Add rev.c for reversing strings and file lines

Implement a command-line utility to reverse strings or lines from a file.

src/userland/cli/rev.c

@@ -0,0 +1,43 @@
+// Copyright (c) 2026 janevers (https://github.com/zeyadhost)
+// This software is released under the GNU General Public License v3.0. See LICENSE file for details.
+// This header needs to maintain in any file it is present in, as per the GPL license terms.
+
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdlib.h>
+
+// reverse function
+void reverse(char *str) {
+    int len = strlen(str);
+    for (int i = 0; i < len / 2; i++) {
+        char tmp = str[i];
+        str[i] = str[len - 1 - i];
+        str[len - 1 - i] = tmp;
+    }
+}
+
+int main(int argc, char *argv[]) {
+    // check if there are too much flags
+    if (argc != 2) {
+        printf("Wrong input, use --help for more info\n");
+        return 1;
+    }
+
+    FILE *fp = fopen(argv[1], "r");
+    // check if the argument is a file or not by opening it
+    if (fp == NULL) {
+        reverse(argv[1]);
+        printf("%s\n", argv[1]);
+        return 0;
+    }
+    // reverse if flag is a file
+    char line[1024];
+    while (fgets(line, sizeof(line), fp)) {
+        line[strcspn(line, "\n")] = '\0';
+        reverse(line);
+        printf("%s\n", line);
+    }
+    fclose(fp);
+    return 0;
+}