additions to bash vars and arrays

Programming_Languages/Shell/Lists_and_arrays.md

@@ -69,17 +69,48 @@ echo preExistingArray
 
 > Note that we have to put the item we want to addend within array brackets
 
-## Looping through file system
+## Check if array empty
 
-The following script loops through all files in a directory that begin with `l` and which are of the bash file type (`.sh`) :
+```sh
+an_array=()
 
-```bash
-for x in ./l*.sh; do
-    echo -n "$x "
-done
-echo
+if [ -z "${another_array[@]}" ]; then
+    echo "Array is empty"
+else
+    echo "Array is not empty"
+fi
 ```
 
+Here we pass all the elements of the array to a [test](/Programming_Languages/Shell/Test_values_in_Bash.md) condition which tests for an empty string.
+
+> NB: This will not immediately work in the context of a function. See below.
+
+## Weirdness with functions
+
+When you pass an array as an argument to a [function](/Programming_Languages/Shell/Functions_in_Bash.md) it will not immediately be understood to be an array.
+
+When we use `$1` to individuate the first function argument this is read as string. So if you parsed an array argument as `$1`, any logic you have in the function will work on the assumption that the argument is a string, not an array.
+
+To get round this we have to effectively _redeclare_ the argument as an array before running any array logic. We do this through a **nameref** (a reference to a variable). The nameref resolves to the value of the variable it references. This allows you to indirectly manipulate the value of the original variable through the nameref.
+
+A nameref is created with the `-n` flag. The following function uses this method to check if an array is empty:
+
+```sh
+function array_empty() {
+    declare -n arr=$1
+    if [ ${#arr[@]} -gt 0 ]; then
+        echo "array is not empty"
+    else
+        echo "array is empty"
+    fi
+}
+
+my_array=()
+array_empty "my_array"
+```
+
+You'll notice when we invoke the function we pass the array as a string, this facilitates the nameref operation in the function.
+
 ## Associational arrays / maps
 
 With Bash 4 we gained an additional array-like data structure that is key-value based and similar to maps in other languages.

Programming_Languages/Shell/Variables_and_data_types.md

@@ -29,6 +29,28 @@ echo ${boolVar} # true
 
 - Quotation marks at declaration are also not strictly necessary however they can help avoid bugs. Also serves as a reminder that every type is basically a string at the end of the day
 
+## Common operations
+
+### Check if a variable exists
+
+```sh
+if [ -v variable_name ]; then
+    echo "The variable exists."
+else
+    echo "The variable does not exist."
+fi
+```
+
+### Check if a variable is set (defined)
+
+```sh
+if [ -z "${variable_name+x}" ]; then
+    echo "The variable does not exist."
+else
+    echo "The variable exists."
+fi
+```
+
 ## Parameter expansion
 
 // TODO: What is the difference betweeen `$var`, `${var}` and `"${var}"` ?