Add "which" method to Processes and ProcessHelper

src/main/java/org/kiwiproject/base/process/ProcessHelper.java

@@ -337,4 +337,15 @@ private Process launchPgrepWithParentPidFlag(long parentProcessId, ProcessHelper
         return processHelper.launch("pgrep", "-P", String.valueOf(parentProcessId));
     }
 
+    /**
+     * Locate a program in the user's path.
+     *
+     * @param program the program to locate
+     * @return an Optional containing the full path to the program, or an empty Optional if not found
+     * @implNote If there is more than program found, only the first one is returned
+     * @see Processes#which(String)
+     */
+    public Optional<String> which(String program) {
+        return Processes.which(program);
+    }
 }

src/main/java/org/kiwiproject/base/process/Processes.java

@@ -676,4 +676,17 @@ private static void validateKilledBeforeTimeout(long processId, boolean killedBe
                     format("Process %s was not killed before 1 second timeout expired", processId));
         }
     }
+
+    /**
+     * Locate a program in the user's path.
+     *
+     * @param program the program to locate
+     * @return an Optional containing the full path to the program, or an empty Optional if not found
+     * @implNote If there is more than program found, only the first one is returned
+     */
+    public static Optional<String> which(String program) {
+        var whichProc = launch("which", program);
+        var stdOut = readLinesFromInputStreamOf(whichProc);
+        return stdOut.stream().findFirst();
+    }
 }

src/test/java/org/kiwiproject/base/process/ProcessHelperTest.java

@@ -22,6 +22,7 @@
 import org.apache.commons.lang3.tuple.Pair;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
 
@@ -542,4 +543,23 @@ public int read() throws IOException {
             throw new IOException(message);
         }
     }
+
+    @Nested
+    class Which {
+
+        /**
+         * @implNote This test assumes {@link Processes#which(String)} works. This is a trade-off to make this
+         * test much simpler than having to replicate its logic in this test.
+         */
+        @Test
+        void shouldFindProgramThatExists() {
+            var lsPath = Processes.which("ls").orElseThrow();
+            assertThat(processes.which("ls")).contains(lsPath);
+        }
+
+        @Test
+        void shouldReturnEmptyOptional_WhenProgramDoesNotExistInPath() {
+            assertThat(processes.which("killify")).isEmpty();
+        }
+    }
 }

src/test/java/org/kiwiproject/base/process/ProcessesTest.java

@@ -250,4 +250,21 @@ void shouldThrowIllegalArgument_WhenPidIsNotNumeric(String pidString) {
                     .isThrownBy(() -> Processes.getPidOrThrow(pidString));
         }
     }
+
+    @Nested
+    class Which {
+
+        @ParameterizedTest
+        @ValueSource(strings = {"cp", "ls", "mv"})
+        void shouldFindProgramThatExists(String program) {
+            assertThat(Processes.which(program)).hasValueSatisfying(value -> assertThat(value).endsWith(program));
+        }
+
+        @ParameterizedTest
+        @ValueSource(strings = {"foobar", "abc-xyz", "clunkerate"})
+        void shouldReturnEmptyOptional_WhenProgramDoesNotExistInPath(String program) {
+            assertThat(Processes.which(program)).isEmpty();
+        }
+    }
+
 }