Interactively toggle output on and off in Linux? - linux

Question

For controlling output on Linux there is control-s and control-t, which provides a method for temporarily halting terminal output and then resuming it. On VMS in addition there was control-O, which would toggle all output on and off. This didn't pause output, it discarded it.
Is there an equivalent keyboard shortcut in Linux?
This comes up most often for me in gdb, when debugging programs which output millions of status lines. It would be very convenient to be able to temporarily send most of that to /dev/null rather than the screen, and then pick up with the output stream further on, having dispensed with a couple of million lines in between.
(Edited: The termios(3) man page mentions VDISCARD - and then says that it isn't going to work in POSIX or Linux. So it looks like this is out of the question for general command line use on linux. gdb might still be able to discard output though, through one of its own commands. Can it?)
Thanks.

Answer 1

On VMS in addition there was control-O ...
This functionality doesn't appear to exist on any UNIX system I've ever dealt with (or maybe I just never knew it existed; it's documented in e.g. FreeBSD man page, and is referenced by Solaris and HP-UX docs as well).
gdb might still be able to discard output though, through one of its own commands. Can it?
I don't believe so: GDB doesn't actually intercept the output from the inferior (being debugged) process, it simply makes it run (between breakpoints) with the inferior output going to wherever it's going.
That said, you could do it yourself:
#include <stdio.h>
int main()
{
int i;
for (i = 0; i < 1000; ++i) {
printf("%d\n", i);
}
}
gcc -g foo.c
gdb -q ./a.out
(gdb) break 6
Breakpoint 1 at 0x40053e: file foo.c, line 6.
(gdb) run 20>/dev/null # run the program, file descriptor 20 goes to /dev/null
Starting program: /tmp/a.out 20>/dev/null
Breakpoint 1, main () at foo.c:6
6 printf("%d\n", i);
(gdb) c
Continuing.
0
Breakpoint 1, main () at foo.c:6
6 printf("%d\n", i);
We've now run two iterations. Let's prevent further output for 100 iterations:
(gdb) call dup2(20, 1)
$1 = 1
(gdb) ign 1 100
Will ignore next 100 crossings of breakpoint 1.
(gdb) c
Continuing.
Breakpoint 1, main () at foo.c:6
6 printf("%d\n", i);
(gdb) p i
$2 = 102
No output, as desired. Now let's restore output:
(gdb) call dup2(2, 1)
$3 = 1
(gdb) ign 1 10
Will ignore next 10 crossings of breakpoint 1.
(gdb) c
Continuing.
102
103
104
105
106
107
108
109
110
111
112
Breakpoint 1, main () at foo.c:6
6 printf("%d\n", i);
Output restored!