Easiest way to check for an index or a key in an array?

Using:

set -o nounset

1. Having an indexed array like:

   myArray=( "red" "black" "blue" )
   

   What is the shortest way to check if element 1 is set?
   I sometimes use the following:

   test "${#myArray[@]}" -gt "1" && echo "1 exists" || echo "1 doesn't exist"
   

   I would like to know if there's a preferred one.

2. How to deal with non-consecutive indexes?

   myArray=()
   myArray[12]="red"
   myArray[51]="black"
   myArray[129]="blue"
   

   How to quick check that 51 is already set for example?

3. How to deal with associative arrays?

   declare -A myArray
   myArray["key1"]="red"
   myArray["key2"]="black"
   myArray["key3"]="blue"
   

   How to quick check that key2 is already used for example?

Solution 1:

To check if the element is set (applies to both indexed and associative array)

[ "${array[key]+abc}" ] && echo "exists"

Basically what ${array[key]+abc} does is

• if array[key] is set, return abc
• if array[key] is not set, return nothing

1. See Parameter Expansion in Bash manual and the little note

if the colon is omitted, the operator tests only for existence [of parameter]

2. This answer is actually adapted from the answers for this SO question: How to tell if a string is not defined in a bash shell script?

A wrapper function:

exists(){
  if [ "$2" != in ]; then
    echo "Incorrect usage."
    echo "Correct usage: exists {key} in {array}"
    return
  fi   
  eval '[ ${'$3'[$1]+muahaha} ]'  
}

For example

if ! exists key in array; then echo "No such array element"; fi 

Solution 2:

From man bash, conditional expressions:

-v varname
              True if the shell variable varname is set (has been assigned a value).

example:

declare -A foo
foo[bar]="this is bar"
foo[baz]=""
if [[ -v "foo[bar]" ]] ; then
  echo "foo[bar] is set"
fi
if [[ -v "foo[baz]" ]] ; then
  echo "foo[baz] is set"
fi
if [[ -v "foo[quux]" ]] ; then
  echo "foo[quux] is set"
fi

This will show that both foo[bar] and foo[baz] are set (even though the latter is set to an empty value) and foo[quux] is not.