Grep value of a specific key from a String, concatenated of key : value pairs

Use awk, read key:value pair as record, read key, value as 1st and 2nd field.

awk -v RS=' *, *' -v FS=' *: *' '$1=="KEY2"{print $2}'  <<<$str

• -v RS=' *, *' set record separator to , and it's surrounding space
• -v FS=' *: *' set field separator to : and it's surrounding space
• '$1=="KEY2"{print $2}' print value if key found. change "KEY2" to your desired key value.

To grep only the value

printf '%sn' "$myString" | grep -Po "(?<=KEY2:)[^,]*"

or

grep -Po "(?<=KEY2:)[^,]*" <<< "$myString"

With regular grep assuming VALUE doesn't contain a colon:

grep -o 'KEY2:[^,]+' | grep -o '[^:]+$'

Using PCRE-enabled grep implementations:

grep -Po '(^|[ ,])KEY1:K[^,]*'

or egrep and cut:

grep -Eo '(^|[ ,])KEY2:[^,]*' | cut -d: -f2-

For both methods, the Value is not allowed to contain comma.

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If you had proper json, e.g.

{ "KEY1":"VALUE1", "KEY2":"VALUE2", "KEY3":"VALUE3" }

you could use jq:

$ jq .KEY2
"VALUE2"
$ jq -r .KEY2
VALUE2