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

Question

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?

Answer 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

---

References:

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