I'm trying to make a shell script to monitor a log file, but I have a problem which is I can't do two tail at the same time.
the script basically is to search for a word if it matches it will redirect the 3 lines include the matched word into a file then I will pruning the useless information to extract what I want.
I tried the commands below and it's working fine but when I mirage it in a file it doesn't work
please advise :)
below is a part of the script,
#!/bin/bash
#grep error log
tail -f /FileLogging.log | grep 'error' >>/home/hello/tech.txt
#pruning useless information
tail -f /home/hello/tech.txt perl -nle "print $1 if /sam-(.+?)\",\"jack/" >>/home/hello/non.txt
Now I detected, the there only one source is watched. So this command should combine both of your example:
tail -f /FileLogging.log | grep 'error' | tee -a /home/hello/tech.txt | perl ... >>/home/hello/non.txt
Related
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 am currently learning a little more about using Bash shell on OSX terminal. I am trying to pipe the output of a cut command into a grep command, but the grep command is not giving any output even though I know there are matches. I am using the following command:
cut -d'|' -f2 <filename.txt> > <temp.txt> | grep -Ff <temp.txt> <searchfile.txt> > <filematches.txt>
I was thinking that this should work, but most of the examples I have seen normally pipe grep output into the cut. My goal was to cut field 2 from the file and use that as the pattern to search for in . However, using the command produced no output.
When I generated the temp.txt first with the cut command and then ran the grep on it manually with no pipe, the grep seemed to run fine. I am not sure why this is?
You can use process substitution here:
grep -Ff <(cut -d'|' -f2 filename.txt) searchfile.txt > filematches.txt
<(cut -d'|' -f2 filename.txt) is feeding cut command's output to grep as a file.
Okay, a reason this line doesn't behave as you expect
cut -d'|' -f2 <filename.txt> > <temp.txt> | grep -Ff <temp.txt> <searchfile.txt> > <filematches.txt>
is that the output of your cut is going to temp.txt. You're not sending anything to the pipe. Now, conveniently pipe also starts a new commend, so it doesn't matter much -- grep runs and reads searchfile.txt.
But what are you trying to do? Here's what your command line is trying to do:
take the second pipe-delimited field from filename.txt
write it to a file
run grep ...
... using the contents of the file from 2 as a grep search string (which isn't going to do what you think either, as you're effectively asking grep to look for the pattern match1\nmatch2...)
You'd be closer with
cut ... && grep ...
as that runs grep assuming cut completes effectively. Or you could use
grep -f `cut ...`
which would put the results on the command line. You need to mess with quoting, but you're still going to be looking for a line containing ALL of your match fields from cut.
I'd recommend maybe you mean something like this:
for match in `cut ...`
do
grep -f $match >> filematches.txt
done
I'm trying to use grep to go through some logs and only select the most recent entries. The logs have years of heavy traffic on them so it's silly to do
tac error.log | grep 2012
tac error.log | grep "Jan.2012"
etc.
and wait for 10 minutes while it goes through several million lines which I already know are not going to match. I know there is the -m option to stop at the first match but I don't know of a way to make it stop at first non-match. I could do something like grep -B MAX_INT -m 1 2011 but that's hardly an optimal solution.
Can grep handle this or would awk make more sense?
How about using awk like this:
tac error.log | awk '{if(/2012/)print;else exit}'
This should exit as soon as a line not matching 2012 is found.
Here is a solution in python:
# foo.py
import sys, re
for line in sys.stdin:
if re.match(r'2012', line):
print line,
continue
break
you#host> tac foo.txt | python foo.py
I don't think grep supports this.
But here is my "why did we have awk again" answer:
tail -n `tac biglogfile | grep -vnm1 2012 | sed 's/:.*//' | xargs expr -1 +` biglogfile
Note that this isn't going to be exact if your log is being written to.
The excellent one-line scripts for sed page to the rescue:
# print section of file between two regular expressions (inclusive)
sed -n '/Iowa/,/Montana/p' # case sensitive
In other words, you should be able to do the following:
sed -n '/Jan 01 2012/,/Feb 01 2012/p' error.log | grep whatevs
Here is an example that parses the user's dot-plan file and stops at the first non-matching line:
PID=$$
while read ln; do
echo $ln | {
if grep "^[-*+] " >/dev/null; then
# matched
echo -e $ln
elif grep "^[#]" >/dev/null; then
# ignore comment line
:
else
# stop at first non-matching line
kill $PID
fi
}
done <$HOME/.plan
Of course this approach is considerably slower than if Grep reads the lines but at least you can incorporate several cases (not just the non-match).
For more complex scripts, it is worth noting that Bash can also apply regular expressions to variables, i.e. you can also do completely without grep.
I want to grab data from this kind of log.
Nov 12 13:46:14 Home cxxd[8892]: 208 11/12 13:46:14| qc=IN (1), qt=A (1), query="www.yahoo.com."
Implemented this which gives me the URL. But does not work with "TAIL -F" so that I could monitor live just the urls.
tail -100 /var/log/system.log | grep "query=" | sed -e "s/.*query=//" | sed -e "s/\"//g" | sed -e "s/.$/ /"
Please suggest or enhance
I expect your multiple sed scripts do work with tail -F output, just not as you expect.
The C standard IO libraries will perform buffering to improve performance. The IO library can do (a) no buffering (b) line-buffering (c) block-buffering. The line-buffering is normally chosen if the output is going to a terminal. But if the output is going to a file or pipe, then block buffering is normally chosen. (It's more complicated than this -- the behavior changes if the file descriptor in question is being used for stdout or stderr or another file. See setvbuf(3) for full details.)
So, while the block-buffering you're seeing now is probably better for performance, it does mean you can wait a while before ever seeing any output, as each command will eventually accumulate a block of data. At least grep(1) allows the --line-buffered command line option to use line-buffering -- and sed(1) allows the --unbuffered command line option to flush output buffers more often. So try this:
tail -f /var/log/system.log | grep --line-buffered "query=" | sed -u -e "s/.*query=//" | sed -u -e "s/\"//g" | sed -u -e "s/.$/ /"
(I didn't find any similar options for tail(1), but even if it sends blocks of data to the others, the changes to grep(1) and sed(1) will drastically help.)
Try reducing the number of pipes by replacing multiple calls to grep and sed to one with awk:
tail -f /var/log/system.log | awk -F'=' '/query=/ { sub(/^"/, "", $NF); sub(/."$/, "", $NF); print $NF }'
...which takes every line matching "query=" and grabs everything after the last '=', replaces the first '"' and the trailing '."' and prints the result.
Try the tail -f and grep argument --line-buffered
I am using 'tail -f' to follow a log file as it's updated; next I pipe the output of that to grep to show only the lines containing a search term ("org.springframework" in this case); finally I'd like to make is piping the output from grep to a third command, 'cut':
tail -f logfile | grep org.springframework | cut -c 25-
The cut command would remove the first 25 characters of each line for me if it could get the input from grep! (It works as expected if I eliminate 'grep' from the chain.)
I'm using cygwin with bash.
Actual results: When I add the second pipe to connect to the 'cut' command, the result is that it hangs, as if it's waiting for input (in case you were wondering).
Assuming GNU grep, add --line-buffered to your command line, eg.
tail -f logfile | grep --line-buffered org.springframework | cut -c 25-
Edit:
I see grep buffering isn't the only problem here, as cut doesn't allow linewise buffering.
you might want to try replacing it with something you can control, such as sed:
tail -f logfile | sed -u -n -e '/org\.springframework/ s/\(.\{0,25\}\).*$/\1/p'
or awk
tail -f logfile | awk '/org\.springframework/ {print substr($0, 0, 25);fflush("")}'
On my system, about 8K was buffered before I got any output. This sequence worked to follow the file immediately:
tail -f logfile | while read line ; do echo "$line"| grep 'org.springframework'|cut -c 25- ; done
What you have should work fine -- that's the whole idea of pipelines. The only problem I see is that, in the version of cut I have (GNU coreutiles 6.10), you should use the syntax cut -c 25- (i.e. use a minus sign instead of a plus sign) to remove the first 24 characters.
You're also searching for different patterns in your two examples, in case that's relevant.