Count number of elements in bash array, where the name of the array is dynamic (i.e. stored in a variable)

Question

Brief statement of the question:

Is there built-in bash method to count number of elements in bash array, where the name of the array is dynamic (i.e. stored in a variable), without resorting to making a fully copy of the array or using eval?

More information:

Using bash parameter substitution, one can do the following:

• Determine the length of an array:
  myArr=(A B C); echo ${#myArr[@]}.
• Indirectly reference a variable by name:
  NAME=myVar; echo ${!NAME}
  (this also applies to array elements):
  NAME=myArr[1]; echo ${!NAME}

But if the name of an array is stored in another variable, how can one determine the number of elements in the array? (One might consider this a combination of the above two parameter substitutions.) For example:

myArr=(A B C D)
NAME=myArr
# Get the number of elements in the array indirectly referenced by NAME.
count=${#$NAME[@]}  # This syntax is invalid. What is the right way?

Below are multiple attempts that all FAIL:

  # Setup for following attempts:
  myArr=(A B C D)
  NAME=myArr
  EXPR1=$NAME[@]          # i.e. EXPR1='myArr[@]'
  EXPR2=#$NAME[@]         # i.e. EXPR2='#myArr[@]'

  # Failed attempts to get the lengh of the array indirectly:
  1.  count=${#$NAME[@]}  # ERROR: bash: ...: bad substitution
  2.  count=${#!EXPR1}    # ERROR: bash: !EXPR}: event not found
  3.  count=${#\!EXPR1}   # ERROR: bash: ...: bad substitution
  4.  count=${!#EXPR1}    # ERROR: bash: ...: bad substitution
  5.  count=${!EXPR2}     # Returns NULL

I've also tried some other variants of the above, but have not yet found anything that works without either: (A) making a copy of the array or (B) by using eval.

Working Methods:

There are a couple ways of solving this that are probably not optimal (but correct me if I'm wrong):

Method 1: Copy the Array

Assign the array to another (statically-named) variable and get the number of elements in it.

EXPR=$NAME[@]
arrCopy=( "${!EXPR}" )
count=${#arrCopy}

Method 2: Use eval

EXPR="count=\${#$NAME[@]}"  # i.e. 'count=${myArr[@]}'
eval $EXPR
# Now count is set to the length of the array

Summary:

Is there any built-in method (i.e. parameter substitution syntax) in bash to determine the length of an array indirectly? If not, what is the most efficient way to do this? I assume it is the eval method above, but are there security or performance issues with eval?

Answer 1

you should handle that stuff in the index evals. and you can indirect through your indirection variable's indices if you make it an array.

a=(abc1 def2 ghi3 jkl4 mno5)
r=('a[c=${#a[@]}]' a\[i] a\[@])
for   i in   0 1 2 3 4 5
do    c=
      printf "<%s>\n" "${!r-${!r[i<c?1:2]}}"
      printf "\n\tindex is $i and count is $c\n\n"
done

---

<abc1>

    index is 0 and count is 5

<def2>

    index is 1 and count is 5

<ghi3>

    index is 2 and count is 5

<jkl4>

    index is 3 and count is 5

<mno5>

    index is 4 and count is 5

<abc1>
<def2>
<ghi3>
<jkl4>
<mno5>

    index is 5 and count is 5

Because bash's indices are 0-based, the total count of array objects will always work out to one more than than the highest set index, and so:

c=
echo "${a[c=${#a[@]}]-this index is unset}" "$c"

---

this index is unset 5

...the parameter expands out to the default word if any is provided.

If one is not provided:

c=
${!r}
echo "$c"

---

5

...there's no harm done.

In the loop I track an $index variable and check if it is at least as large as $count. When it is lesser I expand the $reference var to a[i] because it is a valid index, but when it is equal or greater I expand the $ref to the entire $array.

Here it is in a function:

ref_arr(){
    local    index=-1 count=
    local    ref=(   "$1[ count= \${#$1[@]}  ]"
                     "$1[ index ]"    "$1[ @ ]"
    )  &&    printf  "input array '%s' has '%d' members.\n" \
                     "$1"  "${!ref-${count:?invalid array name: "'$1'"}}"
    while    [ "$((index+=1))" -lt "$count"  ]
    do       printf  "$1[$index]  ==  '%s'\n"  "${!ref[1]}"
    done
}
some_array=(some "dumb
            stuff" 12345\'67890 "" \
          '$(kill my computer)')
ref_arr some_array
ref_arr '$(echo won'\''t work)'

---

input array 'some_array' has '5' members.
some_array[0]  ==  'some'
some_array[1]  ==  'dumb
                stuff'
some_array[2]  ==  '12345'67890'
some_array[3]  ==  ''
some_array[4]  ==  '$(kill my computer)'
bash: count: invalid array name: '$(echo won't work)'

Answer 2

bash 4.3 namerefs are a godsend. However, you can do this:

$ myArr=(A B C D)
$ NAME=myArr
$ tmp="${NAME}[@]"
$ copy=( "${!tmp}" )
$ echo "${#copy[@]}"
4