Multi tool use

Shell subtract one alphabet letter from current letter

I have a file called "current_letter.txt" which displays today's letter (A-K)

cat current_letter.txt
C


Each day the letter increments by one letter (Yesterday's letter would have been "B" Tomorrow's letter will be "D"

How can I put yesterdays letter into a variable?

Example:
A=K
B=A
C=B
D=C
E=D
F=E
G=F
H=G
I=H
J=I
K=J


I'm trying to put yesterday's letter into a variable like

yesterdays_letter=`cat current_letter.txt - 1`


So it uses a - k then repeats itself

2 Answers
2

You can use tr for all your Caesar cipher needs:

tr

#!/bin/bash
for c in {A..Z}
do
b=$(printf '%s' "$c" | tr 'B-ZA' 'A-Z')
echo "One letter before $c is $b"
done


This outputs:

One letter before A is Z
One letter before B is A
One letter before C is B
One letter before D is C
(etc)


The example section you give makes no sense, is that a file's content?

here is the solution:

a='ABCDEFGHIJK';
v=$(cat current_letter.txt);
if [ "$v" = 'A' ]; then
var='K';
else
var=${a[$(($(expr index $a $v)-2))]};
fi;
echo $var;


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