Category: Linux Notes

If you’re looking for a quick way to keep your CentOS machine patched at all times, become friends with yum-cron.

To install, in terminal, use:

#yum -y install yum-cron

The -y forces the install, answering any Yes / No “are you sure” questions.

Once installed, be sure to start the service using:

# systemctl start yum-cron.service

You can verify the status (make sure it’s running) using:

# systemctl status yum-cron.service

Output should look something like:

yum-cron.service – Run automatic yum updates as a cron job
Loaded: loaded (/usr/lib/systemd/system/yum-cron.service; enabled; vendor preset: disabled)
Active: active (exited) since Mon 2016-11-07 19:16:01 UTC; 2s ago
Process: 5385 ExecStart=/bin/touch /var/lock/subsys/yum-cron (code=exited, status=0/SUCCESS)
Main PID: 5385 (code=exited, status=0/SUCCESS)

Last step is to make sure it’s enabled, this will make sure it restarts in the event the machines reboots later.

# systemctl enable yum-cron.service

Sharing is caring!

If you’re working in linux you will be introduced to writing shell scripts at some point. When you do, you’ll become very familiar with the echo.

If you open your terminal, and run an echo command it’ll literally echo whatever you put after the echo:

$ echo "Hello World" Hello World

Sometimes you need to compress an entire directory, but sometimes you want to ignore specific file types.

Here is a basic command that you can use to remove specific file types before you compress it. In this example we use the find command to look for, and ignore, all image file types. That then gets piped into the tar command:

There are instances where you want to quickly locate and move specific file types from point A to Point B.

Example:

Move all PHP files from /home/$user/php-files/ to /home/$user/php-mv-files

Easiest way to do this is to use the Find command in terminal.

$ find ./ -name "*.php"

./php-files/file3.php
./php-files/file2.php
./php-files/file1.php

This lists all the PHP files in the /php-files directory.

To move the files, you run the following command:

$ find ./php-files/ -name "*.php" -exec mv {} ./php-mv-files/ \;

This will move all files from ./php-files to ./php-vmv-files. Running the original Find command will get you the following:

$ find ./ -name "*.php"
./php-mv-files/file3.php
./php-mv-files/file2.php
./php-mv-files/file1.php

In this instance, -exec mv {} says, run the MV command on the files selected from the original find, that’s what the {} implies. You then escape, and close the command using \;. This final piece is critical to stopping the command.

Sharing is caring!

Path environmental variables are variables that function like a shortcut for your system. They allow you to define directories where executables are located.

Assume you installed this application picc-9.82.9453-linux.run. Without setting the path you’d have to run this every time in your terminal:

# /usr/hitech/picc/9.82/bin/picc

Instead of:

# picc

Running picc without having to define the path every time is so much easier, and faster. To achieve this experience you have to add the application executable to the Path environmental variable.

The first option is edit your user profile:

$ vim ~/.bash_profile

It will look something like this:

# .bash_profile

# Get the aliases and functions
if [ -f ~/.bashrc ]; then
. ~/.bashrc
fi

# User specific environment and startup programs

PATH=$PATH:$HOME/bin

export PATH

You are looking to edit this section:

PATH=$PATH:$HOME/bin

Update the path with new application location (e.g., /usr/hitech/picc/9.82/bin) using the colon (:) as the separator.

PATH="$HOME/bin:$PATH:/usr/hitech/picc/9.82/bin"

Save the file, and try running the command in your terminal.

# picc

Sharing is caring!

Whenever you’re working with linux you’ll find yourself in need of running tasks repeatedly. The easiest way to do this is to set up a recurring job using Cron.

A crontab file contains instructions for the cron(8) daemon in the following simplified manner: “run this command at this time on this
date”. – Cron Man Page

You can edit your cron jobs using crontab, specifically:

# crontab -e

If you have any other cron jobs you’ll see other entries. If not, you’ll be able to create your own entries using your favorite editor (e.g., VIM, Nano, VI, etc…).

The format for a cron entry is as follows:

minute hour day-of-month month day-of-week command

There is a great tutorial available here: https://crontab.guru/every-1-hour

If I want to run a job every hour, this is the entry I’d add to Cron:

0 * * * * cd /root/scripts && ./checkrunningprocess.sh

If you ever want to see what jobs you have running you can run:

crontab -l

and you’d see something like this:

# crontab -l
0 * * * * cd /root/scripts && ./checkrunningprocess.sh

Happy automating!

Sharing is caring!

When you’re working with linux you might have to add a user to different group.

Let’s say that you want to create user John, and you want John to have root abilities. To do this you’re going to want to add the user, John, to the sudoers group.

The sudoers group controls who has Sudo privileges on a machine. Sudo is a command that allows a user to run commands based on another users privileges, in most instances it is configured to run as root. Fun fact, SUDO was established from “superuser do.”

To add user John to the sudoers group you would do something like this:

# usermod -aG sudoers John

The two options used include:

• a – Add the user to the supplementary group(s). Use only with the -G option.
• G – A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace.

You have to use both options in conjunction with each other. You can find a more detailed list of options here: https://linux.die.net/man/8/usermod

This same model applies to any group and can be used for any user. A great example is Apache on a web server. 🙂

Sharing is caring!

If you are looking for specific content but can’t remember where it’s located on the server, this post is for you. It will show you how to find specific text anywhere in a folder, or server.

We are going to use the GREP command when doing searches in linux.

grep searches input files for lines containing a match to a given pattern list. When it finds a match in a line, it copies the line to standard output (by default), or produces whatever other sort of output you have requested with options.