Paul Eggert <egg...@cs.ucla.edu> writes:
> On 2025-07-28 10:36, Collin Funk wrote:
>> I don't really like the idea of changing '-f' depending on whether
>> POSIXLY_CORRECT is defined. So I would prefer this as well.
>
> On second thought (sorry...) I now think I understand why GNU pr
> behaves the way it does. The GNU coding standards[1] say "...please
> don’t make the behavior of a command-line program depend on the type
> of output device it gets as standard output or standard input. Device
> independence is an important principle of the system’s design; do not
> compromise it merely to save someone from typing an option now and
> then."
>
> The standards go on to list some exceptions, such as 'ls'. 'pr' is not
> one of the exceptions. Currently there are a few other exceptions in
> Coreutils ('df', 'mv', 'nohup', 'rm', 'tail') but I guess 'pr' didn't
> make the cut when it was written.
>
> That is, my theory is that when GNU 'pr' was written, its original
> author Pete TerMaat followed the GNU coding standards, and
> deliberately made '-f' and '-F' aliases, because that way the behavior
> of pr would not depend on isatty (STDOUT_FILENO). And Pete did not
> implement -p because it didn't sound useful if we follow the coding
> standards.
>
> If this theory is correct, then a fix that would be more in the spirit
> of GNU would be something like this:
Interesting, I did not know about this part of the GNU Coding Standards.
> (a) Implement -p as if isatty (STDOUT_FILENO) is true. This form of -p
> is useful only when stdout is a terminal, but that's good enough since
> nobody uses (or will use :-) -p.
>
> (b) If POSIXLY_CORRECT is set, change -f and -p so that they both
> worry about isatty (STDOUT_FILENO), as POSIX requires.
>
> This would conform to POSIX, would address Pádraig's concern about
> compatibility, and would follow the GNU coding standards.
That sounds good to me. I attached a v4 patch that should behave this
way and address the other issues that you mentioned.
> It'd be less useful in practice, but that doesn't matter since the
> practical need for this behavior vanished decades ago.
Thanks for again for the thorough review and explanations. I find it
funny that I assumed this change was simple and learned it's purpose was
for logging in on a teletype. The Model 37 predates me ~30 years, so it
never occured to me that the purpose was to not print logins.
Collin
>From bb063435c2019af6a6b9e26c67c4b378b9243919 Mon Sep 17 00:00:00 2001
Message-ID: <bb063435c2019af6a6b9e26c67c4b378b9243919.1753762640.git.collin.fu...@gmail.com>
From: Collin Funk <collin.fu...@gmail.com>
Date: Sun, 27 Jul 2025 15:00:15 -0700
Subject: [PATCH v4] pr: implement '-p' and modify '-f' conforming to POSIX
* src/pr.c (pause_option, pause_on_first_page, tty_fd): New variables.
(short_options): Add '-p'.
(long_options): Add '--pause'.
(main): Add the option. Open and close a file descriptor to
/dev/tty. Disable pausing if POSIXLY_CORRECT is set.
(print_files): If the pausing is enabled, emit '\a' to standard error
and wait until a newline is read to print the next page.
(usage): Mention the new option.
* doc/coreutils.texi (pr invocation): Document the new option. Document
the behaviors of POSIXLY_CORRECT on -p and -f.
* NEWS: Mention the new option. Mention the new behavior of 'pr -f' with
the POSIXLY_CORRECT environment variable set.
---
NEWS | 7 +++++
doc/coreutils.texi | 17 ++++++++++++
src/pr.c | 66 +++++++++++++++++++++++++++++++++++++++++++---
3 files changed, 86 insertions(+), 4 deletions(-)
diff --git a/NEWS b/NEWS
index 0b2be7116..6e78c81d3 100644
--- a/NEWS
+++ b/NEWS
@@ -7,6 +7,9 @@ GNU coreutils NEWS -*- outline -*-
'factor' is now much faster at identifying large prime numbers,
and significantly faster on composite numbers greater than 2^128.
+ 'pr -f' with the POSIXLY_CORRECT environment variable set will pause
+ until a newline is read from /dev/tty before printing the first page.
+
** Bug fixes
cksum was not compilable by Apple LLVM 10.0.0 x86-64, which
@@ -64,6 +67,10 @@ GNU coreutils NEWS -*- outline -*-
Iranian locale (fa_IR) and for the Ethiopian locale (am_ET), and also
does so more consistently for the Thailand locale (th_TH.UTF-8).
+ pr now supports the -p option, to pause upon printing each page until
+ a newline character is read from /dev/tty, as required by POSIX. The
+ corresponding long option is --pause.
+
* Noteworthy changes in release 9.7 (2025-04-09) [stable]
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 7ca5b222c..25e4e4e51 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -2726,6 +2726,11 @@ @node pr invocation
Use a form feed instead of newlines to separate output pages. This does
not alter the default page length of 66 lines.
+@vindex POSIXLY_CORRECT
+If the @env{POSIXLY_CORRECT} environment variable is set and standard
+output is a tty, then using @option{-f} will also pause until a newline
+is read from @file{/dev/tty} before printing the first page.
+
@item -h @var{header}
@itemx --header=@var{header}
@opindex -h
@@ -2825,6 +2830,18 @@ @node pr invocation
set with the @option{-W/-w} option. A limited overflow may occur with
numbered single column output (compare @option{-n} option).
+@item -p
+@itemx --pause
+@opindex -p
+@opindex --pause
+After printing each page, print an alert (bell) to standard error and
+wait for a newline to be read from @file{/dev/tty} before printing the
+next page.
+
+@vindex POSIXLY_CORRECT
+If the @env{POSIXLY_CORRECT} environment variable is set and standard
+output is not a tty, this option will be ignored.
+
@item -r
@itemx --no-file-warnings
@opindex -r
diff --git a/src/pr.c b/src/pr.c
index e7081a059..298be4ebd 100644
--- a/src/pr.c
+++ b/src/pr.c
@@ -711,6 +711,15 @@ static char *custom_header;
/* (-D) Date format for the header. */
static char const *date_format;
+/* If true, pause after each page until a newline is read from /dev/tty. */
+static bool pause_option;
+
+/* If true, pause on only the first page. */
+static bool pause_on_first_page;
+
+/* Used to read from /dev/tty if --p/--pause is in use. */
+static int tty_fd;
+
/* The local time zone rules, as per the TZ environment variable. */
static timezone_t localtz;
@@ -738,7 +747,7 @@ enum
};
static char const short_options[] =
- "-0123456789D:FJN:S::TW:abcde::fh:i::l:mn::o:rs::tvw:";
+ "-0123456789D:FJN:S::TW:abcde::fh:i::l:mn::po:rs::tvw:";
static struct option const long_options[] =
{
@@ -758,6 +767,7 @@ static struct option const long_options[] =
{"number-lines", optional_argument, nullptr, 'n'},
{"first-line-number", required_argument, nullptr, 'N'},
{"indent", required_argument, nullptr, 'o'},
+ {"pause", no_argument, nullptr, 'p'},
{"no-file-warnings", no_argument, nullptr, 'r'},
{"separator", optional_argument, nullptr, 's'},
{"sep-string", optional_argument, nullptr, 'S'},
@@ -868,6 +878,8 @@ main (int argc, char **argv)
idx_t n_digits = 0;
idx_t n_alloc = 0;
+ bool posixly_correct = (getenv ("POSIXLY_CORRECT") != nullptr);
+
initialize_main (&argc, &argv);
set_program_name (argv[0]);
setlocale (LC_ALL, "");
@@ -958,6 +970,8 @@ main (int argc, char **argv)
untabify_input = true;
break;
case 'f':
+ pause_on_first_page = posixly_correct;
+ FALLTHROUGH;
case 'F':
use_form_feed = true;
break;
@@ -999,6 +1013,9 @@ main (int argc, char **argv)
chars_per_margin = getoptnum (optarg, 0,
_("'-o MARGIN' invalid line offset"));
break;
+ case 'p':
+ pause_option = true;
+ break;
case 'r':
ignore_failed_opens = true;
break;
@@ -1061,7 +1078,7 @@ main (int argc, char **argv)
}
if (! date_format)
- date_format = (getenv ("POSIXLY_CORRECT") && !hard_locale (LC_TIME)
+ date_format = (posixly_correct && !hard_locale (LC_TIME)
? "%b %e %H:%M %Y"
: "%Y-%m-%d %H:%M");
@@ -1079,6 +1096,22 @@ main (int argc, char **argv)
error (EXIT_FAILURE, 0,
_("cannot specify both printing across and printing in parallel"));
+ /* POSIX states that the pausing behavior of -f and -p should only occur if
+ standard output is a tty. Only behave this way is POSIXLY_CORRECT is set
+ since the GNU Coding Standards discourages changing program behavior based
+ on output device type. */
+ if ((pause_option || pause_on_first_page) && posixly_correct
+ && ! isatty (STDOUT_FILENO))
+ pause_option = pause_on_first_page = false;
+
+ if (pause_option || pause_on_first_page)
+ {
+ tty_fd = open ("/dev/tty", O_RDONLY);
+ if (tty_fd < 0)
+ error (EXIT_FAILURE, errno, _("cannot open %s for reading"),
+ quoteaf ("/dev/tty"));
+ }
+
/* Translate some old short options to new/long options.
To meet downward compatibility with other UNIX pr utilities
and some POSIX specifications. */
@@ -1149,6 +1182,8 @@ main (int argc, char **argv)
if (have_read_stdin && fclose (stdin) == EOF)
error (EXIT_FAILURE, errno, _("standard input"));
+ if (pause_option && close (tty_fd) < 0)
+ error (EXIT_FAILURE, errno, "%s", quotef ("/dev/tty"));
main_exit (failed_opens ? EXIT_FAILURE : EXIT_SUCCESS);
}
@@ -1636,8 +1671,29 @@ print_files (int number_of_files, char **av)
init_funcs ();
line_number = line_count;
- while (print_page ())
- ;
+ while (true)
+ {
+ if (pause_option || (pause_on_first_page && page_number == 1))
+ {
+ if (putc ('\a', stderr) == EOF || fflush (stderr) != 0)
+ write_error ();
+ while (true)
+ {
+ unsigned char ch;
+ ssize_t bytes_read = read (tty_fd, &ch, sizeof ch);
+ if (bytes_read < 0)
+ error (EXIT_FAILURE, errno, _("error reading %s"),
+ quoteaf ("/dev/tty"));
+ /* Just exit if the user presses Ctrl-D. */
+ if (bytes_read == 0)
+ return;
+ if (ch == '\n')
+ break;
+ }
+ }
+ if (! print_page ())
+ break;
+ }
}
/* Initialize header information.
@@ -2824,6 +2880,8 @@ Paginate or columnate FILE(s) for printing.\n\
-o, --indent=MARGIN\n\
offset each line with MARGIN (zero) spaces, do not\n\
affect -w or -W, MARGIN will be added to PAGE_WIDTH\n\
+ -p, --pause pause at the beginning of each page until a newline\n\
+ is read from /dev/tty.\n\
-r, --no-file-warnings\n\
omit warning when a file cannot be opened\n\
"), stdout);
--
2.50.1
