2   col3

I dont want V1 exists in first

I have tried

1. shift and reset colname, it could worked, but now I plan to use select to select columns when I read the file, so I can't do that

2. use data.table::fread(sep='\t', cmd=paste0('pigz -d -c ', input, ' | sed \'s/\t$//\''), select = select_col), Its worked, but seems very slower than process gz by data.table::fread

Starting with this result:

fread("input.tab", sep="\t")
#     col1  col2  col3     V4
#    <int> <int> <int> <lgcl>
# 1:     1     2     9     NA
# 2:     1     3     3     NA
# 3:     3     9     6     NA

if you know how many columns exist ahead of time, then you can simply select= them:

fread("input.tab", select=1:3)
#     col1  col2  col3
#    <int> <int> <int>
# 1:     1     2     9
# 2:     1     3     3
# 3:     3     9     6

Alternatively, if it's a large file and you want to be flexible, then you can do something like this:

# read in just enough rows so that you are confident-enough 
# that the last column is all NA, a symptom of the trailing-tab
tmp <- fread("input.tab", sep="\t", nrows=10)
tmp
#     col1  col2  col3     V4
#    <int> <int> <int> <lgcl>
# 1:     1     2     9     NA
# 2:     1     3     3     NA
# 3:     3     9     6     NA
ncols <- ncol(tmp) - all(is.na(tmp[[ncol(tmp)]]))
ncols
# [1] 3
fread("input.tab", select=seq(ncols))
#     col1  col2  col3
#    <int> <int> <int>
# 1:     1     2     9
# 2:     1     3     3
# 3:     3     9     6

If you have predetermined column indices in select_col, then you can use

fread(..., select = select_col[select_col < ncols])

This naive heuristic can be fooled if the tab-problem does not exist but the first n rows of the table are legitimately empty/null.

Using drop from fread

Finding the desired number of columns through system with e.g. awk, assuming the unwanted column is the last one.

NUM = as.numeric(system('awk \'{print NF; exit}\' file', intern=T))

data.table::fread("file", header=T, drop=NUM + 1)
   col1 col2 col3
1:    1    2    9
2:    1    3    3
3:    3    9    6