Filter function for less +F

Question

When watching a growing log file with e.g. "less -iS +F service.log" I want to limit the display to lines matching a certain pattern.

I tried something like

less +F service.log | grep <pattern> | less +F

which doesn't work. Also

cat < service.log | grep <pattern> | less +F

doesn't do what I want. It looks like the input is already closed and less doesn't show changes.

How can I limit the display to lines matching a certain pattern?

score 28 · Answer 1 · answered May 13 '19 at 10:16

28

This question is ages old, but I still think it's worth adding a solution. Instead of trying to grep first and then use less, what about using filtering inside less?

In brief:

use less +F on your file
CTRL-C to temporarily break the "following" action
Type & and your pattern to enable filtering
Issue +F to re-enable the "following" action

More details on this answer on the Unix&Linux StackExchange

answered May 13 '19 at 10:16

Jir

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thaaanks !! I found the Linux stackexchange and didn't understand it. Glad to find your step by step explanation !! :) – Sylvain Jun 27 '20 at 13:17
3

To do this directly from the command line: `less '+&^MF' service.log` where the `^M` is obtained by typing ctrl-v and then enter – Jack Valmadre Nov 19 '20 at 13:38
2

From bash, without the C-v (quoted-insert): `less $'+&\nF' service.log` – vdm Feb 09 '21 at 20:59

score 2 · Answer 2 · answered Dec 29 '11 at 02:06

I haven't yet worked out how to do this without a temp file, but here is a script that demonstrates a functional grep-filtered less +F (which cleans up its temp file). I call it lessf.

One of the key elements is the --line-buffered argument to grep that permits tail output to continue to flow through the pipeline (the unbuffer command provided by expect provides similar functionality for any program).

#!/bin/sh
LOGFILE=$1
shift
PATTERN=$@
TEMP_DIR=/tmp/lesstmp
TEMP_FILE="$TEMP_DIR/$(basename $LOGFILE)"
[ ! -d $TEMP_DIR ] && mkdir $TEMP_DIR
trap 'rm -rf "$TEMP_DIR"; exit' INT TERM EXIT
( tail -f "$LOGFILE" | grep --line-buffered $PATTERN ) > "$TEMP_FILE" | less +F "$TEMP_FILE"
trap - INT TERM EXIT

Example usage:

lessf /var/log/system.log foobar

lessf /var/log/system.log -v nobar

I'd like to point out how using the pipe between tail|grep and less +F makes sure that Ctrl-C will kill them all in one go. I had something similar but using background jobs, it doesn't work as well. — Spidey, Dec 29 '20 at 17:46

score 1 · Answer 3 · edited Aug 12 '11 at 11:53

1

If you don't mind spawning and tearing down a couple of processes each line, use a read while loop

tail -f filename.log|while read line; do echo $line | grep pattern; done

edited Aug 12 '11 at 11:53

MPelletier

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answered Aug 11 '11 at 16:10

joe

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score -1 · Answer 4 · answered Nov 19 '09 at 11:15

-1

tail -f service.log | grep <pattern>

answered Nov 19 '09 at 11:15

Chris May

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Thanks for pondering my question though this is not exactly what I want. I want to have "less" functionality, i.e. scrolling, searching, hightlighting within the resulting output. Something like "tail -f service.log | grep | less +F" which unfortunalely doesn't do it. – axelrose Nov 19 '09 at 11:28

axelrose · Answer 5 · 2009-11-20T16:16:24.883

-1

The solution seemed simply

LESSOPEN='|grep <pattern> %s' less +F service.log

but less doesn't continue to read new lines from the growing log file.

edited Nov 20 '09 at 16:16

answered Nov 20 '09 at 12:53

axelrose

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my original answer from 2009 doesn't seem to be valid anymore, i.e. the simple solution does watch for a growing log file – axelrose May 01 '19 at 15:36

Filter function for less +F

5 Answers5