Compgen doesn't complete words containing colons correctly - bash

Question

I'm encountering a problem with creating a Bash completion function, when the command is expected to contain colons. When you type a command and press tab, Bash inserts the contents of the command line into an array, only these arrays are broken up by colons. So the command:
dummy foo:apple
Will become:
('dummy' 'foo' ':' 'apple')
I'm aware that one solution is to change COMP_WORDBREAKS, but this isn't an option as it's a team environment, where I could potentially break other code by messing with COMP_WORDBREAKS.
Then this answer suggests using the _get_comp_words_by_ref and __ltrim_colon_completions variables, but it is not remotely clear to me from the answer how to use these.
So I've tried a different solution below. Basically, read the command line as a string, and figure out which word the user's cursor is currently selecting by calculating an "offset". If there is a colon in the command line with text to the left or right of it, it will add 1 each to the offset, and then subtract this from the COMP_CWORD variable.
1 #!/bin/bash
2 _comp() {
3 #set -xv
4 local a=("${COMP_WORDS[#]}")
5 local index=`expr $COMP_CWORD`
6 local c_line="$COMP_LINE"
7
8 # Work out the offset to change the cursor by
9 # This is needed to compensate for colon completions
10 # Because COMP_WORDS splits words containing colons
11 # E.g. 'foo:bar' becomes 'foo' ':' 'bar'.
12
13 # First delete anything in the command to the right of the cursor
14 # We only need from the cursor backwards to work out the offset.
15 for ((i = ${#a[#]}-1 ; i > $index ; i--));
16 do
17 regex="*([[:space:]])"${a[$i]}"*([[:space:]])"
18 c_line="${c_line%$regex}"
19 done
20
21 # Count instances of text adjacent to colons, add that to offset.
22 # E.g. for foo:bar:baz, offset is 4 (bar is counted twice.)
23 # Explanation: foo:bar:baz foo
24 # 0 12 34 5 <-- Standard Bash behaviour
25 # 0 1 <-- Desired Bash behaviour
26 # To create the desired behaviour we subtract offset from cursor index.
27 left=$( echo $c_line | grep -o "[[:alnum:]]:" | wc -l )
28 right=$( echo $c_line | grep -o ":[[:alnum:]]" | wc -l )
29 offset=`expr $left + $right`
30 index=`expr $COMP_CWORD - $offset`
31
32 # We use COMP_LINE (not COMP_WORDS) to get an array of space-separated
33 # words in the command because it will treat foo:bar as one string.
34 local comp_words=($COMP_LINE)
35
36 # If current word is a space, add an empty element to array
37 if [ "${COMP_WORDS[$COMP_CWORD]}" == "" ]; then
38 comp_words=("${comp_words[#]:0:$index}" "" "${comp_words[#]:$index}" )
39 fi
40
41
42 local cur=${comp_words[$index]}
43
44 local arr=(foo:apple foo:banana foo:mango pineapple)
45 COMPREPLY=()
46 COMPREPLY=($(compgen -W "${arr[*]}" -- $cur))
47 #set +xv
48 }
49
50 complete -F _comp dummy
Problem is, this still doesn't work correctly. If I type:
dummy pine<TAB>
Then it will correctly complete dummy pineapple. If I type:
dummy fo<TAB>
Then it will show the three available options, foo:apple foo:banana foo:mango. So far so good. But if I type:
dummy foo:<TAB>
Then the output I get is dummy foo:foo: And then further tabs don't work, because it interprets foo:foo: as a cur, which doesn't match any completion.
When I test the compgen command on its own, like so:
compgen -W 'foo:apple foo:banana foo:mango pineapple' -- foo:
Then it will return the three matching results:
foo:apple
foo:banana
foo:mango
So what I assume is happening is that the Bash completion sees that it has an empty string and three available candidates for completion, so adds the prefix foo: to the end of the command line - even though foo: is already the cursor to be completed.
What I don't understand is how to fix this. When colons aren't involved, this works fine - "pine" will always complete to pineapple. If I go and change the array to add a few more options:
local arr=(foo:apple foo:banana foo:mango pineapple pinecone pinetree)
COMPREPLY=()
COMPREPLY=($(compgen -W "${arr[*]}" -- $cur))
Then when I type dummy pine<TAB> it still happily shows me pineapple pinecone pinetree, and doesn't try to add a superfluous pine on the end.
Is there any fix for this behaviour?

Answer 1

One approach that's worked for me in the past is to wrap the output of compgen in single quotes, e.g.:
__foo_completions() {
COMPREPLY=($(compgen -W "$(echo -e 'pine:cone\npine:apple\npine:tree')" -- "${COMP_WORDS[1]}" \
| awk '{ print "'\''"$0"'\''" }'))
}
foo() {
echo "arg is $1"
}
complete -F __foo_completions foo
Then:
$ foo <TAB>
$ foo 'pine:<TAB>
'pine:apple' 'pine:cone' 'pine:tree'
$ foo 'pine:a<TAB>
$ foo 'pine:apple'<RET>
arg is pine:apple
$ foo pi<TAB>
$ foo 'pine: