substract variable from each line in a file

Shell Programming and Scripting
1

Hi everyone,

I have a file containing one column and I want to subtract the first value (value of first line) from each line's value.

79304
99299
119294
139289
159284
179279
199274
219269
239264
259259
279254
299249
319244
339239
359234

I tried working on an awk solution with first defining the variable for the value of the first line

awk D='NR==1' pulse_time.txt

and then subtracting this from the first column of the file

awk '{print $1 -D}' pulse_time.txt

But unfortunately it fully ignores the D in that case.

I also tried to find a shell solution by defining the variables and then subtracting them. But it doesn't do what it should (subtraction incorrect).

S=$(head -1 pulse_time.txt)
OP=$(cat pulse_time.txt)
$(($OP-$NM))

Any help would be highly appreciated!
many thanks in advance!

2 
awk 'NR==1{x=$0;next}{$0=$1 - x}1' file