System: Linux 4.13.0-43-generic #48~16.04.1-Ubuntu
BASH_VERSION='4.3.48(1)-release'
The command:
while sleep 5
do
date +%T
done | awk -F: '{print $3}'
Should print the 3rd field (seconds) of the "date" output, one line every 5 seconds. Problem: awk reads from the pipe, and processes its input, only when the pipe's buffer is full. i.e. when more than 4K of input is generated.
When awk is replaced by cat, a line is printed every 5 seconds as expected.
This code snippet is simplified from a shell script which had worked ok on other systems, so there must be something about bash, awk and their configuration in this system.
In short, is there a way to convince awk to behave like cat when reading from a pipe?
#Ed Morton : I did try to add fflush() after each print, but it does not work -- that's what showed that the problem is with awk's input, not output.
I also tried to add calls to system("date"), which showed that indeed awk gets all input lines at once, not immediately when they are produced.
For those who asked:
$ awk -W version
mawk 1.3.3 Nov 1996, Copyright (C) Michael D. Brennan
compiled limits:
max NF 32767
sprintf buffer 2040
While trying to find out how to make awk print its version, I discovered that it is really mawk, and that it has the following flag:
-W interactive -- sets unbuffered writes to stdout and line buffered reads from stdin.
Records from stdin are lines regardless of the value of RS.
This seems to solve the problem!
Thanks to all repliers.
Is that possible to use grep on a continuous stream?
What I mean is sort of a tail -f <file> command, but with grep on the output in order to keep only the lines that interest me.
I've tried tail -f <file> | grep pattern but it seems that grep can only be executed once tail finishes, that is to say never.
Turn on grep's line buffering mode when using BSD grep (FreeBSD, Mac OS X etc.)
tail -f file | grep --line-buffered my_pattern
It looks like a while ago --line-buffered didn't matter for GNU grep (used on pretty much any Linux) as it flushed by default (YMMV for other Unix-likes such as SmartOS, AIX or QNX). However, as of November 2020, --line-buffered is needed (at least with GNU grep 3.5 in openSUSE, but it seems generally needed based on comments below).
I use the tail -f <file> | grep <pattern> all the time.
It will wait till grep flushes, not till it finishes (I'm using Ubuntu).
I think that your problem is that grep uses some output buffering. Try
tail -f file | stdbuf -o0 grep my_pattern
it will set output buffering mode of grep to unbuffered.
If you want to find matches in the entire file (not just the tail), and you want it to sit and wait for any new matches, this works nicely:
tail -c +0 -f <file> | grep --line-buffered <pattern>
The -c +0 flag says that the output should start 0 bytes (-c) from the beginning (+) of the file.
In most cases, you can tail -f /var/log/some.log |grep foo and it will work just fine.
If you need to use multiple greps on a running log file and you find that you get no output, you may need to stick the --line-buffered switch into your middle grep(s), like so:
tail -f /var/log/some.log | grep --line-buffered foo | grep bar
you may consider this answer as enhancement .. usually I am using
tail -F <fileName> | grep --line-buffered <pattern> -A 3 -B 5
-F is better in case of file rotate (-f will not work properly if file rotated)
-A and -B is useful to get lines just before and after the pattern occurrence .. these blocks will appeared between dashed line separators
But For me I prefer doing the following
tail -F <file> | less
this is very useful if you want to search inside streamed logs. I mean go back and forward and look deeply
Didn't see anyone offer my usual go-to for this:
less +F <file>
ctrl + c
/<search term>
<enter>
shift + f
I prefer this, because you can use ctrl + c to stop and navigate through the file whenever, and then just hit shift + f to return to the live, streaming search.
sed would be a better choice (stream editor)
tail -n0 -f <file> | sed -n '/search string/p'
and then if you wanted the tail command to exit once you found a particular string:
tail --pid=$(($BASHPID+1)) -n0 -f <file> | sed -n '/search string/{p; q}'
Obviously a bashism: $BASHPID will be the process id of the tail command. The sed command is next after tail in the pipe, so the sed process id will be $BASHPID+1.
Yes, this will actually work just fine. Grep and most Unix commands operate on streams one line at a time. Each line that comes out of tail will be analyzed and passed on if it matches.
This one command workes for me (Suse):
mail-srv:/var/log # tail -f /var/log/mail.info |grep --line-buffered LOGIN >> logins_to_mail
collecting logins to mail service
Coming some late on this question, considering this kind of work as an important part of monitoring job, here is my (not so short) answer...
Following logs using bash
1. Command tail
This command is a little more porewfull than read on already published answer
Difference between follow option tail -f and tail -F, from manpage:
-f, --follow[={name|descriptor}]
output appended data as the file grows;
...
-F same as --follow=name --retry
...
--retry
keep trying to open a file if it is inaccessible
This mean: by using -F instead of -f, tail will re-open file(s) when removed (on log rotation, for sample).
This is usefull for watching logfile over many days.
Ability of following more than one file simultaneously
I've already used:
tail -F /var/www/clients/client*/web*/log/{error,access}.log /var/log/{mail,auth}.log \
/var/log/apache2/{,ssl_,other_vhosts_}access.log \
/var/log/pure-ftpd/transfer.log
For following events through hundreds of files... (consider rest of this answer to understand how to make it readable... ;)
Using switches -n (Don't use -c for line buffering!).By default tail will show 10 last lines. This can be tunned:
tail -n 0 -F file
Will follow file, but only new lines will be printed
tail -n +0 -F file
Will print whole file before following his progression.
2. Buffer issues when piping:
If you plan to filter ouptuts, consider buffering! See -u option for sed, --line-buffered for grep, or stdbuf command:
tail -F /some/files | sed -une '/Regular Expression/p'
Is (a lot more efficient than using grep) a lot more reactive than if you does'nt use -u switch in sed command.
tail -F /some/files |
sed -une '/Regular Expression/p' |
stdbuf -i0 -o0 tee /some/resultfile
3. Recent journaling system
On recent system, instead of tail -f /var/log/syslog you have to run journalctl -xf, in near same way...
journalctl -axf | sed -une '/Regular Expression/p'
But read man page, this tool was built for log analyses!
4. Integrating this in a bash script
Colored output of two files (or more)
Here is a sample of script watching for many files, coloring ouptut differently for 1st file than others:
#!/bin/bash
tail -F "$#" |
sed -une "
/^==> /{h;};
//!{
G;
s/^\\(.*\\)\\n==>.*${1//\//\\\/}.*<==/\\o33[47m\\1\\o33[0m/;
s/^\\(.*\\)\\n==> .* <==/\\o33[47;31m\\1\\o33[0m/;
p;}"
They work fine on my host, running:
sudo ./myColoredTail /var/log/{kern.,sys}log
Interactive script
You may be watching logs for reacting on events?
Here is a little script playing some sound when some USB device appear or disappear, but same script could send mail, or any other interaction, like powering on coffe machine...
#!/bin/bash
exec {tailF}< <(tail -F /var/log/kern.log)
tailPid=$!
while :;do
read -rsn 1 -t .3 keyboard
[ "${keyboard,}" = "q" ] && break
if read -ru $tailF -t 0 _ ;then
read -ru $tailF line
case $line in
*New\ USB\ device\ found* ) play /some/sound.ogg ;;
*USB\ disconnect* ) play /some/othersound.ogg ;;
esac
printf "\r%s\e[K" "$line"
fi
done
echo
exec {tailF}<&-
kill $tailPid
You could quit by pressing Q key.
you certainly won't succeed with
tail -f /var/log/foo.log |grep --line-buffered string2search
when you use "colortail" as an alias for tail, eg. in bash
alias tail='colortail -n 30'
you can check by
type alias
if this outputs something like
tail isan alias of colortail -n 30.
then you have your culprit :)
Solution:
remove the alias with
unalias tail
ensure that you're using the 'real' tail binary by this command
type tail
which should output something like:
tail is /usr/bin/tail
and then you can run your command
tail -f foo.log |grep --line-buffered something
Good luck.
Use awk(another great bash utility) instead of grep where you dont have the line buffered option! It will continuously stream your data from tail.
this is how you use grep
tail -f <file> | grep pattern
This is how you would use awk
tail -f <file> | awk '/pattern/{print $0}'
In Linux, I want to monitor the output of some tool, e.g. dbus-monitor's output. I hope to cat some special key word of its output, and then use the key word to be as input argument of other program. Like below, but it is not good.
dbus-monitor --system > d.log &
var=`cat d.log | grep some-key-word`
my_script.sh $var
I hope to monitor the output flow in real time, and not to cat the whole log from beginning. Just to cat its latest change. E.g. dmesg provides an option, dmesg -w, which meets what I want.
-w, --follow wait for new messages
So how to make such script? To cat the latest new output and use it continuously.
Instead of cat, use tail -F <file> | grep <something>. This option makes tail to wait for and output all incoming data. Most likely, you also will need to modify buffering mode for standard streams with stdbuf -oL (by default, stdout is fully buffered meaning that data is written into file each couple of kilobytes and not after each line).
I am reading 'advanced bash script', in Chapter 31, there is a problem. I can not figure it out.
tail -f /var/log/msg | grep 'error' >> logfile
Why is there nothing to output into logfile?
can you offer me an explanation?
thank you in advance
As #chepner comments, grep is using a larger buffer (perhaps 4k or more) to buffer its stdout. Most of the standard utilities do this when piping or redirecting to a file. They typically only switch to line-buffered mode when outputting directly to the terminal.
You can use the stdbuf utility to force grep to do line buffering of its output:
tail -f /var/log/msg | stdbuf -oL grep 'error' >> logfile
As an easily observable demonstration of this effect, you can try the following two commands:
for ((i=0;;i++)); do echo $i; sleep 0.001; done | grep . | cat
and
for ((i=0;;i++)); do echo $i; sleep 0.001; done | stdbuf -oL grep . | cat
In the first command, the output from grep . (i.e. match all lines) be buffered going into the pipe to cat. On mine the buffer appears to be about 4k. You will see the ascending numbers output in chunks as the buffer gets filled and then flushed.
In the second command, grep's output into the pipe to cat is line-buffered, so you should see terminal output for every line, i.e. more-or-less continuous output.
I created a new FIFO using the mkfifo command. I have a text file f.txt.
I want to write the text file into my FIFO. How? Is there a unix command for that?
You can use cat:
mkfifo /tmp/foo
cat f.txt > /tmp/foo
You'll see that it hangs, because you also need a reader process, as cat.
cat /tmp/foo
You can also start first the reader process and then the writer one.
Just redirect into the pipe:
mkfifo /tmp/pipe
cat f.txt > /tmp/pipe &
cat /tmp/pipe
Note, this is roughly what cat f.txt | cat does, but this a named pipe instead of an anonymous pipe.
Same as any file I think:
cat f.txt > myfifo
Most things can be treated like files in Linux/Unix