watch without blank line

Question

I want to execute a program periodically and watch when something changes. It sounds like watch would be the right command for that but it prints out annoying empty lines which makes it impossible to see the history. The -t switch does not really help here.

What i want:

user@machine ~$ #some previous command
user@machine ~$ watch -t date -Is
2020-10-09T12:50:13
2020-10-09T12:50:15
2020-10-09T12:50:17

What i get:

2020-10-09T12:50:17

(all the previous lines are gone).

I know i can use a while loop with a sleep command, but this takes longer to type and has other disadvantages. This loop: while true; do date -Is ; sleep 2; done works but i would prefer a simple command. I also read about a trick using tee, like this watch -t ' date -Is | tee -a ./somefile, but that is not always a viable solution for me since this only works with a writable filesystem and if it is a tempfs i can not read it after a reboot, which is a problem on a embedded system where i sometimes have no writable file system mounted and i can't read a tempfs after a crash/reboot.

I also tried to filter out empty lines with grep: watch -t date -Is | grep -v '^$' but then i have no output.

Answer 1

A bit convoluted, but you could do:

yes 'date -Is' | pv -qlL1 | sh

(also has the advantage to run the command every second, instead of every 2 seconds plus however long it takes to run it)

1 above is the rate in number of lines per second and can only be an integer number. So 1 per second is the slowest you can get. You could drop the -l though in which case, the rate would be in number of bytes per second. So at 9 bytes per line, that'd be one command every 9 seconds.

Answer 2

perl -e '$t=shift; while(1){print $o=$n if ($n=qx(@ARGV)) ne $o; sleep $t}' 1 date -Is
2020-10-07T06:49:07+03:00
2020-10-07T06:49:08+03:00
2020-10-07T06:49:09+03:00
2020-10-07T06:49:10+03:00

An improved version which does fractional delays and correctly handles spaces and special characters in the command & args:

#! /usr/bin/perl
use Time::HiRes qw(sleep);
die "usage: $0 delay cmd [args ...]\n" if @ARGV < 2;
sub slurpcmd {
   open my $h, '-|', @_ or die "open $_[0]|: $!";
   local $/ unless wantarray;
   <$h>
}
my $t = shift; my ($o, $n);
while(1){print $o=$n if ($n=slurpcmd @ARGV) ne $o; sleep $t}

Adding options to format the output of the command (nicely separate it if multiline, etc) is left as exercise to the reader ;-)