Bash variable not decrementing in a pipeline

Question

The variable x in the first example doesn't get decremented, while in the second example it works. Why?

Non working example:

#!/bin/bash

x=100
f() {
  echo $((x--)) | tr 0-9 A-J
  # this also wouldn't work: tr 0-9 A-J <<< $((x--))
  f
}
f

Working example:

#!/bin/bash

x=100
f() {
  echo $x | tr 0-9 A-J
  ((x--))
  # this also works: a=$((x--))
  f
}
f

I think it's related to subshells since I think that the individual commands in the pipeline are running in subshells.

Answer 1

It does decrement if you don't use a pipeline (and avoid a sub shell forking):

x=10

f() {
   if ((x)); then
      echo $((x--))
      f
   fi
}

Then call it as:

f

it will print:

10
9
8
7
6
5
4
3
2
1

Since decrement is happening inside the subshell hence current shell doesn't see the decremented value of x and goes in infinite recursion.

---

EDIT: You can try this work around:

x=10

f() {
   if ((x)); then
      x=$(tr 0-9 A-J <<< $x >&2; echo $((--x)))
      f
   fi
}

f

To get this output:

BA
J
I
H
G
F
E
D
C
B