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I wish to perform a command ever 10 seconds, and have it executed in the background (thereby eliminating watch?). All the answers show something like the following, but this will execute ever 11 to 14 seconds. How can this be accomplished?
while true; do
# perform command that takes between 1 and 4 seconds
sleep 10
done
How about:
( # In a subshell, for isolation, protecting $!
while true; do
perform-command & # in the background
sleep 10 ;
### If you want to wait for a perform-command
### that happens to run for more than ten seconds,
### uncomment the following line:
# wait $! ;
### If you prefer to kill a perform-command
### that happens to run for more than ten seconds,
### uncomment the following line instead:
# kill $! ;
### (If you prefer to ignore it, uncomment neither.)
done
)
ETA: With all those comments, alternatives, and the subshell for extra protection, that looks a whole lot more complicated than it started. So, for comparison, here's what it looked like before I started worrying about wait or kill, with their $! and need for isolation:
while true; do perform-command & sleep 10 ; done
The rest is really just for when you need it.
$!: "Expands to the process ID of the job most recently placed into the background," and the sleep is not placed into the background, so no, it won't. :)
Oct 11, 2015 at 13:28
You can do something like the following in bash, zsh, or ksh:
SECONDS=0
while command
do sleep "$((10-(SECONDS%10)))"
done
Here's what the bash manual says about $SECONDS:
$SECONDS
- Each time this parameter is referenced, the number of seconds since shell invocation is returned. If a value is assigned to
$SECONDS, the value returned upon subsequent references is the number of seconds since the assignment plus the value assigned. If$SECONDSisunset, it loses its special properties, even if it is subsequently reset.
Here's a working example:
( SECONDS=0
while sleep "$((RANDOM%10))"
do sleep "$((10-(SECONDS%10)))"
echo "$SECONDS"
done
)
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SECONDS do sleep "$((10-(($SECONDS-$start_time)%10)))". You will have to do start_time=$SECONDS before loop.
Oct 10, 2015 at 21:15
SECONDS. Spawning a sub-shell as in your latest edit, should also avoid this problem.
Oct 10, 2015 at 21:31
while true loop I didn't figure any shell state was expected to survive it. Wrapping a loop like that in its own context is usually best practice, anyway. In a situation like that I would be more concerned about doing so for trap's sake than for $SECONDS' sake, though.
BASH only - You could also compute time spent by your command and subtract from 10:
TIMEFORMAT=$'%0R'
while true; do
T=$({ time command; } 2>&1)
sleep $(( 10-T ))
From BASH man:
TIMEFORMAT The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed. The % character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the braces denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output.
