How can I create a set-like array in bash?

Question

Suppose I have the following list of files:

/aaa/bbb/file1.txt
/aaa/ccc/file2.txt
/aaa/bbb/file3.txt
/aaa/bbb/file4.txt
/aaa/ccc/file5.txt

And I'd like to have a set of all of the unique dirnames in an array. The resulting array would look something like this:

dirs=( "/aaa/bbb" "/aaa/ccc" )

I think I can do something like this, but it feels really verbose (pardon the syntax errors, i don't have a shell handy):

dirs=()
for f in filelist do
    dir=$(dirname $f)
    i=0
    while [$i -lt ${#dirs[@]} ]; do
        if [ dirs[$i] == $dir ]
            break
        fi
        i=$[i + 1]
    done
    if [ $i -eq ${dirs[@]} ]
        dirs+=($dir)
    fi
 done

konsolebox · Accepted Answer · 2014-08-12 19:53:39Z

2

Use associative arrays:

declare -A dirs

for f in "${filelist[@]}"; do
    dir=$(exec dirname "$f") ## Or dir=${f%/*}
    dirs[$dir]=$dir
done

printf '%s\n' "${dirs[@]}"

Or if input is from file:

readarray -t files < filelist
for f in "${files[@]}"; do
    dir=$(exec dirname "$f") ## Or dir=${f%/*}
    dirs[$dir]=$dir
done

Let's keep unnecessary forks on a subshell minimum with exec.

edited Aug 12, 2014 at 19:53

answered Aug 12, 2014 at 18:24

konsolebox

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15 Comments

Keith Thompson Over a year ago

You don't need to set the value to $dir, just the key: dirs[$dir]=1. You can then use "${!dirs[@]}" to get the list of keys, and if [ "${dirs[FOO]:-}" ] to test whether "FOO" is in the set.

konsolebox Over a year ago

@KeithThompson Yes I considered that. However I still chose to add the value anyway and use "${dirs[@]}".

kojiro Over a year ago

You could apply that parameter expansion to the array itself in the second example: for file in "${files[@]%/*}"… (Also, you confuse f and file.)

konsolebox Over a year ago

@KeithThompson For less complexity. It's easier to process values over keys when you want to do other things like "${x[@]//something}".

clt60 Over a year ago

@kojiro see this stackoverflow.com/a/22402242/632407 for the comparison of the basename/dirname and variable substituions. ;)

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