bash calculations with numbers from files - bash

Question

I am trying to do a simple thing:
To get the second number in the the line with the second occurence of the word TER and lower it by one and further process it. The tr -s ' ' is there because the file is not delimited by tabs, but by different amounts of whitespaces.
My script:
first_res_atombumb= grep 'TER' tata_sbox_cuda.pdb | head -n 2 | tail -1 |tr -s ' '| cut -f 2 -d ' '
echo $((first_res_atombumb-1))
but this only returnes:
255
-1
Of course I want to have 254.
adding | tr -d '\n' does not help either, what on earth is going on? I have already asked several people at work noone seems to know.
the lines in question look linke this
TER 128 DA3 4
TER 255 DA3 8
and if I apply grep 'TER' tata_sbox_cuda.pdb | head -n 2 | tail -1 | tr -s ' '| cut -f 2 -d ' ' in the command line i get what i expect, just 255

Answer 1

With bash, I'd write
n_ter=0
while read -a words; do
if [[ ${words[0]} == TER ]] && (( ++n_ter == 2 )); then
echo $(( ${words[1]} - 1 ))
fi
done < file
but I'd use awk
awk '$1 == "TER" && ++n == 2 {print $2 - 1}' file
The problem with your code: you forgot to use the $() command substitution syntax
first_res_atombumb= grep 'TER' tata_sbox_cuda.pdb | head -n 2 | tail -1 |tr -s ' '| cut -f 2 -d ' '
# .................^...............................................................................^
echo $((first_res_atombumb-1))
You're setting the variable to an empty string in the environment of the grep command. Then, since you're not capturing the output of that pipeline, "255" is printed to the terminal. Because the variable is unset in your current shell, you get echo $((-1))
All you need is:
first_res_atombumb=$(grep 'TER' tata_sbox_cuda.pdb | head -n 2 | tail -1 |tr -s ' '| cut -f 2 -d ' ')
# .................^^...............................................................................^
But I'd still use awk.

Answer 2

If I understand your problem correctly you can solve it using AWK:
awk 'BEGIN{v=0} $1 == "TER" {v++;if (v==2) {print $2-1 ;exit}}' tata_sbox_cuda.pdb
Explanation:
BEGIN{v=0} declaring and nulling the variable.
$1 == "TER" execute the command in {} only if it's the second occurence of TER.
{v++;if (v==2) {print $2-1 ;exit}}' increase the value of v and check if it's 2, in this case subtract 1 from the second field and display, exit afterwards (will make the processing faster and will skip unnecessary lines).