diff --git a/src/lexer/lexer.h b/src/lexer/lexer.h
index e06bec0..8c5b93e 100644
--- a/src/lexer/lexer.h
+++ b/src/lexer/lexer.h
@@ -4,6 +4,12 @@
 #include <sys/types.h>
 
 #include "lexer_utils.h"
+
+/*
+ * @brief returns true if token is a redir type, false otherwise
+ */
+bool is_token_redir(struct token *token);
+
 /*
  * @brief: returns the next (newly allocated) token without consuming it.
  *         if end of input is reached, returns a token of type TOKEN_EOF.
diff --git a/src/lexer/lexer_utils.c b/src/lexer/lexer_utils.c
index 833b274..9883bf7 100644
--- a/src/lexer/lexer_utils.c
+++ b/src/lexer/lexer_utils.c
@@ -178,6 +178,25 @@ static bool is_end_of_line(char c)
     return c == EOF || c == '\n';
 }
 
+// === Functions
+
+bool is_token_redir(struct token *token)
+{
+    switch (token->type)
+    {
+    case TOKEN_REDIR_LEFT:
+    case TOKEN_REDIR_RIGHT:
+    case TOKEN_REDIR_LEFT_RIGHT:
+    case TOKEN_REDIR_DOUBLE_RIGHT:
+    case TOKEN_REDIR_LEFT_AMP:
+    case TOKEN_REDIR_RIGHT_AMP:
+    case TOKEN_REDIR_RIGHT_PIPE:
+        return true;
+    default:
+        return false;
+    }
+}
+
 bool is_special_char(char *stream, ssize_t i)
 {
     char c = stream[i];
diff --git a/src/lexer/lexer_utils.h b/src/lexer/lexer_utils.h
index 6173485..29d444d 100644
--- a/src/lexer/lexer_utils.h
+++ b/src/lexer/lexer_utils.h
@@ -31,16 +31,13 @@ enum lexing_mode
 
 enum token_type
 {
-    // Special characters
+    // Blanks
     TOKEN_NULL = 0,
     TOKEN_EOF,
     TOKEN_WORD,
     TOKEN_NEWLINE,
 
-    // WARNING: quote and double quote should never be used inside a token.
-    TOKEN_QUOTE,
-    TOKEN_DOUBLE_QUOTE,
-
+    // SPecial characters
     TOKEN_GRAVE,
     TOKEN_SEMICOLON,
     TOKEN_COMMENT,
@@ -51,8 +48,14 @@ enum token_type
     TOKEN_RIGHT_PAREN,
     TOKEN_LEFT_BRACKET,
     TOKEN_RIGHT_BRACKET,
+    TOKEN_PIPE,
+    TOKEN_NEGATION, // TODO handle
 
-    // redirections
+    // TODO merge into one and use the data field
+    // (Too difficult to handle in the parser because of firsts)
+    // TOKEN_REDIRECTION
+    // 
+    // Redirections
     TOKEN_REDIR_LEFT,
     TOKEN_REDIR_RIGHT,
     TOKEN_REDIR_LEFT_RIGHT,
@@ -62,7 +65,6 @@ enum token_type
     TOKEN_REDIR_RIGHT_PIPE,
 
     TOKEN_IONUMBER,
-    TOKEN_PIPE,
 
     // Keywords
     TOKEN_IF,
diff --git a/src/parser/grammar.c b/src/parser/grammar.c
index c1457eb..6a192db 100644
--- a/src/parser/grammar.c
+++ b/src/parser/grammar.c
@@ -4,8 +4,8 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#include "grammar_basic.h"
 #include "../lexer/lexer.h"
+#include "grammar_basic.h"
 
 // === Static variables
 
@@ -34,8 +34,8 @@ static bool add_first(enum rule rule, enum token_type token)
 
         // Append
         item->list_length++;
-        item->tokens = realloc(
-            item->tokens, (item->list_length) * sizeof(enum token_type));
+        item->tokens = realloc(item->tokens,
+                               (item->list_length) * sizeof(enum token_type));
     }
     else
     {
@@ -91,24 +91,6 @@ static void add_first_redir(enum rule rule)
 
 // === Functions
 
-bool is_token_redir(struct token *token)
-{
-    switch (token->type)
-    {
-        case TOKEN_REDIR_LEFT:
-        case TOKEN_REDIR_RIGHT:
-        case TOKEN_REDIR_LEFT_RIGHT:
-        case TOKEN_REDIR_DOUBLE_RIGHT:
-        case TOKEN_REDIR_LEFT_AMP:
-        case TOKEN_REDIR_RIGHT_AMP:
-        case TOKEN_REDIR_RIGHT_PIPE:
-            return true;
-        default:
-            return false;
-    }
-}
-
-
 bool grammar_init(void)
 {
     // Initialize the firsts map
@@ -119,17 +101,21 @@ bool grammar_init(void)
     // Populate the firsts map
     add_first(RULE_INPUT, TOKEN_WORD);
     add_first(RULE_INPUT, TOKEN_IF);
+    add_first(RULE_COMMAND, TOKEN_NEGATION);
     add_first(RULE_INPUT, TOKEN_NEWLINE);
     add_first(RULE_INPUT, TOKEN_EOF);
 
     add_first(RULE_LIST, TOKEN_WORD);
     add_first(RULE_LIST, TOKEN_IF);
+    add_first(RULE_COMMAND, TOKEN_NEGATION);
 
     add_first(RULE_AND_OR, TOKEN_WORD);
     add_first(RULE_AND_OR, TOKEN_IF);
+    add_first(RULE_COMMAND, TOKEN_NEGATION);
 
     add_first(RULE_PIPELINE, TOKEN_WORD);
     add_first(RULE_PIPELINE, TOKEN_IF);
+    add_first(RULE_COMMAND, TOKEN_NEGATION);
 
     add_first(RULE_COMMAND, TOKEN_WORD);
     add_first(RULE_COMMAND, TOKEN_IF);
diff --git a/src/parser/grammar.h b/src/parser/grammar.h
index 3faf39d..9538848 100644
--- a/src/parser/grammar.h
+++ b/src/parser/grammar.h
@@ -50,10 +50,6 @@ struct firsts_list {
 
 // === Functions
 
-/* @brief: returns true if token has a type redirection (not pipe or IONUMBER).
- */
-bool is_token_redir(struct token *token);
-
 /*
  * @brief Initializes the grammar submodule
  * @return PARSER_INIT_SUCCESS on success PARSER_INIT_ERROR on error
diff --git a/src/parser/grammar_basic.c b/src/parser/grammar_basic.c
index d99ba6d..0b3b7c6 100644
--- a/src/parser/grammar_basic.c
+++ b/src/parser/grammar_basic.c
@@ -1,13 +1,13 @@
+#include <stdbool.h>
 #define _POSIX_C_SOURCE 200809L
 
-#include "grammar_basic.h"
-
 #include <stdio.h>
 #include <string.h>
 
 #include "../utils/lists/lists.h"
 #include "grammar.h"
 #include "grammar_advanced.h"
+#include "grammar_basic.h"
 
 // === Static functions
 
@@ -108,7 +108,42 @@ struct ast *parse_and_or(struct lexer_context *ctx)
 
 struct ast *parse_pipeline(struct lexer_context *ctx)
 {
-    return parse_command(ctx);
+    bool negation = false;
+    struct token *token = PEEK_TOKEN();
+
+    // Eventual '!'
+    if (token->type == TOKEN_NEGATION)
+    {
+        negation = true;
+        POP_TOKEN();
+        token = PEEK_TOKEN();
+    }
+
+    // TODO handle negation (new AST type)
+
+    struct ast *left = parse_command(ctx);
+
+    token = PEEK_TOKEN();
+    while (token->type == TOKEN_PIPE)
+    {
+        POP_TOKEN();
+
+        // skip newlines
+        token = PEEK_TOKEN();
+        while (token->type == TOKEN_NEWLINE)
+        {
+            POP_TOKEN();
+            token = PEEK_TOKEN();
+        }
+
+        struct ast *right = parse_command(ctx);
+
+        // Create AST
+        left = ast_create_pipe(left, right);
+        token = PEEK_TOKEN();
+    }
+
+    return left;
 }
 
 struct ast *parse_command(struct lexer_context *ctx)
@@ -125,7 +160,7 @@ struct ast *parse_command(struct lexer_context *ctx)
     }
     else
     {
-        puts("Syntax error: expected command");
+        puts("Syntax error: unexpected token");
         return NULL;
     }
 }
@@ -324,7 +359,7 @@ struct ast *parse_compound_list(struct lexer_context *ctx)
                 return NULL;
             result_list = list_append(result_list, current_cmd);
         }
-        
+
         token = PEEK_TOKEN();
     }
 
diff --git a/src/parser/grammar_basic.h b/src/parser/grammar_basic.h
index f1c2f77..5ef6a56 100644
--- a/src/parser/grammar_basic.h
+++ b/src/parser/grammar_basic.h
@@ -28,9 +28,9 @@ struct ast *parse_and_or(struct lexer_context *ctx);
 /*
  * @brief Only parses a command rule for the moment
  *
- * @code pipeline = command ;
+ * @code pipeline = ['!'] command { '|' {'\n'} command } ;
  *
- * @first first(command)
+ * @first '!', first(command)
  */
 struct ast *parse_pipeline(struct lexer_context *ctx);
 
diff --git a/src/utils/ast/ast.c b/src/utils/ast/ast.c
index b4afb9c..a507a33 100644
--- a/src/utils/ast/ast.c
+++ b/src/utils/ast/ast.c
@@ -2,7 +2,6 @@
 #define _POSIX_C_SOURCE 200809L
 #include "ast.h"
 
-#include <assert.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdio.h>
@@ -11,18 +10,45 @@
 void ast_free(struct ast **node)
 {
     if (node == NULL || *node == NULL)
+    {
+        puts(
+            "WARNING: Internal error: failed to free AST node (NULL argument)");
         return;
-    // ast void does not need to be freed.
-    if (ast_is_if(*node))
+    }
+
+    switch ((*node)->type)
+    {
+    case AST_IF:
         ast_free_if(ast_get_if(*node));
-    else if (ast_is_command(*node))
+        break;
+    case AST_CMD:
         ast_free_command(ast_get_command(*node));
-    else if (ast_is_list(*node))
+        break;
+    case AST_LIST:
         ast_free_list(ast_get_list(*node));
-    else if (ast_is_and_or(*node))
+        break;
+    case AST_AND_OR:
         ast_free_and_or(ast_get_and_or(*node));
-    else if (ast_is_redir(*node))
+        break;
+    case AST_REDIR:
         ast_free_redir(ast_get_redir(*node));
+        break;
+    case AST_PIPE:
+        ast_free_pipe(ast_get_pipe(*node));
+        break;
+    case AST_WORD:
+        ast_free_word(ast_get_word(*node));
+        break;
+
+    case AST_VOID:
+    case AST_END:
+        break;
+
+    default:
+        puts("WARNING: Internal error: failed to free an AST node (Unknown "
+             "type)");
+        return;
+    }
 
     free(*node);
     *node = NULL;
@@ -40,6 +66,7 @@ struct ast *ast_create(enum ast_type type, void *data)
     return node;
 }
 
+// TODO handle new types (AST_WORD, AST_PIPE, etc.)
 static void ast_print_dot_recursive(struct ast *node, FILE *out)
 {
     if (!node)
diff --git a/src/utils/ast/ast.h b/src/utils/ast/ast.h
index fc35666..cbb5efc 100644
--- a/src/utils/ast/ast.h
+++ b/src/utils/ast/ast.h
@@ -10,6 +10,7 @@
 #include "ast_redir.h"
 #include "ast_void.h"
 #include "ast_word.h"
+#include "ast_pipe.h"
 
 /**
  * Prints the Graphviz DOT representation of the given AST to stdout.
diff --git a/src/utils/ast/ast_and_or.h b/src/utils/ast/ast_and_or.h
index 344cfbf..4813592 100644
--- a/src/utils/ast/ast_and_or.h
+++ b/src/utils/ast/ast_and_or.h
@@ -1,8 +1,6 @@
 #ifndef AST_AND_OR_H
 #define AST_AND_OR_H
 
-#include <stdbool.h>
-
 #include "ast_base.h"
 
 enum ast_and_or_type
diff --git a/src/utils/ast/ast_base.h b/src/utils/ast/ast_base.h
index 4ae00ec..341b380 100644
--- a/src/utils/ast/ast_base.h
+++ b/src/utils/ast/ast_base.h
@@ -2,6 +2,7 @@
 #define AST_BASE_H
 
 #include <stdlib.h>
+#include <stdbool.h>
 
 enum ast_type
 {
@@ -12,7 +13,9 @@ enum ast_type
     AST_REDIR,
     AST_VOID,
     AST_CMD,
-    AST_WORD
+    AST_WORD,
+    AST_PIPE,
+    AST_NEG
 };
 
 struct ast
diff --git a/src/utils/ast/ast_command.c b/src/utils/ast/ast_command.c
index e52787e..2b6bbf6 100644
--- a/src/utils/ast/ast_command.c
+++ b/src/utils/ast/ast_command.c
@@ -1,6 +1,5 @@
 #include "ast_command.h"
 
-#include <assert.h>
 #include <stdbool.h>
 #include <stdlib.h>
 
@@ -19,15 +18,14 @@ struct ast *ast_create_command(struct list *command)
 
 struct ast_command *ast_get_command(struct ast *node)
 {
-    assert(node != NULL);
-    assert(node->type == AST_CMD);
+    if (node == NULL || node->type == AST_CMD)
+        return NULL;
     return (struct ast_command *)node->data;
 }
 
 bool ast_is_command(struct ast *node)
 {
-    assert(node != NULL);
-    return node->type == AST_CMD;
+    return node != NULL && node->type == AST_CMD;
 }
 
 void ast_free_command(struct ast_command *command_data)
diff --git a/src/utils/ast/ast_command.h b/src/utils/ast/ast_command.h
index 089225f..cf6b913 100644
--- a/src/utils/ast/ast_command.h
+++ b/src/utils/ast/ast_command.h
@@ -1,8 +1,6 @@
 #ifndef AST_COMMAND_H
 #define AST_COMMAND_H
 
-#include <stdbool.h>
-
 #include "../lists/lists.h"
 #include "ast_base.h"
 
diff --git a/src/utils/ast/ast_end.c b/src/utils/ast/ast_end.c
index fbde72f..0d314bc 100644
--- a/src/utils/ast/ast_end.c
+++ b/src/utils/ast/ast_end.c
@@ -1,13 +1,11 @@
 #include "ast_end.h"
 
-#include <assert.h>
 #include <stdbool.h>
 #include <stdlib.h>
 
 bool ast_is_end(struct ast *node)
 {
-    assert(node != NULL);
-    return node->type == AST_END;
+    return node != NULL && node->type == AST_END;
 }
 
 struct ast *ast_create_end(void)
diff --git a/src/utils/ast/ast_end.h b/src/utils/ast/ast_end.h
index d846cad..55b5322 100644
--- a/src/utils/ast/ast_end.h
+++ b/src/utils/ast/ast_end.h
@@ -1,8 +1,6 @@
 #ifndef AST_END_H
 #define AST_END_H
 
-#include <stdbool.h>
-
 #include "../lists/lists.h"
 #include "ast_base.h"
 
diff --git a/src/utils/ast/ast_if.c b/src/utils/ast/ast_if.c
index cff0320..1402ff6 100644
--- a/src/utils/ast/ast_if.c
+++ b/src/utils/ast/ast_if.c
@@ -1,6 +1,5 @@
 #include "ast_if.h"
 
-#include <assert.h>
 #include <stdbool.h>
 #include <stdlib.h>
 
@@ -20,15 +19,14 @@ struct ast *ast_create_if(struct ast *condition, struct ast *then_clause,
 
 struct ast_if *ast_get_if(struct ast *node)
 {
-    assert(node != NULL);
-    assert(node->type == AST_IF);
-    return (struct ast_if *)node->data;
+    if (node == NULL || node->type == AST_IF)
+        return NULL;
+    return node->data;
 }
 
 bool ast_is_if(struct ast *node)
 {
-    assert(node != NULL);
-    return node->type == AST_IF;
+    return node != NULL && node->type == AST_IF;
 }
 
 void ast_free_if(struct ast_if *if_data)
diff --git a/src/utils/ast/ast_if.h b/src/utils/ast/ast_if.h
index f54a795..f1842bd 100644
--- a/src/utils/ast/ast_if.h
+++ b/src/utils/ast/ast_if.h
@@ -1,8 +1,6 @@
 #ifndef AST_IF_H
 #define AST_IF_H
 
-#include <stdbool.h>
-
 #include "ast_base.h"
 
 struct ast_if
diff --git a/src/utils/ast/ast_list.c b/src/utils/ast/ast_list.c
index 2b96a23..cb4aaa6 100644
--- a/src/utils/ast/ast_list.c
+++ b/src/utils/ast/ast_list.c
@@ -1,6 +1,4 @@
-#include <assert.h>
-
-#include "ast.h"
+#include "ast_list.h"
 
 struct ast *ast_create_list(struct list *list)
 {
@@ -15,13 +13,14 @@ struct ast *ast_create_list(struct list *list)
 
 struct ast_list *ast_get_list(struct ast *node)
 {
-    assert(node != NULL);
-    return (struct ast_list *)node->data;
+    if (node == NULL)
+        return NULL;
+    return node->data;
 }
 
 bool ast_is_list(struct ast *node)
 {
-    return node->type == AST_LIST;
+    return node != NULL && node->type == AST_LIST;
 }
 
 void ast_free_list(struct ast_list *ast_list)
diff --git a/src/utils/ast/ast_list.h b/src/utils/ast/ast_list.h
index ee98ea5..21b24fb 100644
--- a/src/utils/ast/ast_list.h
+++ b/src/utils/ast/ast_list.h
@@ -1,8 +1,6 @@
 #ifndef AST_LIST_H
 #define AST_LIST_H
 
-#include <stdbool.h>
-
 #include "../lists/lists.h"
 #include "ast_base.h"
 
diff --git a/src/utils/ast/ast_neg.c b/src/utils/ast/ast_neg.c
new file mode 100644
index 0000000..f5817fc
--- /dev/null
+++ b/src/utils/ast/ast_neg.c
@@ -0,0 +1,32 @@
+#include "ast_neg.h"
+
+#include <stdlib.h>
+
+bool ast_is_neg(struct ast *node)
+{
+    return node != NULL && node->type == AST_REDIR;
+}
+
+struct ast_neg *ast_get_neg(struct ast *node)
+{
+    if (ast_is_neg(node))
+        return node->data;
+    return NULL;
+}
+
+struct ast *ast_create_neg(bool negation, struct ast *child)
+{
+    struct ast_neg *node = malloc(sizeof(struct ast_neg));
+    if (!node)
+        return NULL;
+
+    return ast_create(AST_NEG, node);
+}
+
+void ast_free_neg(struct ast_neg *node)
+{
+    if (!node)
+        return;
+    ast_free(&node->child);
+    free(node);
+}
diff --git a/src/utils/ast/ast_neg.h b/src/utils/ast/ast_neg.h
new file mode 100644
index 0000000..8c2d4e8
--- /dev/null
+++ b/src/utils/ast/ast_neg.h
@@ -0,0 +1,17 @@
+#ifndef AST_NEG_H
+#define AST_NEG_H
+
+#include "ast_base.h"
+
+struct ast_neg
+{
+    bool negation; // True negates the child's output
+    struct ast* child;
+};
+
+bool ast_is_neg(struct ast *node);
+struct ast_neg *ast_get_neg(struct ast *node);
+struct ast *ast_create_neg(bool negation, struct ast* child);
+void ast_free_neg(struct ast_neg* node);
+
+#endif /* ! AST_NEG_H */
diff --git a/src/utils/ast/ast_pipe.c b/src/utils/ast/ast_pipe.c
new file mode 100644
index 0000000..92754cf
--- /dev/null
+++ b/src/utils/ast/ast_pipe.c
@@ -0,0 +1,35 @@
+#include "ast_pipe.h"
+
+#include <stdlib.h>
+
+bool ast_is_pipe(struct ast *node)
+{
+    return node != NULL && node->type == AST_REDIR;
+}
+
+struct ast_pipe *ast_get_pipe(struct ast *node)
+{
+    if (ast_is_pipe(node))
+        return node->data;
+    return NULL;
+}
+
+struct ast *ast_create_pipe(struct ast *left, struct ast *right)
+{
+    struct ast_pipe *node = malloc(sizeof(struct ast_pipe));
+    if (!node)
+        return NULL;
+    node->left = left;
+    node->right = right;
+
+    return ast_create(AST_PIPE, node);
+}
+
+void ast_free_pipe(struct ast_pipe *node)
+{
+    if (!node)
+        return;
+    ast_free(&node->left);
+    ast_free(&node->right);
+    free(node);
+}
diff --git a/src/utils/ast/ast_pipe.h b/src/utils/ast/ast_pipe.h
new file mode 100644
index 0000000..c5e1e39
--- /dev/null
+++ b/src/utils/ast/ast_pipe.h
@@ -0,0 +1,18 @@
+#ifndef AST_PIPE_H
+#define AST_PIPE_H
+
+#include "ast_base.h"
+
+struct ast_pipe
+{
+    struct ast* left;
+    struct ast* right;
+    // Output of left will be redirected to right stdin
+};
+
+bool ast_is_pipe(struct ast *node);
+struct ast_pipe *ast_get_pipe(struct ast *node);
+struct ast *ast_create_pipe(struct ast* left, struct ast* right);
+void ast_free_pipe(struct ast_pipe* node);
+
+#endif /* ! AST_PIPE_H */
diff --git a/src/utils/ast/ast_redir.h b/src/utils/ast/ast_redir.h
index 4b2eb63..4c1008a 100644
--- a/src/utils/ast/ast_redir.h
+++ b/src/utils/ast/ast_redir.h
@@ -1,8 +1,6 @@
 #ifndef AST_REDIR_H
 #define AST_REDIR_H
 
-#include <stdbool.h>
-
 #include "ast_base.h"
 
 enum ast_redir_type
diff --git a/src/utils/ast/ast_void.c b/src/utils/ast/ast_void.c
index 213413b..e7d2dee 100644
--- a/src/utils/ast/ast_void.c
+++ b/src/utils/ast/ast_void.c
@@ -1,13 +1,11 @@
 #include "ast_void.h"
 
-#include <assert.h>
 #include <stdbool.h>
 #include <stdlib.h>
 
 bool ast_is_void(struct ast *node)
 {
-    assert(node != NULL);
-    return node->type == AST_VOID;
+    return node != NULL && node->type == AST_VOID;
 }
 
 struct ast *ast_create_void(void)
diff --git a/src/utils/ast/ast_void.h b/src/utils/ast/ast_void.h
index ecbdcf8..05a5933 100644
--- a/src/utils/ast/ast_void.h
+++ b/src/utils/ast/ast_void.h
@@ -1,8 +1,6 @@
 #ifndef AST_VOID_H
 #define AST_VOID_H
 
-#include <stdbool.h>
-
 #include "../lists/lists.h"
 #include "ast_base.h"
 
diff --git a/src/utils/ast/ast_word.h b/src/utils/ast/ast_word.h
index d5e0e1d..571f869 100644
--- a/src/utils/ast/ast_word.h
+++ b/src/utils/ast/ast_word.h
@@ -1,8 +1,6 @@
 #ifndef AST_WORD_H
 #define AST_WORD_H
 
-#include <stdbool.h>
-
 #include "ast_base.h"
 
 struct ast_word