This is really over-thought.

#!/usr/bin/env bash

IFS='
'
select x in $(cd /some/data/directory && du -sh -- *); do
    break
done
unset IFS  # or save/restore it explicitly

As an added bonus, the only bashism here is the select.

Typescript:

$ ./cg
 1) 0   0
 2) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 3) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 4) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000a
 5) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000ą
 6) 60k 1028501896.pdf
 7) 0   a
 8) 16k a.bkp
 9) 36k a.cpio
10) 10k a.d
11) 36k a.patch
12) 60k a.pax
13) 84k a.png
14) 4k  b.cpio
15) 32k b.pax
16) 12k b.tar
17) 0   bugreport.cgi?bug=910770;mbox=yes;mboxmaint=yes
18) 74.2M       build-output
19) 796k        busybox
20) 428k        busybox_1%3a1.30.1-6+b3_amd64.deb
21) 0   CB_Unix
22) 4k  cg
#? 

The quoting like you're doing doesn't work because quote removal applies only oto the original word; the result of parameter expansion is only field-split (and globbed). To do that, you need to tokenise the string as input:

eval "select x in $sel
do
  break
done"

and for obvious reasons you really shouldn't do this, since you haven't actually escaped the filenames:

$ ./cg
./cg: eval: line 11: unexpected EOF while looking for matching `''
./cg: eval: line 15: syntax error: unexpected end of file

if you really need to fork a process for each line and really need to store the du output in a big string then you should've done

#!/usr/bin/env bash

sel=
while read -r l; do
    sel="$(printf '%s %q' "$sel" "$l")"
done < <(cd /some/data/directory && du -sh -- *) 

eval "select x in $sel; do
  break
done"

(I've disabled escape-mangling the paths, which you had on for some reason).

This is really over-thought.

#!/usr/bin/env bash

IFS='
'
select x in $(cd /some/data/directory && du -sh -- *); do
    break
done
unset IFS  # or save/restore it explicitly

As an added bonus, the only bashism here is the select.

Typescript:

$ ./cg
 1) 0   0
 2) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 3) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 4) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000a
 5) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000ą
 6) 60k 1028501896.pdf
 7) 0   a
 8) 16k a.bkp
 9) 36k a.cpio
10) 10k a.d
11) 36k a.patch
12) 60k a.pax
13) 84k a.png
14) 4k  b.cpio
15) 32k b.pax
16) 12k b.tar
17) 0   bugreport.cgi?bug=910770;mbox=yes;mboxmaint=yes
18) 74.2M       build-output
19) 796k        busybox
20) 428k        busybox_1%3a1.30.1-6+b3_amd64.deb
21) 0   CB_Unix
22) 4k  cg
#? 

The quoting like you're doing doesn't work because quote removal applies only to the original word; the result of parameter expansion is only field-split (and globbed). To do that, you need to tokenise the string as input:

eval "select x in $sel
do
  break
done"

and for obvious reasons you really shouldn't do this, since you haven't actually escaped the filenames:

$ ./cg
./cg: eval: line 11: unexpected EOF while looking for matching `''
./cg: eval: line 15: syntax error: unexpected end of file

if you really need to fork a process for each line and really need to store the du output in a big string then you should've done

#!/usr/bin/env bash

sel=
while read -r l; do
    sel="$(printf '%s %q' "$sel" "$l")"
done < <(cd /some/data/directory && du -sh -- *) 

eval "select x in $sel; do
  break
done"

(I've disabled escape-mangling the paths, which you had on for some reason.)

This is really over-thought.

#!/usr/bin/env bash

IFS='
'
select x in $(cd /some/data/directory && du -sh -- *); do
    break
done
unset IFS  # or save/restore it explicitly

As an added bonus, the only bashism here is the select.

Typescript:

$ ./cg
 1) 0   0
 2) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 3) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 4) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000a
 5) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000ą
 6) 60k 1028501896.pdf
 7) 0   a
 8) 16k a.bkp
 9) 36k a.cpio
10) 10k a.d
11) 36k a.patch
12) 60k a.pax
13) 84k a.png
14) 4k  b.cpio
15) 32k b.pax
16) 12k b.tar
17) 0   bugreport.cgi?bug=910770;mbox=yes;mboxmaint=yes
18) 74.2M       build-output
19) 796k        busybox
20) 428k        busybox_1%3a1.30.1-6+b3_amd64.deb
21) 0   CB_Unix
22) 4k  cg
#? 

The quoting like you're doing doesn't work because quote removal doesn't happen on the result of parameter expansion, only on the original word. To do that, you need to tokenise the string as input:

eval "select x in $sel
do
  break
done"

and for obvious reasons you really shouldn't do this, since you haven't actually escaped the filenames:

$ ./cg
./cg: eval: line 11: unexpected EOF while looking for matching `''
./cg: eval: line 15: syntax error: unexpected end of file

if you really need to fork a process for each line and really need to store the du output in a big string then you should've done

#!/usr/bin/env bash

sel=
while read -r l; do
    sel="$(printf '%s %q' "$sel" "$l")"
done < <(cd /some/data/directory && du -sh -- *) 

eval "select x in $sel; do
  break
done"

(I've disabled escape-mangling the paths, which you had on for some reason).

This is really over-thought.

#!/usr/bin/env bash

IFS='
'
select x in $(cd /some/data/directory && du -sh -- *); do
    break
done
unset IFS  # or save/restore it explicitly

As an added bonus, the only bashism here is the select.

Typescript:

$ ./cg
 1) 0   0
 2) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 3) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 4) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000a
 5) 0   000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000ą
 6) 60k 1028501896.pdf
 7) 0   a
 8) 16k a.bkp
 9) 36k a.cpio
10) 10k a.d
11) 36k a.patch
12) 60k a.pax
13) 84k a.png
14) 4k  b.cpio
15) 32k b.pax
16) 12k b.tar
17) 0   bugreport.cgi?bug=910770;mbox=yes;mboxmaint=yes
18) 74.2M       build-output
19) 796k        busybox
20) 428k        busybox_1%3a1.30.1-6+b3_amd64.deb
21) 0   CB_Unix
22) 4k  cg
#? 

The quoting like you're doing doesn't work because quote removal applies only oto the original word; the result of parameter expansion is only field-split (and globbed). To do that, you need to tokenise the string as input:

eval "select x in $sel
do
  break
done"

and for obvious reasons you really shouldn't do this, since you haven't actually escaped the filenames:

$ ./cg
./cg: eval: line 11: unexpected EOF while looking for matching `''
./cg: eval: line 15: syntax error: unexpected end of file

if you really need to fork a process for each line and really need to store the du output in a big string then you should've done

#!/usr/bin/env bash

sel=
while read -r l; do
    sel="$(printf '%s %q' "$sel" "$l")"
done < <(cd /some/data/directory && du -sh -- *) 

eval "select x in $sel; do
  break
done"

(I've disabled escape-mangling the paths, which you had on for some reason).