bash - increment variables that contain letters - bash

Question

I have a set of valid characters [0-9a-z_] and a variable that is assigned one of these characters. What I want to do is to be able to increment that variable to the next in the set.
If need be I can handle the "special" cases where it would increment from '9' to 'a' and 'z' to '_', but I can't figure out how to increment letters.
#!/bin/bash
y=b
echo $y # this shows 'b'
y=$((y+1))
echo $y # this shows '1', but I want it to be 'c'

Answer 1

y=b
echo "$y" # this shows 'b'
y=$(echo "$y" | tr "0-9a-z" "1-9a-z_")
echo "$y" # this shows 'c'
Note that this does not handle the case where $y = "_" (not sure what you want then, and in any case it'll probably require separate handling), and if $y is more than one character long it'll "increment" all of them (i.e. "10" -> "21", "09g" -> "1ah", etc).

Answer 2

Maybe this can be a solution:
a=({0..9} {a..z} _)
echo ${a[*]}
yc=11
echo ${a[yc]}
((++yc))
echo ${a[yc]}
echo ${a[++yc]}
#Alternative
declare -A h
# Fill the has point to the next character
for((i=0;((i+1))<${#a[*]};++i)) { h[${a[i]}]=${a[i+1]};}
y=b
echo $y, ${h[$y]}
Output:
0 1 2 3 4 5 6 7 8 9 a b c d e f g h i j k l m n o p q r s t u v w x y z _
b
c
d
b, c

Answer 3

for those who would like to print incremented letter by execution of a function:
ALPHA=( {A..Z} )
alpha_increment () { echo ${ALPHA[${i:-0}]}; ((i++)) ;}
alpha_increment
A
alpha_increment
B
alpha_increment
C

Answer 4

You can start with this:
echo 0x$(( $(printf "%x" "'b'") + 1)) | xxd -r

Answer 5

I wrote this for a project, it uses the chr and ord fucntions(also found here somewhere) and some pure bash(only external called in the function is tr), if you are doing more than 100 characters I would use something else, but for short strings in my testing its actually slightly faster than python.
Also this script lower cases any input, you will have to modify it for upper case.
after putting these functions in your script(or cut and paste into a shell) you can just do
inc_string abz9z9
and get back.
aca0a0
chr() {
[ "$1" -lt 256 ] || return 1
printf "\\$(printf '%03o' "$1")"
}
ord() {
LC_CTYPE=C printf '%d' "'$1"
}
inc_string ()
{
string="$1";
lcstring=$(echo $string | tr '[:upper:]' '[:lower:]');
for ((position=$((${#lcstring}-1));position>=0;position--));do
if [ "${lcstring:position:1}" = 'z' ]; then
if [ "$position" -eq "$((${#lcstring}-1))" ]; then
newstring="${lcstring:0:$(($position))}a";
lcstring="$newstring";
elif [ "$position" -eq "0" ]; then
newstring="a${lcstring:$((position+1))}";
echo $newstring;
break;
else
newstring="${lcstring:0:$(($position))}a${lcstring:$((position+1))}";
lcstring="$newstring";
fi
elif [ "${lcstring:position:1}" = '9' ]; then
if [ "$position" -eq "$((${#lcstring}-1))" ]; then
newstring="${lcstring:0:$(($position))}0";
lcstring="$newstring";
elif [ "$position" -eq "0" ]; then
newstring="0${lcstring:$((position+1))}";
echo $newstring;
break;
else
newstring="${lcstring:0:$(($position))}0${lcstring:$((position+1))}";
lcstring="$newstring";
fi
else
if [ "$position" -eq "$((${#lcstring}-1))" ]; then
newstring="${lcstring:0:$(($position))}$(chr $(($(ord ${lcstring:position})+1)))";
echo $newstring;
break;
elif [ "$position" -eq "0" ]; then
newstring="$(chr $(($(ord ${lcstring:position})+1)))${lcstring:$((position+1))}";
echo $newstring;
break;
else
newstring="${lcstring:0:$(($position))}$(chr $(($(ord ${lcstring:position})+1)))${lcstring:$(($position+1))}";
echo $newstring;
break;
fi
fi
done
}