Using Variables - A Practical Application With Logs¶
In lesson two, "Bash - Using Variables", you've seen some ways to use variables and learned a lot about what variables can be used for. This is just one practical example of using variables within your bash scripts.
When a system administrator has to deal with log files, there are sometimes different formats that come into play. Let's say that you want to get some information out of the
/var/log/dnf.log). Let's take a quick look at what that log file looks like using
2022-05-04T09:02:18-0400 DEBUG extras: using metadata from Thu 28 Apr 2022 04:25:35 PM EDT. 2022-05-04T09:02:18-0400 DEBUG repo: using cache for: powertools 2022-05-04T09:02:18-0400 DEBUG powertools: using metadata from Thu 28 Apr 2022 04:25:36 PM EDT. 2022-05-04T09:02:18-0400 DEBUG repo: using cache for: epel 2022-05-04T09:02:18-0400 DEBUG epel: using metadata from Tue 03 May 2022 11:55:16 AM EDT. 2022-05-04T09:02:18-0400 DEBUG repo: using cache for: epel-modular 2022-05-04T09:02:18-0400 DEBUG epel-modular: using metadata from Sun 17 Apr 2022 07:09:16 PM EDT. 2022-05-04T09:02:18-0400 INFO Last metadata expiration check: 3:07:06 ago on Wed 04 May 2022 05:55:12 AM EDT. 2022-05-04T09:02:18-0400 DDEBUG timer: sack setup: 512 ms 2022-05-04T09:02:18-0400 DDEBUG Cleaning up.
Now take a look at the
messages log file
May 4 08:47:19 localhost systemd: Starting dnf makecache... May 4 08:47:19 localhost dnf: Metadata cache refreshed recently. May 4 08:47:19 localhost systemd: dnf-makecache.service: Succeeded. May 4 08:47:19 localhost systemd: Started dnf makecache. May 4 08:51:59 localhost NetworkManager: <info> [1651668719.5310] dhcp4 (eno1): state changed extended -> extended, address=192.168.1.141 May 4 08:51:59 localhost dbus-daemon: [system] Activating via systemd: service name='org.freedesktop.nm_dispatcher' unit='dbus-org.freedesktop.nm-dispatcher.service' requested by ':1.10' (uid=0 pid=981 comm="/usr/sbin/NetworkManager --no-daemon " label="system_u:system_r:NetworkManager_t:s0") May 4 08:51:59 localhost systemd: Starting Network Manager Script Dispatcher Service... May 4 08:51:59 localhost dbus-daemon: [system] Successfully activated service 'org.freedesktop.nm_dispatcher' May 4 08:51:59 localhost systemd: Started Network Manager Script Dispatcher Service. May 4 08:52:09 localhost systemd: NetworkManager-dispatcher.service: Succeeded.
And finally let's take a look at the output of the
Wed May 4 09:47:00 EDT 2022
Findings and Goals¶
What we can see here is that the two log files,
messages display the date in entirely different ways. If we wanted to grab the information from the
messages log in a bash script using
date we could do so without much trouble, but getting the same information out of the
dnf.log would take some doing. Let's say that as a system administrator, you need to review the
dnf.log daily to make sure that nothing was introduced to the system that you weren't aware of or that might cause problems. You want this information to be grabbed from the
dnf.log file by date and then emailed to you daily. You will use a
cron job to automate this, but first we need to get a script that will do what we want it to do.
To accomplish what we want, we are going to use a variable in our script called "today" that will format the date according to the date displayed in the
dnf.log. To get the correct
date format, we are using the
+%F which will get us the yyyy-mm-dd format we are looking for. Since all we are concerned with is the day, not the times or any other information, that's all we will need to get the correct information out of the
dnf.log. Try just this much of the script:
#!/usr/bin/env bash # script to grab dnf.log data and send it to administrator daily today=`date +%F` echo $today
Here we are using the
echo command to see if we have been successful with our date formatting. When you run the script, you should get an output with today's date that looks something like this:
If so then great, we can remove our "debug" line and continue. Let's add another variable called "logfile" that we will set to
/var/log/dnf.log and then let's see if we can
grep that using our "today" variable. For now, let's just let it run to standard output:
!/usr/bin/env bash # script to grab dnf.log data and send it to administrator daily today=`date +%F` logfile=/var/log/dnf.log /bin/grep $today $logfile
dnf.log has a lot of information in it every day, so we are not posting that to the screen here, but you should see output that has only today's data in it. Give the script a try and if it works, then we can move on to the next step. After we've checked the output, the next step is that we want to do a pipe redirect to send the information to email.
mailx and a mail daemon such as
postfix installed to accomplish this next step. There's also some configuration that will probably be necessary for you to receive email from your server to your companies email address. Don't worry about those steps at this point, because you can check the
maillog to see if the attempt was made and then work from there to get email from your server to your email address working. That's not something that this document is going to deal with. For now do:
dnf install mailx postfix systemctl enable --now postfix
#!/usr/bin/env bash # script to grab dnf.log data and send it to administrator daily today=`date +%F` logfile=/var/log/dnf.log /bin/grep $today $logfile | /bin/mail -s "DNF logfile data for $today" firstname.lastname@example.org
Let's take a look at the additions to the script here. We've added a pipe
| to redirect output to
/bin/mail set the subject of the email (
-s) with what is in double quotes and set the recipient to be "email@example.com". Replace that last bit with your email address and then try running the script again.
As noted, you probably won't get the email without some changes to your Postfix mail setup, but you should see the attempt in
The next thing you need to do is to get sending email from your server working. You can take a look at Postfix for Reporting to get you started on that front. We also need to automate this script to run daily, to do that we will use
cron. There are multiple references here: cron, anacron, and cronie. For more information on date formatting, check out
man date or this link.
Author: Steven Spencer
Contributors: Antoine Le Morvan, Ganna Zhyrnova