Why does a Bash while loop result in PIPESTATUS "1" instead of "0"?

GNU bash, version 5.2.15(1)-release (x86_64-pc-linux-gnu)

Consider this simple while loop:

while false; do true; done; echo "${PIPESTATUS[0]}"

It prints 1 instead of 0. This is unexpected to me and I can't find any documentation about this.

The same loop, when checked for actual exit code:

while false; do true; done; echo $?

prints 0, as expected.

I also noticed that if a break is used in the body:

while true; do true; break; done; echo "${PIPESTATUS[0]}"

then both the PIPESTATUS as well as the exit code are both 0, as expected.

Setting pipefail option does not seem to change this behaviour.

My guess: I guess the loop condition is technically the last statement to be executed, and hence its exit code is returned in PIPESTATUS. This explains why break did not return 1 while false loop condition does. However, this leads to more questions:

Why the disparity between PIPESTATUS and actual exit code?
Won't all while loops end with PIPESTATUS 1, since every loop has to stop due loop condition being false? (Except the loops that end due to a break)
Does this mean it is a reliable way to tell whether the loop exited to loop condition becoming false or because of a break?

Thanks in advance for your help.

Solution

From man bash:

man bash | grep -A3 PIPESTATUS
  PIPESTATUS
         An  array  variable (see Arrays below) containing a list of exit
         status values from the processes in  the  most-recently-executed
         foreground pipeline (which may contain only a single command).

man bash | sed -e '/^ *? /{N;q};d'
  ?      Expands  to  the exit status of the most recently executed fore‐
         ground pipeline.

So,

$PIPESTATUS holds the result of the last forked command (false in this case),
$? contains the status of the last script command (while ...;do ...;done).

Then

if ! :; then :;fi ; echo ${?@Q} ${PIPESTATUS[@]@Q}
'0' '0'

while ! :; do :;done ; echo ${?@Q} ${PIPESTATUS[@]@Q}
'0' '0'

if /bin/false; then :;fi ; echo ${?@Q} ${PIPESTATUS[@]@Q}
'0' '1'

while false; do :;done ; echo ${?@Q} ${PIPESTATUS[@]@Q}
'0' '1'

More at this difference of $? and ${PIPESTATUS[0]} @ lists.gnu.org.

`$?` is POSIX, while PIPESTATUS is a bash array...

if ls /wrong/path | wc | sed 'w/wrong/path' >/dev/null ; then
    echo Don't print this'
fi ; echo ${?@Q} ${PIPESTATUS[@]@A}

ls: cannot access '/wrong/path': No such file or directory
sed: couldn't open file /wrong/path: No such file or directory
'0' declare -a PIPESTATUS=([0]="2" [1]="0" [2]="4")

Where $PIPESTATUS hold result of all 3 commands ls, wc and sed.

if ls /wrong/path | wc | cat - /wrong/path | sed 'w/wrong/path' >/dev/null ; then
    echo Don't print this'
fi ; echo ${?@Q} ${PIPESTATUS[@]@A}  $(( $? ${PIPESTATUS[@]/#/+} ))

ls: cannot access '/wrong/path': No such file or directory
cat: /wrong/path: No such file or directory
sed: couldn't open file /wrong/path: No such file or directory
'0' declare -a PIPESTATUS=([0]="2" [1]="0" [2]="1" [3]="4") 7

The group command if ... then ...(elif ... then ) ... (else ... then) ... fi was executed successfully. His result code is 0, but
ls's result code was 2,
wc's result code was 0,
cat's result code was 1 and
sed's result code was 4.

until ls -ld / | wc | cat - | sed -n '/[a-z]/p' ; do echo Don't print this' done ; echo ${?@Q} ${PIPESTATUS[@]@A} $(( $? ${PIPESTATUS[@]/#/+} ))

'0' declare -a PIPESTATUS=([0]="0" [1]="0" [2]="0" [3]="0") 0

or with some cosmetic:

until ls -ld / | wc | cat - | sed -n '/[a-z]/p' ; do
    echo Don't print this'
done; printf 'RC: $?=%s PIPESTATUS=(%s) Total=%d\n' \
    $? "${PIPESTATUS[*]}"  $(( $? ${PIPESTATUS[*]/#/+} ))

RC: $?=0 PIPESTATUS=(0 0 0 0) Total=0

while ls -ld /wrong/path | wc | cat /tmp/noperm - | sed -n '/[a-z]/w/wrong/path' ; do
    echo Don't print this'
done ;printf 'RC: $?=%s PIPESTATUS=(%s) Total=%d\n' \
    $? "${PIPESTATUS[*]}"  $(( $? ${PIPESTATUS[*]/#/+} ))

ls: cannot access '/wrong/path': No such file or directory
cat: /tmp/noperm: Permission denied
sed: couldn't open file /wrong/path: No such file or directory
RC: $?=0 PIPESTATUS=(2 0 141 4) Total=147

... But if not is a group command:

ls -ld /wrong/path | wc | cat /tmp/noperm - | 
  sed -n '/[a-z]/w/wrong/path'; printf 'RC: $?=%s PIPESTATUS=(%s) Total=%d\n' \
    $? "${PIPESTATUS[*]}"  $(( $? ${PIPESTATUS[*]/#/+} ))

ls: cannot access '/wrong/path': No such file or directory
sed: couldn't open file /wrong/path: No such file or directory
cat: /tmp/noperm: Permission denied
RC: $?=4 PIPESTATUS=(2 0 141 4) Total=151

Why does a Bash while loop result in PIPESTATUS "1" instead of "0"?

$? is POSIX, while PIPESTATUS is a bash array...

`$?` is POSIX, while PIPESTATUS is a bash array...