[RFC PATCH v4 19/28] cli: hush_2021: Enable using \< and \> as string compare operators

[Francis Laniel <francis.laniel@amarulasolutions.com>]

In Busybox hush, '<' and '>' are used as redirection operators.
For example, cat foo > bar will write content of file foo inside file bar.
In U-Boot, we do not have file system, so we can hardly redirect command output
inside a file.

But, in actual U-Boot hush, these operators ('<' and '>') are used as string
compare operators.
For example, test aaa < bbb returns 0 as aaa is before bbb in the dictionary.
Busybox hush also permits this, but operators need to be escaped ('\<' and
'\>'), so we stick here to this behavior.
Also, if escaping is needed it permits the developer to think about its code, as
in a lot of case, we want to compare integers (using '-lt' or '-gt') rather than
strings.

Signed-off-by: Francis Laniel <francis.laniel@amarulasolutions.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
---
 common/cli_hush_upstream.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/common/cli_hush_upstream.c b/common/cli_hush_upstream.c
index a768fd6197..6ef6bb6bf4 100644
--- a/common/cli_hush_upstream.c
+++ b/common/cli_hush_upstream.c
@@ -6161,7 +6161,33 @@ static struct pipe *parse_stream(char **pstring,
 			if (parse_redirect(&ctx, redir_fd, redir_style, input))
 				goto parse_error_exitcode1;
 			continue; /* get next char */
-#endif /* !__U_BOOT__ */
+#else /* __U_BOOT__ */
+			/*
+			 * In U-Boot, '<' and '>' can be used in test command to test if
+			 * a string is, alphabetically, before or after another.
+			 * In 2021 Busybox hush, using test foo < bar will show the
+			 * following error:
+			 * hush: can't open 'bar': No such file or directory
+			 * Indeed, in this context, '>' and '<' are used as redirection
+			 * operators, to use them as sorting operators, one should use:
+			 * [[ foo < bar ]]
+			 * Or:
+			 * test foo \< bar
+			 *
+			 * For the moment, we will not implement [[ ]], so we will need
+			 * the user to escape these operators to use them as string
+			 * comparaison operators.
+			 * NOTE In my opinion, when you use '<' or '>' I am almost sure
+			 * you wanted to use "-gt" or "-lt" in place, so thinking to
+			 * escape these will make you should check your code (sh syntax
+			 * at this level is, for me, error prone).
+			 */
+			case '>':
+				/* fallthrough */
+			case '<':
+				printf("Redirection operator ('%c') is not supported!\nPlease escape it ('\\%c') to use it as string comparaison operator.\n", (char) ch, (char) ch);
+				goto parse_error_exitcode1;
+#endif /* __U_BOOT__ */
 		case '#':
 			if (ctx.word.length == 0 && !ctx.word.has_quoted_part) {
 				/* skip "#comment" */
-- 
2.25.1