Just another update

Task/Queue-Definition/UNIX-Shell/queue-definition-1.sh

@@ -0,0 +1,25 @@
+queue_push() {
+    typeset -n q=$1
+    shift
+    q+=("$@")
+}
+
+queue_pop() {
+    if queue_empty $1; then
+        print -u2 "queue $1 is empty"
+        return 1
+    fi
+    typeset -n q=$1
+    print "${q[0]}"     # emit the value of the popped element
+    q=( "${q[@]:1}" )   # and remove the first element from the queue
+}
+
+queue_empty() {
+    typeset -n q=$1
+    (( ${#q[@]} == 0 ))
+}
+
+queue_peek() {
+    typeset -n q=$1
+    print "${q[0]}"
+}

Task/Queue-Definition/UNIX-Shell/queue-definition-2.sh

@@ -0,0 +1,14 @@
+# any valid variable name can be used as a queue without initialization
+
+queue_empty foo && echo foo is empty || echo foo is not empty
+
+queue_push foo bar
+queue_push foo baz
+queue_push foo "element with spaces"
+
+queue_empty foo && echo foo is empty || echo foo is not empty
+
+print "peek: $(queue_peek foo)"; queue_pop foo
+print "peek: $(queue_peek foo)"; queue_pop foo
+print "peek: $(queue_peek foo)"; queue_pop foo
+print "peek: $(queue_peek foo)"; queue_pop foo