When doing a tail -f error.log, how to insert programmatically a line break after nothing has been appened to the file for 3 seconds ?
(obviously, once one line break has been added, no other line break should be added until other line(s) of text is added to the log file)
For instance, these lines are appened to error.log :
foo bar boo [[wait 4 seconds]] 2far 2foo 2bar 2boo [[wait 40 seconds]] 2far This would be the output in the console :
foo bar boo 2far 2foo 2bar 2boo 2far You could always implement the tail -f (well here, unless you uncomment the seek(), more like tail -n +1 -f as we're dumping the whole file) by hand with perl for instance:
perl -e ' $| = 1; # seek STDIN, 0, 2; # uncomment if you want to skip the text that is # already there. Or if using the ksh93 shell, add # a <((EOF)) after < your-file while (1) { if ($_ = <STDIN>) { print; $t = 0 } else { print "\n" if $t == 3; # and a line of "-"s after 10 seconds: print "-" x 72 . "\n" if $t == 10; sleep 1; $t++; } }' < your-file Or let tail -f do the tailing and use perl to insert the newlines if there's no input for 3 seconds:
tail -f file | perl -pe 'BEGIN{$SIG{ALRM} = sub {print "\n"}} alarm 3' Those assume that the output itself is not slowed down (like when the output goes to a pipe that is not actively read).
bash + date solution:
while IFS= read -r line; do prev=$t # get previous timestamp value t=$(date +%s) # get current timestamp value [[ ! -z "$prev" ]] && [[ "$((t-prev))" -ge 3 ]] && echo "" echo "$line" # print current line done < <(tail -f error.log) 
$SECONDS to count time intervals. I think it's the number of seconds since the shell started, not that it matters when taking a difference. read -t or $TMOUT. $SECONDS is broken in bash and mksh. time bash -c 'while ((SECONDS < 3)); do :; done' will last in between 2 and 3 seconds. Better to use zsh or ksh93 instead here (with typeset -F SECONDS) date +%s. Both give the time in full seconds, which does have the effect that the interval from, say, 1.9 to 4.0 looks like 3 full seconds, even though it's really 2.1. It's hard to work around that if all you can't access the fractional seconds. But yes, they should probably sleep here instead of busylooping, and then read -t could just as well be used. Even if you do sleep manually, time bash -c 'while [[ $SECONDS -lt 3 ]]; do sleep 1; done' works just fine. SECONDS=0 ensures that $SECONDS will reach 1 in exactly 1 second. That's not the case with bash as it uses time() to track $SECONDS instead of gettimeofday(). I reported bugs to mksh, zsh and bash some time ago, only zsh was fixed. (good point about the issue being the same with date +%s). Note that it's not a busyloop here, as we're reading from the output of tail -f over a pipe. printf to emulate date without external tools or command substitution: printf -v t '%(%s)T' -1. Python solution (with dynamic time gap argument):
tailing_by_time.py script:
import time, sys t_gap = int(sys.argv[1]) # time gap argument ts = 0 while True: line = sys.stdin.readline().strip() # get/read current line from stdin curr_ts = time.time() # get current timestamp if ts and curr_ts - ts >= t_gap: print("") # print empty line/newline ts = curr_ts if line: print(line) # print current line if it's not empty Usage:
tail -f error.log | python tailing_by_time.py 3
tsto add timestamping to the output and process the timestamps