Creating and populating nested associative array in bash

Question

I want to create a nested associative array in bash and populate it in a loop. Here is the sample code that should print all the file names along with the corresponding Last Modification Time of that file in the current directory.

declare -A file_map
for file in *
do
    declare -A file_attr
    uuid=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 16 | head -n 1)
    file_attr['name']="$file"
    file_attr['mod_date']=$(stat -c %y "$file")
    file_map["$uuid"]=file_attr
done


for key in "${!file_map[@]}"
do
    echo $(eval echo \${${file_map[$key]}['name']}) "->" $(eval echo \${${file_map[$key]}['mod_date']})
done

But it's only printing the information for a single file in the directory.

The output is coming as:

test.sh -> 2017-03-10 18:46:52.832356165 +0530
test.sh -> 2017-03-10 18:46:52.832356165 +0530
test.sh -> 2017-03-10 18:46:52.832356165 +0530
test.sh -> 2017-03-10 18:46:52.832356165 +0530

Whereas it should be, test.sh and 3 other different files.

Seems like declare -A file_attr is not creating any new associative array and as a result the previous values are getting overwritten. Any hep ?

a{k1}.b{k2} could be represented as x{k1.k2}. Isn't the flattening of the hash easier? — clt60
– clt60, Commented Mar 20, 2017 at 6:37
Only a function creates a new scope in shell, and file_map["$uuid"]=file_attr just assigns adds the string "file_attr", not an array reference, to the array. I strongly recommend using a language with proper data structures instead of bash. (As an additional benefit, you'l probably get a library that wraps the stat system call instead of having to run an external program for every file.) — chepner
– chepner, Commented Mar 20, 2017 at 12:04

chepner · Accepted Answer · 2017-03-20 13:00:45Z

1

You need to create a distinct, global array in each loop iteration, and store its name in the "outer" array. You'll need declare to assign to the dynamically generated arrays as well.

declare -A file_map
for file in *
do
    declare -A file_attr$((i++))
    # Consider using something like uuidgen instead
    # uuid=$(uuidgen)
    uuid=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 16 | head -n 1)
    declare -A "file_attr$i[name]=$file"
    declare -A "file_attr$i[mod_date]=$(stat -c %y "$file")"
    file_map[$uuid]="file_attr$i"
done

Use indirect variable expansion, not `eval`, to access the elements.

for uuid in "${!file_map[@]}"
do
    arr=${file_map[$uuid]}
    file_map_name=$arr[name]
    file_map_date=$arr[mod_date]
    echo "$uuid: ${!file_map_name} -> ${!file_map_date}"
done

answered Mar 20, 2017 at 13:00

chepner

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