diff --git a/modules/bash-commons/src/assert.sh b/modules/bash-commons/src/assert.sh
index 4ec6028..1e767e3 100644
--- a/modules/bash-commons/src/assert.sh
+++ b/modules/bash-commons/src/assert.sh
@@ -91,7 +91,7 @@ function assert_exactly_one_of {
   # Determine how many arg_vals are non-empty
   for (( i=0; i<$((num_args)); i+=2 )); do
     arg_names+=("${args[i]}")
-    if [[ ! -z "${args[i+1]}" ]]; then
+    if [[ -n "${args[i+1]}" ]]; then
       num_non_empty=$((num_non_empty+1))
     fi
   done
@@ -108,4 +108,12 @@ function assert_uid_is_root_or_sudo {
     log_error "This script should be run using sudo or as the root user"
     exit 1
   fi
-}
\ No newline at end of file
+}
+
+# Assert that the user running this script has permissions to run sudo.
+function assert_user_has_sudo_perms {
+  if ! sudo -n true >/dev/null 2>&1; then
+    log_error "This script should be run using sudo, as the root user, or as a user with sudo permissions."
+    exit 1
+  fi
+}
diff --git a/modules/bash-commons/src/string.sh b/modules/bash-commons/src/string.sh
index a7eca79..40467eb 100644
--- a/modules/bash-commons/src/string.sh
+++ b/modules/bash-commons/src/string.sh
@@ -56,3 +56,32 @@ function string_is_empty_or_null {
   local -r response="$1"
   [[ -z "$response" || "$response" == "null" ]]
 }
+
+
+# Given a string $str, return the substring beginning at index $start and ending at index $end.
+#
+# Example:
+#
+# string_substr "hello world" 0 5
+#   Returns "hello"
+function string_substr {
+  local -r str="$1"
+  local -r start="$2"
+  local end="$3"
+
+  if [[ "$start" -lt 0 || "$end" -lt 0 ]]; then
+    log_error "In the string_substr bash function, each of \$start and \$end must be >= 0."
+    exit 1
+  fi
+
+  if [[ "$start" -gt "$end" ]]; then
+    log_error "In the string_substr bash function, \$start must be < \$end."
+    exit 1
+  fi
+
+  if [[ -z "$end" ]]; then
+    end="${#str}"
+  fi
+
+  echo "${str:$start:$end}"
+}
diff --git a/test/string.bats b/test/string.bats
index 05526cf..aa84011 100644
--- a/test/string.bats
+++ b/test/string.bats
@@ -186,4 +186,13 @@ load "test-helper"
   assert_failure
 }
 
+@test "string_substr empty string" {
+    run string_substr "" 0 0
+    assert_success
+}
 
+@test "string_substr non empty string" {
+    run string_substr "hello world" 0 5
+    assert_success
+    assert_output "hello"
+}