bash: unexpected array member handle

Question

I found weird the following bash behaviour in the last command line. For me, it is totally unexpected.

$ set | grep ^BASH_VERSINFO             # Show the array
BASH_VERSINFO=([0]="3" [1]="2" [2]="25" [3]="1" [4]="release" [5]="x86_64-redhat-linux-gnu")

$ echo "${BASH_VERSINFO[@]}"             # Print all array
3 2 25 1 release x86_64-redhat-linux-gnu

$ echo "${BASH_VERSINFO[@]%e}"           # Print all array removing final 'e' of every member
3 2 25 1 releas x86_64-redhat-linux-gnu

$ i=4; echo "${BASH_VERSINFO[i]}"        # Print an array member
release
$ i=4; echo "${BASH_VERSINFO[i++]}"      # Print an array member and increase the 'i' variable
release
$ i=4; echo "${BASH_VERSINFO[i]%e}"      # Print an array member removing the final 'e'
releas
$ i=4; echo "${BASH_VERSINFO[i++]%e}"    # Why does bash use the next member?
x86_64-redhat-linux-gnu

It seems that bash preincrements the 'i' variable.

Similar weird behaviour:

$ i=5; echo "${BASH_VERSINFO[--i]}"    # Print array member ${BASH_VERSINFO[4]}. 'i' is descreased one time.
release
$ i=5; echo "${BASH_VERSINFO[--i]%e}"  # Print array member ${BASH_VERSINFO[3]%e}. 'i' is descreased two times.
1

Does anyone can tell me that behaviour?

Thanx in advance

Answer 1

This seems to be a bug that has been fixed, possibly this one:

(from the bash ChangeLog -- changes between bash-4.2-alpha and bash-4.1-release)

Fixed a bug in arithmetic expansion that caused the index in an array expansion to be evaluated twice under certain circumstances.

Worksforme:

$ echo "${BASH_VERSINFO[*]}"
4 2 42 1 release i586-pc-linux-gnu
$ i=4; echo "${BASH_VERSINFO[i++]}"; echo "$i"
release
5
$ i=4; echo "${BASH_VERSINFO[i++]%e}"; echo "$i"
releas
5