Completion for options with comma-separated values loses its prefix

[Lekensteyn]

commit 021058b tried to extend completion such that usermod -G root,systemd-<TAB> suggests possible completions for systemd-.

It works when only a single candidate is available (e.g. root,systemd-core<TAB> -> root,systemd-coredump), but when multiple completions are available, the prefix (root,) is dropped (e.g. root,systemd-<TAB> -> systemd- with initially no further suggestions).

This problem was first noticed when using the tshark -O http,t<TAB> option.

Reproducer (change XDG_DATA_DIRS to avoid loading completions from default path):

$ bash --rcfile <(echo 'XDG_DATA_DIRS=/nonexistent; . ./bash_completion')
$ usermod -G root,systemd-<TAB>

Environment:

• Distribution: Arch Linux
• bash 4.4.023-1
• bash-completion 2.8-1 and git master 2.1-871-gc9c14da4

[scop]

It doesn't fail that way for me on Ubuntu 18, bash 4.4.19, current bash-completion master. Neither does tshark, it keeps the preceding comma separated completions as well.

$ usermod -G root,systemd-<TAB>
systemd-journal   systemd-network   systemd-resolve   systemd-timesync  
$ usermod -G root,systemd-

[Lekensteyn]

Odd, perhaps it is some shell option that causes this? I reproduced the issue from the git tree with:

bash --rcfile <(echo . ./bash_completion)

so it should not be caused by any form of customization.

Edit: I was also able to reproduce this issue on Ubuntu 18.04 which ships with bash 4.4.18-2ubuntu1.
Do you have any special readline options?

Setting COMP_WORDBREAKS+=, seems to "improve" things as it matches the expected behavior, but now usermod -G root,--<TAB> is also being completed as option. Related question here with (unsatisfatory) answers: https://unix.stackexchange.com/questions/124539/bash-completion-for-comma-separated-values

[scop]

I tried to reproduce with the same command as you, ditto in a new shell started with bash --norc, but failed, it Just Works for me. The only customized readline setting I have is set show-all-if-ambiguous on in ~/.inputrc. Turning that off didn't seem to make a difference.

[Lekensteyn]

Really strange, perhaps it is a terminal thing? I can still reproduce it from Kubuntu 18.04.1 (no updates installed):

script -c "bash --rcfile <(echo 'XDG_DATA_DIRS=/nonexistent; . ./bash_completion') -x" output.txt
# Then type: "usermod -G root,systemd-<TAB>"
# followed by Ctrl-C and Ctrl-D (exit)

Over SSH or through Konsole (in the GUI) does not make a difference and produces the same output:
output.txt

Observe the last line: ^G^H^H^H^H^H^H^H^H^H^H^H^H^H^[[5Psystemd-
That's a bell, 13 times backspace (erases root,systemd-), CSI 5 P (delete 5 characters on the current line according to console_codes(4)), then insert systemd-. IOW, it replaces usermod -G root,systemd- by usermod -G systemd- which is what I am observing (without set -x).

[scop]

Here's my output for comparison. Didn't spot anything obvious in the diff on a quick look: output.txt

[Lekensteyn]

Can you reproduce it with a different user on the same system? Have you set the INPUTRC environment variable? strace reveals that files are search in this order (first match);

• $INPUTRC
• ~/.inputrc
• /etc/inputrc

What is your output for echo $TERM? Mine says xterm-256color.

[scop]

INPUTRC is not set, and TERM is the same as yours.

~/.inputrc contents:

$include /etc/inputrc
set show-all-if-ambiguous on

...but I can reproduce with another user on this system. Cool, now I have something to look into when I find some time.

[scop]

It's the set show-all-if-ambiguous on. Commenting that out I can reproduce with my user too.