So i'm currently trying to grep a single result from a random file in a specific directory. The grepping works just fine and the expected output file is populated as expected, but for some reason, even after the output file has already been filled, the process won't stop. This is the grep command where the program seems to be getting stuck.
searchFILE(){
case $2 in
pref)
echo "Populating output file: $3-$1.data.out"
dataOutputFile="$3-$1.data.out"
zgrep -a "\"someParameter\"\:\"$1\"" /folder/anotherFolder/filetemplate.log.* | zgrep -a "\"parameter2\"\:\"$3\"" | head -1 > $dataOutputFile
;;
*)
echo "Unrecognized command"
;;
esac
echo "Query finished"
}
What is currently happening is that the output file is being populated as expected with the head pipe, but for some reason I'm not getting the "Query finished" message, and the process seems not to stop at all.
grep does not know that head -n1 is no longer reading from the pipe until it attempts to write to the pipe, which it will only do if another match is found. There is no direct communication between the processes. It will eventually stop, but only once all the data is read, a second match is found and write fails with EPIPE, or some other error occurs.
You can watch this happen in a simple pipeline like this:
cat /dev/urandom | grep -ao "12[0-9]" | head -n1
With a sufficiently rare pattern, you will observe a delay between output and exit.
One solution is to change your stop condition. Instead of waiting for SIGPIPE as your pipeline does, wait for grep to match once using the -m1 option:
cat /dev/urandom | grep -ao -m1 "12[0-9]"
I saw better performance results with zcat myZippedFile | grep whatever paradigm...
The first difference you need to try is pipe with | head -z --lines=1
The reason is null terminated lines instead of newlines (just in case).
My example script below worked (drop the case statement to make it more simple). If I hold onto $1 $2 inside functions things go wrong. I use parameter $names and only use the $1 $2 $# once, because it also goes wrong for me if I don't and in any case you can then shift over $# and catch arguments. The $# in the script itself are not the same as arguments in bash functions.
grep searching for 2 or multiple parameters in any order means using grep twice; in your case zgrep | grep. The second grep is a normal grep! You only need the first grep to be zgrep to do the unzip. Your question is simpler if you drop the case statement as bash case scares off people: bash was always an ugly lady that works good for short scripts.
zgrep searches text or compressed text, but newlines in LINUX style vs WINDOWS are not the same. So use dos2unix to convert files so that newlines work. I use compressed file simply because it is strange and rare to see zgrep, so it is demonstrated in a shell script with a compressed file! It works for me. I changed a few things, like >> and "sort -u" but you can obviously change them back.
#!/usr/bin/env bash
# Search for egA AND egB using option go
# COMMAND LINE: ./zgrp egA go egB
A="$1"
cOPT="$2" # expecting case go
B="$3"
LOG="./filetemplate.log" # use parameters for long names.
# Generate some data with gzip and delete the temporary file.
echo "\"pramA\":\"$A\" \"pramB\":\"$B\"" >> $B$A.tmp
rm -f ${LOG}.A; tar czf ${LOG}.A $B$A.tmp
rm -f $B$A.tmp
# Use paramaterise $names not $1 etc because you may want to do shift etc
searchFILE()
{
outFile="$B-$A.data.out"
case $cOPT in
go) # This is zgrep | grep NOT zgrep | zgrep
zgrep -a "\"pramA\":\"$A\"" ${LOG}.* | grep -a "\"pramB\":\"$B\"" | head -z --lines=1 >> $outFile
sort -u $outFile > ${outFile}.sorted # sort unique on your output.
;;
*) echo -e "ERROR second argument must be go.\n Usage: ./zgrp egA go egB"
exit 9
;;
esac
echo -e "\n ============ Done: $0 $# Fin. ============="
}
searchFILE "$#"
cat ${outFile}.sorted
Goal: Needing to use tail with grep search each incoming line for a specific pattern, provided by a while loop
Issue: Despite the proper syntax/placement of the grep command no incoming lines are being filtered and checked for the pattern
function interval ()
{
echo $(date '+%y%m%d %H%M%S') $cnt
cnt=0
}
declare -i cnt=0
trap interval SIGUSR1
shopt -s lastpipe
tail -f --pid=$$ ${1:-access.log} | grep "20051" | while read aline
do
let cnt++
done
If you are open to other options, you might want to take a look at this. Using the watch command could be very useful in this case. Redirecting all your output into a file, say file.txt,
watch -d cat file.txt | grep "20051"
Taking a look at the watch command man page, you can customize the time interval between consecutive executions.
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}'
I have a stream of logs from a specific tool on my server with persistent output.
I've say it's like
task 3 done
all is ok
added another task
something made
I cannot grep from output file by a reason, but I have an opportunity of make something through pipes or output streams:
./myexecutablefile | grep something
In this case I receive highlighted "something" if it appeared in output.
I want to make something if first occurrence is met, but i don't know how, because of continuous stream. For example my goal is to kill process if I will see some error.
Surely this doesn't work, && kill will begin after executable ends
./myexecutable | grep 'Error occured' && kill -9 myexecutable
You need to use -q option in grep:
./myexecutable | grep -q 'Error occured' && kill -9 myexecutable
If you are interested in the first occurrence only, you can use the -m switch of grep:
./myexecutablefile | grep -m1 something && do_something
This will still output the matching line. To suppress that, you can turn on -q, but then, you don't need -m1 anymore:
./myexecutablefile | grep -q something && do_something
I'm currently figuring my way around a bash script (sorry, can't use other languages like perl) to keep track of a running log during a server startup. Basically, I have to trigger certain events depending on whether or not i run into certain strings or patterns while the log is being written. Currently, i have this code:
LOG=path_to_logfile
LINE1="[1-9][0-9]* some string"
LINE2="another string"
LINE3="third string"
tail -fn0 $LOG | \
while read line
do
echo $line | grep "$LINE1" || echo $line | grep "$LINE2" || echo $line | grep "$LINE3"
if [ $? = 0 ]
then
TMP=<echo line above>
... bunch of conditional statements...
fi
done
However, this is kinda slow; by the time the line i need to track is detected by the echo/grep combinations using or, it's waaaay after the server already started up. What's a good alternative to the above? I've read awk should be used but when i tried writing it in awk, either i wrote it wrong or the processing was also taking too much time to finish.
Any help will be appreciated. Thanks!
Rather than calling grep (potentially several times) on each line, let bash do the regular expression matching.
LOG=path_to_logfile
LINE1="[1-9][0-9]* some string"
LINE2="another string"
LINE3="third string"
tail -fn0 $LOG | while read line
do
if [[ $line =~ $LINE1|$LINE2|$LINE3 ]]; then
TMP=<echo line above>
... bunch of conditional statements...
fi
done
I'd try something like this instead:
tail -fn0 $LOG | egrep "$LINE1|$LINE2|$LINE3" | \
while read TMP
do
...
done
That way, the while read loop, which at a guess is going to be the slowest part of this whole operation, is only invoked when egrep actually finds a matching line in the input log.
You can have multiple match statements, which are ORed together to see if the line matches:
tail -f -n0 "$LOG" | grep -e "$LINE1" -e "$LINE2" -e "$LINE3" | while IFS= read -r line
do
# Do something with each matching $line
done