forked from bonsai/harakit
peek(1): strip down to bare essentials, rewrite man page to match
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docs/peek.1
82
docs/peek.1
@ -12,52 +12,17 @@ peek \(en read from standard input, furtively
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.SH SYNOPSIS
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peek
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.RB ( -1enot )
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.RB ( -p
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.RB [ program
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.RB [ arguments... ]])
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.RB ( -i )
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.SH DESCRIPTION
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Peek reads input from standard input with terminal echo disabled, which may be
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useful to prevent secrets being spied upon by adversaries watching a user's
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screen.
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Peek reads input from standard input with terminal echo disabled.
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.SH OPTIONS
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.B -1
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.B -i
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.RS
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Limits input to a single line (stopping when the newline character is read).
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.RE
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.B -e
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.RS
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Configures peek to output to standard error.
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.RE
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.B -n
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.RS
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Prints ("iNcludes") the terminating character in outputs. This is nonsensical
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if not combined with the
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.B -1
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option.
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.RE
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.B -o
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.RS
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Configures peek to output to standard output.
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.RE
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.B -p
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.RS
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Configures peek to pipe output to an executed child program, e.g. a password
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hashing utility. This is provided as a convenience and to avoid potential
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insecurities resulting from programmer error in scripts.
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.RE
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.B -t
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.RS
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Makes peek exit if not run within a terminal.
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Allows input to come from sources other than terminals (i.e. a pipe).
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.RE
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.SH DIAGNOSTICS
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@ -70,36 +35,45 @@ still usable after premature termination; if the signal can't be handled, it
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prints an error message and continues. If peek is interrupted, it exits
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unsuccessfully, without an error message.
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.SH RATIONALE
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This tool was originally written to accept passwords in shell scripts, as an
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extremely simple alternative to the GNU Privacy Guard project's pinentry(1).
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Accepting input without showing what is being typed is useful when keying in
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secrets in public settings or places with installed surveillance cameras.
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.SH BUGS
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Accepting secrets in shell scripts is not adviseable in any context.
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This does nothing to prevent others seeing the keyboard being used to input
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secrets or mask the sound of typing. Audio or video recordings of typing can be
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used to determine what was input without needing to see the characters appear
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on the screen.
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If used in a safety-critical application it must be ensured that the
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environment with which peek is used is not compromised.
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Accepting secrets in shell scripts is probably not adviseable.
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On systems that support it, the ioctl(2) command TIOCSTI can be used to insert
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characters into the standard input going to peek. This doesn't allow snooping
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but can be used for general mischief.
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Peek will happily run without outputs, slurping input and doing nothing with
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it.
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.SH EXAMPLES
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This is a command line for POSIX sh(1), using POSIX env(1) to discard the
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environment except for PATH, and htpassword(1) from Apache's utilities to hash
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the input with the bcrypt algorithm, printing the resulting hash:
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This is an sh(1p) command line that hashes a given password. It uses head(1p)
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to only accept one line of input, xargs(1p) and printf(1p) to strip the
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trailing newline, htpasswd(1) from Apache's utilities to hash the input with
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the bcrypt algorithm, and cut(1p) to print only the resulting hash:
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.RS
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.R $ env -i PATH="$PATH" peek -1tp htpasswd -nBi _ | cut -d : -f 2
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.R $ peek | head -n 1 | xargs printf '%s' | htpasswd -nBi _ | cut -d : -f 2
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.RE
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This is a POSIX sh(1) command line that lets a user blindly write into a text
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file, only able to see written lines. Some writers have the habit of
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prematurely revising their work and use tools like this to prevent it:
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This is an sh(1p) command line that lets a user blindly write into a text file,
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only able to see written lines. Some writers have the habit of prematurely
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revising their work and use tools like this to prevent it. It uses mm(1) to
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pipe the output of peek to both the standard error and the regular file
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writing.txt.
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.RS
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.R $ echo Input ^D to quit. && peek -eot >writing.txt
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.R $ echo Input ^D to quit. && peek | mm -eo - >writing.txt
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.RE
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.SH AUTHOR
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@ -113,4 +87,4 @@ Copyright (c) 2023-2024 DTB. License AGPLv3+: GNU AGPL version 3 or later
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.SH SEE ALSO
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env(1), ioctl(2), ioctl_tty(2), read(1), sh(1)
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ioctl(2), ioctl_tty(2), read(1), sh(1)
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72
src/peek.c
72
src/peek.c
@ -17,75 +17,66 @@
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*/
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#include <signal.h> /* sigaction(2), signal(2), struct sigaction, SIGINT */
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#include <stdio.h> /* fclose(3), fdopen(3), fprintf(3), getc(3), perror(3),
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* putc(3), stderr, stdin, stdout, EOF, NULL */
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#include <stdlib.h> /* exit(3), size_t, EXIT_FAILURE */
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#include <string.h> /* strerror(3) */
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#include <stdio.h> /* fprintf(3), fgetc(3), perror(3), fputc(3), stderr, stdin,
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* stdout, EOF, NULL */
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#include <stdlib.h> /* exit(3), EXIT_FAILURE */
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#if !defined EX_OK || !defined EX_OSERR || !defined EX_USAGE
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# include <sysexits.h>
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#endif
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#include <termios.h> /* tcgetattr(3), tcsetattr(3), struct termios, ECHO */
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#include <unistd.h> /* getopt(3), isatty(3), STDIN_FILENO */
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static int oserr(char *s){ perror(s); return EX_OSERR; }
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static char *program_name = "peek";
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static int usage(char *s){
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fprintf(stderr, "Usage: %s (-1enot) (-p [program [arguments...]])\n",
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s == NULL ? program_name : s);
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return EX_USAGE;
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}
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/* Restores terminal echo; otherwise when a user ^Cs the terminal would
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* continue to not display typed text. If sig isn't zero, this will terminate
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* the program. */
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void restore_echo(int sig){
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static void restore_echo(int sig){
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static struct termios t;
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tcgetattr(STDIN_FILENO, &t);
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t.c_lflag |= ECHO;
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tcsetattr(STDIN_FILENO, TCSAFLUSH, &t);
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/* If, for whatever ungodly reason, exit(3) returns, the user will notice
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* their typed characters on the screen. */
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if(sig != 0)
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exit(EXIT_FAILURE);
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else
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return;
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return;
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}
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static int oserr(char *s){ perror(s); return EX_OSERR; }
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static int ioerr(char *s){ perror(s); restore_echo(0); return EX_IOERR; }
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static int usage(char *s){
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fprintf(stderr, "Usage: %s (-1enot) (-p [program [arguments...]])\n", s);
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return EX_USAGE;
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}
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int main(int argc, char *argv[]){
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int eof;
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char include_eof;
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FILE *outputs[] = {
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NULL /* stdout */,
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NULL /* stderr */
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};
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if(argc < 1)
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return usage(argv[0]);
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return usage(program_name);
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eof = EOF;
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include_eof = 0;
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{ /* options parsing */
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char allow_nonterminals;
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int c;
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while((c = getopt(argc, argv, "1enot")) != -1)
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allow_nonterminals = 0;
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while((c = getopt(argc, argv, "i")) != -1)
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switch(c){
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case '1': eof = '\n'; break;
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case 'n': include_eof = 1; break;
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case 'o': outputs[0] = stdout; break;
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case 'e': outputs[1] = stderr; break;
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case 't':
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if(isatty(STDIN_FILENO) != 1){
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fprintf(stderr, "%s: Must be run in a terminal"
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" (option -t specified)\n", argv[0]);
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return EX_USAGE;
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}
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case 'i': allow_nonterminals = 1; break;
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default: return usage(argv[0]);
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}
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if(argc > optind)
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return usage(argv[0]);
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if(!allow_nonterminals && isatty(STDIN_FILENO) != 1){
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fprintf(stderr, "%s: Must be run in a terminal"
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" (option -i skips this check)\n", argv[0]);
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return EX_USAGE;
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}
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}
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{ /* install signal handler */
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@ -116,15 +107,10 @@ int main(int argc, char *argv[]){
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{ /* actual input loop */
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int c;
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size_t i;
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do{ if((c = getc(stdin)) != eof || include_eof)
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for(i = 0; i < (sizeof outputs)/(sizeof *outputs); ++i)
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if(outputs[i] != NULL && putc(c, outputs[i]) == EOF){
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perror(argv[0]);
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outputs[i] = NULL;
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}
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}while(c != eof);
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while((c = getc(stdin)) != EOF)
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if(putc(c, stdout) == EOF)
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return ioerr(argv[0]);
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}
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restore_echo(0);
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