Bash: Using Array Parameter Expansion to Include Items Matching Pattern

Question

I would like to use bash parameter expansion on an array to include items matching a given pattern, rather than to exclude them.

Here's an example excluding the items matching ba*:

ar=( foo bar baz qux )
for i in ${ar[@]##ba*}; do
    echo "$i"
done

# foo
# qux

That's all well and good, but in this case I'd actually like to include only the items matching ba*.

Here's what I've tried, and frankly, I'm not quite sure how to interpret the results:

ar=( foo bar baz qux )
#shopt -s extglob  # seems to have no effect either way
for i in ${ar[@]##!(ba*)}; do
    echo "$i"
done

# ar
# az

It looks like I'm in this case I'm getting the right items, but their values have been munged.

Here's my current working solution:

ar=( foo bar baz qux )
for i in ${ar[@]}; do
    if [[ "$i" =~ ba* ]]; then
        echo "$i"
    fi
done

# bar
# baz

I am using Mac OSX and have tested this with Mac bundled bash 3.2.51 as well as Homebrew installed bash 4.2.45.

Thanks!

Edit

@casc (rightly) suggests below to just re-append the removed 'b' from the matches, but that's not exactly what I'm looking for.

My actual use case would be more like including items that match *ba*, not just ba*. The point is that I don't necessarily know the full string, just that it will definitely have the pattern ba* somewhere. This may be a better example:

ar=( foo zoobar waybaz qux )
for i in ${ar[@]}; do
    if [[ "$i" =~ ba* ]]; then
        # do something with $i
    fi
done

# zoobar
# waybaz

chepner · Accepted Answer · 2013-12-31 17:28:34Z

1

From each element of the array, you are removing the longest prefix that does not match ba*.

For foo and qua, the entire string does not match.
For bar and baz, b does not match ba*, but ba does, so only b is removed.

answered Dec 31, 2013 at 17:28

chepner

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1 Comment

michaelxor Over a year ago

Ah, that makes sense. So do you know if there's a way to do what I want with parameter expansion? It seems like it may not be possible with bash. I've seen something like what I want accomplished with zsh: for i in ${(M)ar:#ba*}, which is I guess where I got the idea.

casc · Accepted Answer · 2014-01-27 12:43:08Z

1

Why not simply prepend the chopped-off b again?

(
shopt -s extglob
ar=( foo bar baz qux )
for i in ${ar[@]/#!(ba*)}; do
   echo "b${i}"
done
)

# ... or ...

(
export IFS=""
ar=( foo bar baz qux )
ar=( ${ar[@]/#!(ba*)} )
ar=( ${ar[@]/#/b} )
echo ${!ar[*]}
printf '%s\n' "${ar[@]}"
)

Further alternatives include:

ar=( foo bar baz qux )
printf '%s\n' "${ar[@]}" "${ar[@]##ba*}" | sort | uniq -u
printf '%s\n' "${ar[@]}" "${ar[@]/#ba*}" | sort | uniq -u


# alternative to current working solution 
(
array=( foo bar baz qux )
for item in "${ar[@]}"; do
   case "$item" in
      ba*) echo "$item";;
        *) :;;
   esac
done
)

answered Jan 27, 2014 at 12:43

casc

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1 Comment

michaelxor Over a year ago

Thanks for the input. The first two solutions rely on knowing all of the characters that were removed, so they don't work well if I try to replace ba* with *ba*. The last three solutions work with a wildcard at the beginning, but the printfs don't give me much flexibility. The last solution gives the best of both worlds but switch syntax is so ugly in shell -- it seems like a lateral move compared to my working solution. Out of curiosity, what are your paren blocks for? Does this scope local variables?

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