Update MentOs code to the latest development version.

programs/echo.c

@@ -0,0 +1,117 @@
+///                MentOS, The Mentoring Operating system project
+/// @file echo.c
+/// @brief
+/// @copyright (c) 2014-2021 This file is distributed under the MIT License.
+/// See LICENSE.md for details.
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <sys/bitops.h>
+
+#define ENV_NORM 1
+#define ENV_BRAK 2
+#define ENV_PROT 3
+
+void expand_env(char *str, char *buf, size_t buf_len, int first_word, int last_word)
+{
+    // Buffer where we store the name of the variable.
+    char buffer[BUFSIZ] = { 0 };
+    // Flags used to keep track of the special characters.
+    unsigned flags = 0;
+    // We keep track of where teh
+    char *env_start = NULL;
+    // Where we store the retrieved environmental variable value.
+    char *ENV = NULL;
+    // Get the length of the string.
+    size_t str_len = strlen(str);
+    // Position where we are writing on the buffer.
+    int b_pos = 0;
+    // Iterate the string.
+    for (int s_pos = 0; s_pos < str_len; ++s_pos) {
+        if (first_word && (s_pos == 0) && str[s_pos] == '"')
+            continue;
+        if (last_word && (s_pos == (str_len - 1)) && str[s_pos] == '"')
+            continue;
+        // If we find the backslash, we need to protect the next character.
+        if (str[s_pos] == '\\') {
+            if (bit_check(flags, ENV_PROT))
+                buf[b_pos++] = '\\';
+            else
+                bit_set_assign(flags, ENV_PROT);
+            continue;
+        }
+        // If we find the dollar, we need to catch the meaning.
+        if (str[s_pos] == '$') {
+            // If the previous character is a backslash, we just need to print the dollar.
+            if (bit_check(flags, ENV_PROT)) {
+                buf[b_pos++] = '$';
+            } else if ((s_pos < (str_len - 2)) && ((str[s_pos + 1] == '{'))) {
+                // Toggle the open bracket method of accessing env variables.
+                bit_set_assign(flags, ENV_BRAK);
+                // We need to skip both the dollar and the open bracket `${`.
+                env_start = &str[s_pos + 2];
+            } else {
+                // Toggle the normal method of accessing env variables.
+                bit_set_assign(flags, ENV_NORM);
+                // We need to skip the dollar `$`.
+                env_start = &str[s_pos + 1];
+            }
+            continue;
+        }
+        if (bit_check(flags, ENV_BRAK)) {
+            if (str[s_pos] == '}') {
+                // Copy the environmental variable name.
+                strncpy(buffer, env_start, &str[s_pos] - env_start);
+                // Search for the environmental variable, and print it.
+                if ((ENV = getenv(buffer)))
+                    for (int k = 0; k < strlen(ENV); ++k)
+                        buf[b_pos++] = ENV[k];
+                // Remove the flag.
+                bit_clear_assign(flags, ENV_BRAK);
+            }
+            continue;
+        }
+        if (bit_check(flags, ENV_NORM)) {
+            if (str[s_pos] == ':') {
+                // Copy the environmental variable name.
+                strncpy(buffer, env_start, &str[s_pos] - env_start);
+                // Search for the environmental variable, and print it.
+                if ((ENV = getenv(buffer)))
+                    for (int k = 0; k < strlen(ENV); ++k)
+                        buf[b_pos++] = ENV[k];
+                // Copy the `:`.
+                buf[b_pos++] = str[s_pos];
+                // Remove the flag.
+                bit_clear_assign(flags, ENV_NORM);
+            }
+            continue;
+        }
+        buf[b_pos++] = str[s_pos];
+    }
+    if (bit_check(flags, ENV_NORM)) {
+        // Copy the environmental variable name.
+        strcpy(buffer, env_start);
+        // Search for the environmental variable, and print it.
+        if ((ENV = getenv(buffer)))
+            for (int k = 0; k < strlen(ENV); ++k)
+                buf[b_pos++] = ENV[k];
+        // Remove the flag.
+        bit_clear_assign(flags, ENV_NORM);
+    }
+}
+
+int main(int argc, char **argv)
+{
+    char buffer[BUFSIZ];
+    // Iterate all the words.
+    for (int i = 1; i < argc; ++i) {
+        memset(buffer, 0, BUFSIZ);
+        expand_env(argv[i], buffer, BUFSIZ, (i == 1), (i == (argc - 1)));
+        puts(buffer);
+        if (i < (argc - 1))
+            putchar(' ');
+    }
+    printf("\n\n");
+    return 0;
+}