notes: add redirection section

Signed-off-by: Khaïs COLIN <kcolin@student.42lehavre.fr>

NOTES.md

@@ -19,6 +19,11 @@ Parses the tokens into simple and compound commands (see *Shell Commands*)
 Performs the various _Shell Expansions_, breaking the expanded tokens into lists
 of filenames and commands and arguments.
 
+Performs any necessary _redirections_ and removes the **redirection operators**
+and their operands from the argument list.
+
+TODO: add missing operations
+
 ### Quoting Rules
 
 cf. 3.1.2 Quoting
@@ -252,7 +257,7 @@ by a slash in the pattern.
 cf. 3.5.8.1 Pattern Matching
 
 Any character that appears in a pattern, other than the special pattern
-characters described below, matches itself.  The NUL character may not occur in
+characters described below, matches itself. The NUL character may not occur in
 a pattern.
 
 The special pattern characters have the following meanings:
@@ -273,9 +278,83 @@ bash-5.1$ ls *'*'there
 #### Quote Removal
 cf. 3.5.9 Quote Removal
 
-After the preceding expansions, all unquoted occurrences of the
+After the preceding expansions, all unquoted occurrences of the characters '''
-characters ''' and '"' that did not result from one of the above
+and '"' that did not result from one of the above expansions are removed.
-expansions are removed.
+
+### Redirection
+cf. 3.6 Redirections
+
+Before a command is executed, its input and output may be "redirected" using a
+special notation interpreted by the shell.  "Redirection" allows commands' file
+handles to be made to refer to different files, and can change the files the
+command reads from and writes to.
+
+The redirection operators may precede or appear anywhere within a simple command
+or may follow a command.
+
+e.g. this is the correct behaviour
+```shell
+bash-5.1$ ls > hello.txt *here
+bash-5.1$ cat hello.txt
+hello*there
+hi*there
+noonethere
+```
+
+Redirections are processed in the order they appear, from left to right.
+
+e.g. this is the correct behaviour
+```shell
+bash-5.1$ ls > hello.txt share > here.txt *.txt
+bash-5.1$ ls -l hello.txt here.txt
+-rw-r--r-- 1 kcolin 2024_le-havre  0 Feb  7 15:54 hello.txt
+-rw-r--r-- 1 kcolin 2024_le-havre 68 Feb  7 15:54 here.txt
+bash-5.1$ cat here.txt
+hello.txt
+here.txt
+log.txt
+newlog-strict.txt
+newlog.txt
+
+share:
+man
+```
+
+'<' refers to the standard input (fd 0, STDIN\_FILENO)
+
+'>' refers to the standard output (fd 1, STDOUT\_FILENO)
+
+TODO: check unless otherwise noted
+The word following the redirection operator, unless otherwise noted, is
+subjected to parameter expansion, filename expansion, word splitting, and
+quote removal.
+
+If it expands to more than one word, Bash reports an error, except when in posix
+mode.
+TODO: decide if we follow posix default or bash default
+
+Note: This behaviour change is, as far as I can tell, not documented in the Bash
+Reference Manual.
+
+```shell
+# bash behaviour
+bash-5.1$ var="file1 file2"
+bash-5.1$ echo "hello world" > $var
+bash: $var: ambiguous redirect
+# posix behaviour
+bash-5.1$ var="file1 file2"
+bash-5.1$ echo "hello world" > $var
+bash-5.1$ cat "$var"
+hello world
+```
+
+In bash mode and in posix mode, if the variable is not defined, bash prints the
+following error:
+
+```shell
+bash-5.1$ echo "hello world" > $nonexist
+bash: $nonexist: ambiguous redirect
+```
 
 ## Subshell