bash - split string into array WITH empty values

Question

pattern="::a::b::"
oldIFS=$IFS
IFS="::"
read -r -a extractees <<< $pattern
IFS=$oldIFS

this results in

{"a","b"}

however, I need to maintain the indices, so I want

{"","a","b",""}

(for comparison, if I wanted {"a","b"}, I would have written "a::b".

Why? because these elements are later split again (on a different delimiter) and the empty "" values should result in an empty list then.

How do I achieve this?

@Cyrus {"","a","b",""} - as I have written in the post itself. {"","a","","b",""} makes no sense if :: is the delimiter and it changes the indices. — User1291
– User1291, Commented Oct 31, 2016 at 19:30

hidefromkgb · Accepted Answer · 2016-10-31 19:58:49Z

1

No field separator can be longer than 1 character, unfortunately, so '::' → ':'.

Aside of that, globbing should be explicitly turned off to prevent potential filename expansion in an unquoted variable.

set -f # disable globbing
pattern=":a:b c:"
oldIFS=$IFS
IFS=":"
extractees=($pattern)
IFS=$oldIFS

echo "'${extractees[0]}'"
echo "'${extractees[1]}'"
echo "'${extractees[2]}'"
echo "'${extractees[3]}'"

edited Oct 31, 2016 at 19:58

answered Oct 31, 2016 at 17:55

hidefromkgb

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4 Comments

chepner Over a year ago

Unquoted parameter expansions are also not a reliable way to split a string into an array.

hidefromkgb Over a year ago

@chepner okay. $IFS is already set, so the only thing that can affect unquoted expansion is filename globbing. However, it does not occur here; just tried on BASH 4.3. Don`t know what the exact reason is, this case is seemingly undocumented. But to be on the safe side, globbing can always be explicitly disabled via set -f. Edited.

hidefromkgb Over a year ago

@chepner consecutive whitespace characters are removed only if $IFS equals its default value. Prooflink.

chepner Over a year ago

Ah, right. Definitely mention that globbing should be turned off, though.

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