How to get the first line of an input text file, while deleting that line from the text file

Use the sed delete command (here assuming GNU sed as found on Ubuntu and other GNU systems).

 sed -i '/^[@#]/ d' sample.txt

If you need to account for leading space characters:

sed -i '/^\s*[@#]/ d' sample.txt

I think you're looking for expand and/or unexpand. It seems you're trying to ensure a \tab width counts as 8 chars rather than the single one. fold will do that as well, but it will wrap its input to the next line rather than truncating it. I think you want:

expand < input | cut -c -80

expand and unexpand are both POSIX specified:

• The expand utility shall write files or the standard input to the standard output with \tab characters replaced with one or more space characters needed to pad to the next tab stop. Any backspace characters shall be copied to the output and cause the column position count for tab stop calculations to be decremented; the column position count shall not be decremented below zero.

Pretty simple. So, here's a look at what this does:

unset c i; set --;                                                             
until [ "$((i+=1))" -gt 10 ]; do set -- "$@" "$i" "$i"; done                      
for c in 'tr \\t \ ' expand;  do eval '                                           
    { printf "%*s\t" "$@"; echo; } | 
      tee /dev/fd/2 |'"$c"'| { 
      tee /dev/fd/3 | wc -c >&2; } 3>&1 |
      tee /dev/fd/2 | cut -c -80'
done

The until loop at top gets a set of data like...

1 1 2 2 3 3 ...

It printfs this with the %*s arg padding flag so for each of those in the set printf will pad with as many spaces as are in the number of the argument. To each one it appends a \tab character.

All of the tees are used to show the effects of each filter as it is applied.

And the effects are these:

1        2        3        4        5        6        7        8                9               10
1  2   3    4     5      6       7        8         9         10 
1  2   3    4     5      6       7        8         9         10 
66
1        2        3        4        5        6        7        8                9               10
1        2        3        4        5        6        7        8                9               10 
1        2        3        4        5        6        7        8                
105

Those rows are lined up in two sets like...

1. output of printf ...; echo
2. output of tr ... or expand
3. output of cut
4. output of wc

The top four rows are the results of the tr filter - in which each \tab is converted to a single space.

And the bottom four the results of the expand chain.