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How to Tail, Search, and Filter Linux Logs

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Regardless of what language you code in or what type of apps you’re working on, you’re going to end up reading log files. They’re your window into what’s happening inside your code or the server you’re talking to. Linux log management is one of the skills setting an experienced developer apart from the rest.

The good news is Linux has a broad array of tools for searching and filtering log files. Like most system administration tasks, there’s more than one way to tackle this task.

Viewing and Tailing Logs

Let’s start by looking at the last few messages in a file and following a log in real time.

Start With the Tail

This article has tail right there in the title, so let’s start there.

Log files tend to grow in size, so opening them up in an editor like vi can be difficult, if not impossible. Opening a large file in an editor consumes a lot of memory and will start to use disk space if you make any changes. Your system might not handle the load, or it could interfere with a production process.

That’s where tail comes in.

egoebelbecker@latveria:/var/log$ tail auth.log
Nov 30 14:50:23 latveria sshd[84245]: Received disconnect from 192.168.7.53 port 63089:11: disconnected by user
Nov 30 14:50:23 latveria sshd[84245]: Disconnected from user egoebelbecker 192.168.7.53 port 63089
Nov 30 14:50:23 latveria sshd[84164]: pam_unix(sshd:session): session closed for user egoebelbecker
Nov 30 14:50:23 latveria systemd-logind[760]: Session 230 logged out. Waiting for processes to exit.
Nov 30 14:50:23 latveria systemd-logind[760]: Removed session 230.
Nov 30 14:50:24 latveria sshd[84309]: Accepted publickey for egoebelbecker from 192.168.7.53 port 63270 ssh2: RSA SHA256:rzKp/Dyg4Xm2ieYndTB8OWHSXFty4cNuYj7Uj5c0UvE
Nov 30 14:50:24 latveria sshd[84309]: pam_unix(sshd:session): session opened for user egoebelbecker by (uid=0)
Nov 30 14:50:24 latveria systemd-logind[760]: New session 231 of user egoebelbecker.
Nov 30 15:17:01 latveria CRON[84403]: pam_unix(cron:session): session opened for user root by (uid=0)
Nov 30 15:17:01 latveria CRON[84403]: pam_unix(cron:session): session closed for user root

Tail’s default behavior is to read the last 10 lines of a file. It does this without reading the entire file first, so it doesn’t matter how large the file is.

You can view more or less than 10 lines by adding a command-line argument.

egoebelbecker@latveria:/var/log$ tail -5 auth.log
Nov 30 14:44:44 latveria systemd-logind[760]: New session 228 of user egoebelbecker.
Nov 30 14:44:44 latveria systemd: pam_unix(systemd-user:session): session opened for user egoebelbecker by (uid=0)
Nov 30 14:47:12 latveria sshd[84164]: Accepted publickey for egoebelbecker from 192.168.7.53 port 63089 ssh2: RSA SHA256:rzKp/Dyg4Xm2ieYndTB8OWHSXFty4cNuYj7Uj5c0UvE
Nov 30 14:47:12 latveria sshd[84164]: pam_unix(sshd:session): session opened for user egoebelbecker by (uid=0)
Nov 30 14:47:12 latveria systemd-logind[760]: New session 230 of user egoebelbecker.

So, if you’re dealing with a gigantic log file and want to see what’s going on for the last few hundred lines, give tail the number of lines, pipe it to more, and page through the results.

egoebelbecker@latveria:/var/log$ tail -200 auth.log|more
Nov 29 00:17:01 latveria CRON[77383]: pam_unix(cron:session): session opened for user root by (uid=0)
Nov 29 00:17:01 latveria CRON[77383]: pam_unix(cron:session): session closed for user root
Nov 29 01:17:01 latveria CRON[77928]: pam_unix(cron:session): session opened for user root by (uid=0)
Nov 29 01:17:01 latveria CRON[77928]: pam_unix(cron:session): session closed for user root
Nov 29 02:17:01 latveria CRON[78339]: pam_unix(cron:session): session opened for user root by (uid=0)
Nov 29 02:17:01 latveria CRON[78339]: pam_unix(cron:session): session closed for user root
Nov 29 03:10:01 latveria CRON[78380]: pam_unix(cron:session): session opened for user root by (uid=0)
Nov 29 03:10:01 latveria CRON[78380]: pam_unix(cron:session): session closed for user root
Nov 29 03:17:01 latveria CRON[78399]: pam_unix(cron:session): session opened for user root by (uid=0)
Nov 29 03:17:01 latveria CRON[78399]: pam_unix(cron:session): session closed for user root
Nov 29 03:30:01 latveria CRON[78417]: pam_unix(cron:session): session opened for user root by (uid=0)
Nov 29 03:30:01 latveria CRON[78417]: pam_unix(cron:session): session closed for user root
Nov 29 03:47:01 latveria CRON[78435]: pam_unix(cron:session): session opened for user root by (uid=0)
•	More—

Chasing the Tail

You’re probably already familiar with using tail to follow a file as it updates. Add the -f command-line argument and the file to follow.

This is a common use for tail and is useful for integration testing or troubleshooting problems in situ.

But what if you want to go back and look at something you missed while following the file? What if, as mentioned above, the file is too big to open with an editor or page through screen by screen with more?

Less Is More (and More!)

Less works like more. It displays a file’s contents page by page like more, but it does much more than that. You can scroll forward with the space bar like with more. But, you can scroll backward with b and search for text with / and ? similar to vi.

That’s just the tip of the iceberg for less, but there’s one feature particularly handy for logs. It can follow them, similar to tail -f, but it’ll also let you page and search through the file without reopening it, too.

If you already have a file open, press f to start following it. Less will move to the end and wait for an update.

If something scrolls by and you want to scroll back and see it, press ctrl+C, and less will stop waiting for updates. Now you can scroll back or search. If you want to resume reading updates, press f again.

Filtering Logs

Reading logs as they’re updated is useful for testing and can be a good way to isolate a problem, but sometimes you need to go back and search through a log file to see what happened in the past.

This is where searching and filtering comes in. Less can do some of this work for you since you can search forward with / and back with ?.

It’s also worth mentioning view, which runs vi in read-only mode. This makes it safe to open huge files.

Grep

For filtering, grep is the workhorse most people turn to, and for good reason.

You can use it to search for a basic regular expression by pasting it on the command line.

egoebelbecker@latveria:/var/log$ grep egoebelbecker auth.log |more
Nov 29 16:54:32 latveria sshd[79348]: Accepted publickey for egoebelbecker from 192.168.
7.131 port 33366 ssh2: RSA SHA256:P9Ot2rSd5MMXAdTF4nVjEpQQFnO07sEL0AWZX/RDkYk
Nov 29 16:54:32 latveria sshd[79348]: pam_unix(sshd:session): session opened for user eg
oebelbecker by (uid=0)
Nov 29 16:54:32 latveria systemd-logind[760]: New session 200 of user egoebelbecker.
Nov 29 17:01:18 latveria sshd[79490]: Disconnected from user egoebelbecker 192.168.7.131
port 33366
Nov 29 17:01:18 latveria sshd[79348]: pam_unix(sshd:session): session closed for user eg
oebelbecker
Nov 30 00:09:31 latveria sshd[81080]: Accepted publickey for egoebelbecker from 192.168.
7.53 port 64605 ssh2: RSA SHA256:rzKp/Dyg4Xm2ieYndTB8OWHSXFty4cNuYj7Uj5c0UvE
Nov 30 00:09:31 latveria sshd[81080]: pam_unix(sshd:session): session opened for user eg
oebelbecker by (uid=0)
Nov 30 00:09:31 latveria systemd-logind[760]: New session 208 of user egoebelbecker.
Nov 30 00:13:45 latveria sudo: pam_unix(sudo:auth): authentication failure; logname=egoe
belbecker uid=1000 euid=0 tty=/dev/pts/1 ruser=egoebelbecker rhost= user=egoebelbecker
Nov 30 00:13:51 latveria sudo: egoebelbecker : TTY=pts/1 ; PWD=/var/log ; USER=root ; CO
MMAND=/usr/bin/su
Nov 30 00:13:51 latveria sudo: pam_unix(sudo:session): session opened for user root by e
goebelbecker(uid=0)
Nov 30 00:13:52 latveria su: (to root) egoebelbecker on pts/1
Nov 30 00:13:52 latveria su: pam_unix(su:session): session opened for user root by egoeb
elbecker(uid=0)
•	More—

In this case, the result was more than a page long, so I piped it through more. I could have used less, but it would’ve cleared the screen and removed the grep command.

Regex

Basic regular expressions will get you a long way, but sometimes you need to step things up with an extended regular expression. These expressions support a larger set of metacharacters.

Let’s say our example server has connectivity to two different subnets, 192.168.7.X and 192.168.1.X. We want to grep for logins from either one, so we need the or (|) operator.

egoebelbecker@latveria:/var/log$ grep -E ‘192.168.7|192.168.1’ auth.log |more
Nov 29 16:54:32 latveria sshd[79348]: Accepted publickey for egoebelbecker from 192.168.1.131 port 33366 ssh
2: RSA SHA256:P9Ot2rSd5MMXAdTF4nVjEpQQFnO07sEL0AWZX/RDkYk
Nov 29 17:01:18 latveria sshd[79490]: Received disconnect from 192.168.7.131 port 33366:11: disconnected by
user
Nov 29 17:01:18 latveria sshd[79490]: Disconnected from user egoebelbecker 192.168.1.131 port 33366
Nov 30 00:09:31 latveria sshd[81080]: Accepted publickey for egoebelbecker from 192.168.7.53 port 64605 ssh2
: RSA SHA256:rzKp/Dyg4Xm2ieYndTB8OWHSXFty4cNuYj7Uj5c0UvE
Nov 30 00:14:15 latveria sshd[81329]: Accepted publickey for egoebelbecker from 192.168.7.53 port 64715 ssh2
: RSA SHA256:rzKp/Dyg4Xm2ieYndTB8OWHSXFty4cNuYj7Uj5c0UvE
Nov 30 02:26:51 latveria sshd[81227]: Received disconnect from 192.168.7.53 port 64605:11: disconnected by u
ser
Nov 30 02:26:51 latveria sshd[81410]: Received disconnect from 192.168.7.53 port 64715:11: disconnected by u
ser
Nov 30 02:26:51 latveria sshd[81227]: Disconnected from user egoebelbecker 192.168.7.53 port 64605
Nov 30 02:26:51 latveria sshd[81410]: Disconnected from user egoebelbecker 192.168.7.53 port 64715
Nov 30 14:44:44 latveria sshd[83945]: Accepted publickey for egoebelbecker from 192.168.7.53 port 62966 ssh2
: RSA SHA256:rzKp/Dyg4Xm2ieYndTB8OWHSXFty4cNuYj7Uj5c0UvE
•	More—

Line Numbers

Sometimes you want to find a set of lines within a large log file. Crafting a regular expression to catch all of them would be too difficult. So, what we need is a line number. Grep can do this for you, too. Let’s say we’re looking for one of my logins into the sample server. We’ll look for port 53799.

egoebelbecker@latveria:/var/log$ grep -n 53799 auth.log
165:Nov 30 17:06:25 latveria sshd[84705]: Accepted publickey for egoebelbecker from 192.168.1.53 port 53799 ssh2: RSA SHA256:rzKp/Dyg4Xm2ieYndTB8OWHSXFty4cNuYj7Uj5c0UvE

The -n option tells grep to include the port number with the search result. Now we can open the file in view or less and go to line number 165.

What’s Next for Linux Log Management?

We covered how to use tail, less, view, grep, and more to follow and filter logs. With these tools, you have most of what you need for Linux log management. You can follow, search, and filter your log files regardless of their size. Depending on what you need, you can combine these tools to tackle almost any situation.

But this is only the beginning. You can use your logs to generate alerts and stay on top of software problems and application issues in real time without watching a screen. To get started, sign up for a free SolarWinds® Papertrail trial today.

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This post was written by Eric Goebelbecker. Eric has worked in the financial markets in New York City for 25 years, developing infrastructure for market data and financial information exchange (FIX) protocol networks. He loves to talk about what makes teams effective (or not so effective!).

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