bash - print a line every X seconds (like sed every X lines) - bash

Question

I know with sed you can pipe the output of a command so that you can print every X lines.
make all | sed -n '2~5'
Is there an equivalent command to print a line every X seconds?
make all | print_line_every_sec '5'

Answer 1

In 5 seconds timeout read one line and discard anything else:
while
# timeout 5 seconds
! timeout 5 sh -c '
# read one line
if IFS= read -r line; then
# output the line
printf "%s\n" "$line"
# discard the input for the rest of 5 seconds
cat >/dev/null
fi
# will get here only, if there is nothing to read
'
# that means that `timeout` will always return 124 if stdin is still open
# and it will return 0 exit status only if there is nothing to read
# so we loop on nonzero exit status of timeout.
do :; done
and a oneliner:
while ! timeout 0.5 sh -c 'IFS= read -r line && printf "%s\n" "$line" && cat >/dev/null'; do :; done
But maybe something simpler - just discard 5 seconds of data each one line:
while IFS= read -r line; do
printf "%s\n" "$line"
timeout 5 cat >/dev/null
done
or
while IFS= read -r line &&
printf "%s\n" "$line" &&
! timeout 5 cat >/dev/null
do :; done

Answer 2

If you want the most recent message every 5 seconds, this is a try :
make all | {
display(){
if (( $SECONDS >= 5)); then
if test -n "${last_line+x}"; then
# print only if there is a message in the last 5 seconds
echo $last_line; unset last_line
fi
SECONDS=0
fi
}
SECONDS=0
while true; do
while IFS= read -t 0.001 line; do
last_line=$line
display
done
display
done
}

Answer 3

Even if the proposed solutions are interesting and beautiful, the most elegant solution IMHO is a awk solution. If you want to issue
make all | print_line_every_sec 5
then you have to create the script print_line_every_sec as follows, including a test to avoid an infinite loop:
#!/bin/bash
if [ $1 -le 0 ] ; then echo $(basename $0): invalid argument \'$1\'; exit 1; fi
awk -v delay=$1 'BEGIN {t = systime ()}
{if (systime() >= t) {print $0 ; t += delay}}'

Answer 4

This might work for you (GNU sed):
sed 'e sleep 1' file
Print a line every n (in the above example 1 ) second.
To print 5 lines every 2 seconds, use:
sed '1~5e sleep 2' file

Answer 5

You can do it by watch command.
If You need only print your output every X second, you could use something like this:
watch -n X "Your CMD"
if you need to designate any change on your output, it would be useful to use -d switch :
watch -n X -d "Your CMD"