array_problems.md

Array problems

Arrays or commands involving arrays have sometimes an unexpected behavior depending on the array type:

• the usual "is variable defined?" check doesn't work for associative arrays
• the attempt to load a non-numeric index in a numeric array raises no error and returns a value.
  As a consequence, a reliable universal "does the array have an element at index x?" check can't be based on the fact whether a value is returned or not

"Is variable defined?" problem

The usual "does variable exist" syntax snippets don't work with associative arrays.

Take the [ -v <variable name> ] pattern for example (details):

integer_var=2
string_var="string"
numeric_array_var=("1st" "2nd")
declare -A associative_array_var && associative_array_var[index]="value"
[ -v "unknown" ]
echo "[ -v \"unknown\" ] returns $?"
[ -v "integer_var" ]
echo "[ -v \"integer_var\" ] returns $?"
[ -v "string_var" ]
echo "[ -v \"string_var\" ] returns $?"
[ -v "numeric_array_var" ]
echo "[ -v \"numeric_array_var\" ] returns $?"
[ -v "associative_array_var" ]
echo "[ -v \"associative_array_var\" ] returns $?"
[ -v "${associative_array_var}" ]
echo "[ -v \"\${associative_array_var}\" ] returns $?"

It prints:

[ -v "unknown" ] returns 1
[ -v "integer_var" ] returns 0
[ -v "string_var" ] returns 0
[ -v "numeric_array_var" ] returns 0
[ -v "associative_array_var" ] returns 1
[ -v "${associative_array_var}" ] returns 1

Many examples given on the web don't work in this case:

• the [ -n <variable> ] pattern, which gives the same result as [ -v <variable name> ] shown in the example above
• the ${<variable name>:?<fallback>} pattern (details)which fails for arrays

One solution is the pattern

[ -v <variable name> ] || [ "${#<variable name>[*]}" -gt 0 ]

It doesn't produce errors and returns the expected status (continuation from above, supposing identical variable definitions):

[ -v "unknown" ] || [ "${#unknown[*]}" -gt 0 ] returns 1
[ -v "integer_var" ] || [ "${#integer_var[*]}" -gt 0 ] returns 0
[ -v "string_var" ] || [ "${#string_var[*]}" -gt 0 ] returns 0
[ -v "numeric_array_var" ] || [ "${#numeric_array_var[*]}" -gt 0 ] returns 0
[ -v "associative_array_var" ] || [ "${#associative_array_var[*]}" -gt 0 ] returns 0

With one restriction however: since it uses a check for the number of elements of an eventual array, it returns with status 1 if the array is declared but empty.

"Does the array have an element at index x ?" problem

The snippet

numeric_array_var=("1st" "2nd")
echo " \$> numeric_array_var=(\"1st\" \"2nd\")"
declare -A associative_array_var && associative_array_var[index]="value"
echo " \$> declare -A associative_array_var && associative_array_var[index]=\"${associative_array_var[index]}\""
[ -n "${numeric_array_var[0]}" ]
echo "[ -n \"\${numeric_array_var[0]}\" ] returns $?"
[ -n "${numeric_array_var[100]}" ]
echo "[ -n \"\${numeric_array_var[100]}\" ] returns $?"
[ -n "${numeric_array_var[string]}" ]
echo "[ -n \"\${numeric_array_var[string]}\" ] returns $?"
[ -n "${associative_array_var[index]}" ]
echo "[ -n \"\${associative_array_var[index]}\" ] returns $?"
[ -n "${associative_array_var[undefined]}" ]
echo "[ -n \"\${associative_array_var[undefined]}\" ] returns $?"
[ -n "${associative_array_var[0]}" ]
echo "[ -n \"\${associative_array_var[0]}\" ] returns $?"

prints

 $> numeric_array_var=("1st" "2nd")
 $> declare -A associative_array_var && associative_array_var[index]="value"
[ -n "${numeric_array_var[0]}" ] returns 0
[ -n "${numeric_array_var[100]}" ] returns 1
[ -n "${numeric_array_var[string]}" ] returns 0
[ -n "${associative_array_var[index]}" ] returns 0
[ -n "${associative_array_var[undefined]}" ] returns 1
[ -n "${associative_array_var[0]}" ] returns 1

The expansion syntax returns a value if a non-numeric index is used in a numeric array. The pattern ${<variable name>[<index>]-<placeholder>} also gives unconclusive results (supposing the same variable definitions as above):

Numeric array element 0: ${numeric_array_var[0]} gives '1st' - ${numeric_array_var[0]-foo} gives '1st'
Numeric array element 100 (not defined): ${numeric_array_var[100]} gives '' - ${numeric_array_var[100]-foo} gives 'foo'
Numeric array element 'string' (not defined): ${numeric_array_var[string]} gives '1st' - ${numeric_array_var[string]-foo} gives '1st'
Associative array element 'index': ${associative_array_var[index]} gives 'value' - ${associative_array_var[index]-foo} gives 'value'
Associative array element 'undefined' (not defined): ${associative_array_var[undefined]} gives '' - ${associative_array_var[undefined]-foo} gives 'foo'
Associative array element 0 (not defined): ${associative_array_var[0]} gives '' - ${associative_array_var[0]-foo} gives 'foo'

Finally, a solution based on a grep over the expanded array indizes was implemented in bash commons' is_array_index()