1

I have a log file that text is appended to during the program run. I want to know when a specific string was appended to the end of the file.

All the solutions given here: Monitoring a file until a string is found and here: https://stackoverflow.com/questions/25959870/how-to-wait-till-a-particular-line-appears-in-a-file work, but only on Ubuntu.

I'm trying to do the same thing on Centos 7 and it doesn't seem to end the command.

some things I tried that work in Ubuntu but not in Centos 7:

( tail -f -n0 logfile & ) | grep -q "database start"

tail -n0 -f logfile  | sed '/database start/ q'

No idea why it would not work on CentOS 7 but on Ubuntu it does work. I prefer to use a single command instead of sleeping x and grep again, because the line should be outputted multiple times in the file and I want only the one since I started checking the file

I did check the log file and the line did appear, but none of the commands stopped.

Improve this question
5
  • Related? Why isn't tail -f … | grep -q … quitting when it finds a match? Still, even if tail is not patched, I expect the command that runs it in the background (i.e. the one with ( tail … & )) to kinda work; so there must be something else.
    – Kamil Maciorowski
    Commented Feb 28, 2023 at 17:30
  • Test with logfile that is under your total control. Use echo foo >> logfile and echo "database start" >> logfile while tail runs. Do the commands in question fail to do their job in these circumstances?
    – Kamil Maciorowski
    Commented Feb 28, 2023 at 17:45
  • seems to detect >> but not >, any way to also detect > ?
    – Codo
    Commented Mar 1, 2023 at 9:19
  • (1) Why do you want >? Usually for logfiles >> is better. If more than one process writes then >> is definitely better (see this question). (2) Anyway, for me it works also with > but tail exits when the file is truncated, it may be not what you want. Don't use > then. The only case where it "doesn't work" is if the file contains database start as a sole line already acknowledged by tail (but not printed due to -n0); executing echo "database start" > logfile brings the file to the same state; tail cannot notice (cont'd)
    – Kamil Maciorowski
    Commented Mar 1, 2023 at 11:38
  • (cont'd) unless it gets scheduled exactly between truncating and writing, this is unlikely. (3) Sanity check: you say "it doesn't seem to end the command" in your original question. Please investigate if it detects the matching line in the first place (run grep without -q and see if it prints). Maybe the problem is not in "doesn't end", maybe the problem is in "doesn't see". How exactly did you "check the log file and the line did appear"? Was the file removed and created anew? Maybe you need tail -F.
    – Kamil Maciorowski
    Commented Mar 1, 2023 at 11:42

1 Answer 1

Reset to default
0

Is it possible the "database start" is perhaps written as "Database start"?
The default grep may not ignore a change of case, requiring grep -i for doing that.

The following might be safer syntax :

grep -q "database start" <(tail -f logfile)

Reference : How to wait till a particular line appears in a file.

Improve this answer
8
  • This is not the case, I put databse start just as an example, but there is no problem there.
    – Codo
    Commented Feb 28, 2023 at 17:35
  • Noted. Is the above syntax working better on CentOS?
    – harrymc
    Commented Feb 28, 2023 at 17:36
  • Which shell are you using?
    – harrymc
    Commented Mar 1, 2023 at 9:16
  • It doesn't work. fixed the spaces. I use bash.
    – Codo
    Commented Mar 1, 2023 at 9:18
  • Just tested it - the error is in the blank after the redirection. See my correction above.
    – harrymc
    Commented Mar 1, 2023 at 9:22

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .