peek(1): remove unused variables, clean up some typos

Makefile

@@ -102,11 +102,15 @@ mm: build/bin/mm
 build/bin/mm: src/mm.c build
 	$(CC) $(CFLAGS) -o $@ src/mm.c
 
-
 .PHONY: npc
 npc: build/bin/npc
 build/bin/npc: src/npc.c build
-	$(CC) $(CFLAGAS) -o $@ src/npc.c
+	$(CC) $(CFLAGS) -o $@ src/npc.c
+
+.PHONY: peek
+peek: build/bin/peek
+build/bin/peek: src/peek.c build
+	$(CC) $(CFLAGS) -o $@ src/peek.c
 
 .PHONY: rpn
 rpn: build/bin/rpn

docs/peek.1

@@ -0,0 +1,90 @@
+.\" Copyright (c) 2023-2024 DTB <trinity@trinity.moe>
+.\"
+.\" This work is licensed under CC BY-SA 4.0. To see a copy of this license,
+.\" visit <http://creativecommons.org/licenses/by-sa/4.0/>.
+
+.TH PEEK 1
+
+.SH NAME
+
+peek \(en read from the standard input, furtively
+
+.SH SYNOPSIS
+
+peek
+.RB ( -i )
+
+.SH DESCRIPTION
+
+Peek reads input from the standard input with terminal echo disabled.
+
+.SH OPTIONS
+
+.B -i
+.RS
+Allows input to come from sources other than terminals (i.e. a pipe).
+.RE
+
+.SH DIAGNOSTICS
+
+Peek prints an error message and exits with the appropriate status from
+sysexits.h(3) if it encounters an unrecoverable error.
+
+Peek tries to handle the signal SIGINT (^C) to ensure the user's terminal is
+still usable after premature termination; if the signal can't be handled, it
+prints an error message and continues. If peek is interrupted, it exits
+unsuccessfully, without an error message.
+
+.SH RATIONALE
+
+This tool was originally written to accept passwords in shell scripts, as an
+extremely simple alternative to the GNU Privacy Guard project's pinentry(1).
+Accepting input without showing what is being typed is useful when keying in
+secrets in public settings or places with installed surveillance cameras.
+
+.SH BUGS
+
+This does nothing to prevent others seeing the keyboard being used to input
+secrets or mask the sound of typing. Audio or video recordings of typing can be
+used to determine what was input without needing to see the characters appear
+on the screen.
+
+Accepting secrets in shell scripts is probably not adviseable.
+
+On systems that support it, the ioctl(2) command TIOCSTI can be used to insert
+characters into the standard input going to peek. This doesn't allow snooping
+but can be used for general mischief.
+
+.SH EXAMPLES
+
+This is an sh(1p) command line that hashes a given password. It uses head(1p)
+to only accept one line of input, xargs(1p) and printf(1p) to strip the
+trailing newline, htpasswd(1) from Apache's utilities to hash the input with
+the bcrypt algorithm, and cut(1p) to print only the resulting hash:
+
+.RS
+.R $ peek | head -n 1 | xargs printf '%s' | htpasswd -nBi _ | cut -d : -f 2
+.RE
+
+This is an sh(1p) command line that lets a user blindly write into a text file,
+only able to see written lines. Some writers have the habit of prematurely
+revising their work and use tools like this to prevent it. It uses mm(1) to
+pipe the output of peek to both the standard error and the regular file
+writing.txt.
+
+.RS
+.R $ echo Input ^D to quit. && peek | mm -eo - >writing.txt
+.RE
+
+.SH AUTHOR
+
+Written by DTB <trinity@trinity.moe>.
+
+.SH COPYRIGHT
+
+Copyright (c) 2023-2024 DTB. License AGPLv3+: GNU AGPL version 3 or later
+<https://gnu.org/licenses/gpl.html>.
+
+.SH SEE ALSO
+
+ioctl(2), ioctl_tty(2), read(1), sh(1)

src/peek.c

@@ -0,0 +1,118 @@
+/*
+ * Copyright (c) 2023–2024 DTB <trinity@trinity.moe>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ *
+ * This program is free software: you can redistribute it and/or modify it under
+ * the terms of the GNU Affero General Public License as published by the Free
+ * Software Foundation, either version 3 of the License, or (at your option) any
+ * later version.
+ * 
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+ * details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see https://www.gnu.org/licenses/.
+ */
+
+#include <signal.h> /* sigaction(2), signal(2), struct sigaction, SIGINT */
+#include <stdio.h> /* fprintf(3), fgetc(3), perror(3), fputc(3), stderr, stdin,
+                    * stdout, EOF, NULL */
+#include <stdlib.h> /* exit(3), EXIT_FAILURE */
+#if !defined EX_IOERR || !defined EX_OK || !defined EX_USAGE
+#	include <sysexits.h>
+#endif
+#include <termios.h> /* tcgetattr(3), tcsetattr(3), struct termios, ECHO */
+#include <unistd.h> /* getopt(3), isatty(3), STDIN_FILENO */
+
+static char *program_name = "peek";
+
+/* Restores terminal echo; otherwise when a user ^Cs the terminal would
+ * continue to not display typed text. If sig isn't zero, this will terminate
+ * the program. */
+static void restore_echo(int sig){
+	static struct termios t;
+
+	tcgetattr(STDIN_FILENO, &t);
+	t.c_lflag |= ECHO;
+	tcsetattr(STDIN_FILENO, TCSAFLUSH, &t);
+
+	/* If, for whatever ungodly reason, exit(3) returns, the user will notice
+	 * their typed characters on the screen. */
+	if(sig != 0)
+		exit(EXIT_FAILURE);
+
+	return;
+}
+
+static int ioerr(char *s){ perror(s); restore_echo(0); return EX_IOERR; }
+static int usage(char *s){
+	fprintf(stderr, "Usage: %s (-1enot) (-p [program [arguments...]])\n", s);
+	return EX_USAGE;
+}
+
+int main(int argc, char *argv[]){
+
+	if(argc < 1)
+		return usage(program_name);
+
+	{ /* option parsing */
+		char allow_nonterminals;
+		int c;
+
+		allow_nonterminals = 0;
+		while((c = getopt(argc, argv, "i")) != -1)
+			switch(c){
+			case 'i':	allow_nonterminals = 1; break;
+			default:	return usage(argv[0]);
+			}
+
+		if(argc > optind)
+			return usage(argv[0]);
+
+		if(!allow_nonterminals && isatty(STDIN_FILENO) != 1){
+			fprintf(stderr, "%s: Must be run in a terminal"
+			    " (option -i skips this check)\n", argv[0]);
+			return EX_USAGE;
+		}
+	}
+
+	{ /* install signal handler */
+		/* There isn't a difference in functionality between the signal(2) and
+		 * sigaction(2) methods. sigaction(2) is vastly preferred for
+		 * portability but some configurations can only use signal(2). */
+		/* Errors aren't terminating because the worst that happens is some
+		 * terminal phooeyness if things go awry. */
+#if defined _POSIX_C_SOURCE
+		struct sigaction act = { 0 };
+
+		act.sa_handler = restore_echo;
+		if(sigaction(SIGINT, &act, NULL) != 0)
+			perror(argv[0]);
+#else
+		if(signal(SIGINT, restore_echo) == SIG_ERR)
+			perror(argv[0]);
+#endif
+	}
+	
+	{ /* banish terminal echo */
+		struct termios t;
+
+		tcgetattr(STDIN_FILENO, &t);
+		t.c_lflag ^= ECHO;
+		tcsetattr(STDIN_FILENO, TCSAFLUSH, &t);
+	}
+
+	{ /* actual input loop */
+		int c;
+
+		while((c = fgetc(stdin)) != EOF)
+			if(fputc(c, stdout) == EOF)
+				return ioerr(argv[0]);
+	}
+
+	restore_echo(0);
+
+	return EX_OK;
+}