Files – Sick Gaming

Posted on October 5, 2020 by — Leave a comment

Recover your files from Btrfs snapshots

As you have seen in a previous article, Btrfs snapshots are a convenient and fast way to make backups. Please note that these articles do not suggest that you avoid backup software or well-tested backup plans. Their goals are to show a great feature of this file system, snapshots, and to inspire curiosity and invite you to explore, experiment and deepen the subject. Read on for more about how to recover your files from Btrfs snapshots.

A subvolume for your project

Let’s assume that you want to save the documents related to a project inside the directory $HOME/Documents/myproject.

As you have seen, a Btrfs subvolume, as well as a snapshot, looks like a normal directory. Why not use a Btrfs subvolume for your project, in order to take advantage of snapshots? To create the subvolume, use this command:

btrfs subvolume create $HOME/Documents/myproject

You can create an hidden directory where to arrange your snapshots:

mkdir $HOME/.snapshots

As you can see, in this case there’s no need to use sudo. However, sudo is still needed to list the subvolumes, and to use the send and receive commands.

Now you can start writing your documents. Each day (or each hour, or even minute) you can take a snapshot just before you start to work:

btrfs subvolume snapshot -r $HOME/Documents/myproject $HOME/.snapshots/myproject-day1

For better security and consistency, and if you need to send the snapshot to an external drive as shown in the previous article, remember that the snapshot must be read only, using the -r flag.

Note that in this case, a snapshot of the /home subvolume will not snapshot the $HOME/Documents/myproject subvolume.

How to recover a file or a directory

In this example let’s assume a classic error: you deleted a file by mistake. You can recover it from the most recent snapshot, or recover an older version of the file from an older snapshot. Do you remember that a snapshot appears like a regular directory? You can simply use the cp command to restore the deleted file:

cp $HOME/.snapshots/myproject-day1/filename.odt $HOME/Documents/myproject

Or restore an entire directory:

cp -r $HOME/.snapshots/myproject-day1/directory $HOME/Documents/myproject

What if you delete the entire $HOME/Documents/myproject directory (actually, the subvolume)? You can recreate the subvolume as seen before, and again, you can simply use the cp command to restore the entire content from the snapshot:

btrfs subvolume create $HOME/Documents/myproject
cp -rT $HOME/.snapshots/myproject-day1 $HOME/Documents/myproject

Or you could restore the subvolume by using the btrfs snapshot command (yes, a snapshot of a snapshot):

btrfs subvolume snapshot $HOME/.snapshots/myproject-day1 $HOME/Documents/myproject

How to recover btrfs snapshots from an external drive

You can use the cp command even if the snapshot resides on an external drive. For instance:

cp /run/media/user/mydisk/bk/myproject-day1/filename.odt $HOME/Documents/myproject

You can restore an entire snapshot as well. In this case, since you will use the send and receive commands, you must use sudo. In addition, consider that the restored subvolume will be created as read only. Therefore you need to also set the read only property to false:

sudo btrfs send /run/media/user/mydisk/bk/myproject-day1 | sudo btrfs receive $HOME/Documents/
mv Documents/myproject-day1 Documents/myproject
btrfs property set Documents/myproject ro false

Here’s an extra explanation. The command btrfs subvolume snapshot will create an exact copy of a subvolume in the same device. The destination has to reside in the same btrfs device. You can’t use another device as the destination of the snapshot. In that case you need to take a snapshot and use the send and receive commands.

For more information, refer to some of the online documentation:

man btrfs-subvolume
man btrfs-send
man btrfs-receive

Posted on March 16, 2020 by — Leave a comment

Connect your Google Drive to Fedora Workstation

There are plenty of cloud services available where you can store important documents. Google Drive is undoubtedly one of the most popular. It offers a matching set of applications like Docs, Sheets, and Slides to create content. But you can also store arbitrary content in your Google Drive. This article shows you how to connect it to your Fedora Workstation.

Adding an account

Fedora Workstation lets you add an account either after installation during first startup, or at any time afterward. To add your account during first startup, follow the prompts. Among them is a choice of accounts you can add:

Select Google and a login prompt appears for you to login, so use your Google account information.

Be aware this information is only transmitted to Google, not to the GNOME project. The next screen asks you to grant access, which is required so your system’s desktop can interact with Google. Scroll down to review the access requests, and choose Allow to proceed.

You can expect to receive notifications on mobile devices and Gmail that a new device — your system — accessed your Google account. This is normal and expected.

If you didn’t do this at first startup, or you need to re-add your account, open the Settings tool, and select Online Accounts to add the account. The Settings tool is available through the dropdown at right side of the Top Bar (the “gear” icon), or by opening the Overview and typing settings. Then proceed as described above.

Using the Files app with Google Drive

Open the Files app (formerly known as nautilus). Locations the Files app can access appear on the left side. Locate your Google account in the list.

When you select this account, the Files app shows the contents of your Google drive. Some files can be opened using your Fedora Workstation local apps, such as sound files or LibreOffice-compatible files (including Microsoft Office docs). Other files, such as Google app files like Docs, Sheets, and Slides, open using your web browser and the corresponding app.

Remember that if the file is large, it will take some time to receive over the network so you can open it.

You can also copy and paste files in your Google Drive storage from or to other storage connected to your Fedora Workstation. You can also use the built in functions to rename files, create folders, and organize them.

Be aware that the Files app does not refresh contents in real time. If you add or remove files from other Google connected devices like your mobile phone or tablet, you may need to hit Ctrl+R to refresh the Files app view.

Photo by Beatriz Pérez Moya on Unsplash.

Posted on August 30, 2019 by — Leave a comment

Command line quick tips: Using pipes to connect tools

One of the most powerful concepts of Linux is carried on from its predecessor, UNIX. Your Fedora system has a bunch of useful, single-purpose utilities available for all sorts of simple operations. Like building blocks, you can attach them in creative and complex ways. Pipes are key to this concept.

Before you hear about pipes, though, it’s helpful to know the basic concept of input and output. Many utilities in your Fedora system can operate against files. But they can often take input not stored on a disk. You can think of input flowing freely into a process such as a utility as its standard input (also sometimes called stdin).

Similarly, a tool or process can display information to the screen by default. This is often because its default output is connected to the terminal. You can think of the free-flowing output of a process as its standard output (or stdout — go figure!).

Examples of standard input and output

Often when you run a tool, it outputs to the terminal. Take for instance this simple sequence command using the seq tool:

The output, which is simply to count integers up from 1 to 6, one number per line, comes to the screen. But you could also send it to a file using the > character. The shell interpreter uses this character to mean “redirect standard output to a file whose name follows.” So as you can guess, this command puts the output into a file called six.txt:

$ seq 1 6 > six.txt

Notice nothing comes to the screen. You’ve sent the ouptut into a file instead. If you run the command cat six.txt you can verify that.

You probably remember the simple use of the grep command from a previous article. You could ask grep to search for a pattern in a file by simply declaring the file name. But that’s simply a convenience feature in grep. Technically it’s built to take standard input, and search that.

The shell uses the < character similarly to mean “redirect standard input from a file whose name follows.” So you could just as well search for the number 4 in the file six.txt this way:

$ grep 4 < six.txt
4

Of course the output here is, by default, the content of any line with a match. So grep finds the digit 4 in the file and outputs that line to standard output.

Introducing pipes

Now imagine: what if you took the standard output of one tool, and instead of sending it to the terminal, you sent it into another tool’s standard input? This is the essence of the pipe.

Your shell uses the vertical bar character | to represent a pipe between two commands. You can find it on most keyboard above the backslash \ character. It’s used like this:

$ command1 | command2

For most simple utilities, you wouldn’t use an output filename option on command1, nor an input file option on command2. (You might use other options, though.) Instead of using files, you’re sending the output of command1 directly into command2. You can use as many pipes in a row as needed, creating complex pipelines of several commands in a row.

This (relatively useless) example combines the commands above:

$ seq 1 6 | grep 4
4

What happened here? The seq command outputs the integers 1 through 6, one line at a time. The grep command processes that output line by line, searching for a match on the digit 4, and outputs any matching line.

Here’s a slightly more useful example. Let’s say you want to find out if TCP port 22, the ssh port, is open on your system. You could find this out using the ss command* by looking through its copious output. Or you could figure out its filter language and use that. Or you could use pipes. For example, pipe it through grep looking for the ssh port label:

$ ss -tl | grep ssh
LISTEN 0 128 0.0.0.0:ssh 0.0.0.0:* LISTEN 0 128 [::]:ssh [::]:*

* Those readers familiar with the venerable netstat command may note it is mostly obsolete, as stated in its man page.

That’s a lot easier than reading through many lines of output. And of course, you can combine redirectors and pipes, for instance:

$ ss -tl | grep ssh > ssh-listening.txt

This is barely scratching the surface of pipes. Let your imagination run wild. Have fun piping!

Posted on July 29, 2019 by — Leave a comment

Command line quick tips: More about permissions

A previous article covered some basics about file permissions on your Fedora system. This installment shows you additional ways to use permissions to manage file access and sharing. It also builds on the knowledge and examples in the previous article, so if you haven’t read that one, do check it out.

Symbolic and octal

In the previous article you saw how there are three distinct permission sets for a file. The user that owns the file has a set, members of the group that owns the file has a set, and then a final set is for everyone else. These permissions are expressed on screen in a long listing (ls -l) using symbolic mode.

Each set has r, w, and x entries for whether a particular user (owner, group member, or other) can read, write, or execute that file. But there’s another way to express these permissions: in octal mode.

You’re used to the decimal numbering system, which has ten distinct values (0 through 9). The octal system, on the other hand, has eight distinct values (0 through 7). In the case of permissions, octal is used as a shorthand to show the value of the r, w, and x fields. Think of each field as having a value:

r = 4
w = 2
x = 1

Now you can express any combination with a single octal value. For instance, read and write permission, but no execute permission, would have a value of 6. Read and execute permission only would have a value of 5. A file’s rwxr-xr-x symbolic permission has an octal value of 755.

You can use octal values to set file permissions with the chmod command similarly to symbolic values. The following two commands set the same permissions on a file:

chmod u=rw,g=r,o=r myfile1
chmod 644 myfile1

Special permission bits

There are several special permission bits also available on a file. These are called setuid (or suid), setgid (or sgid), and the sticky bit (or delete inhibit). Think of this as yet another set of octal values:

setuid = 4
setgid = 2
sticky = 1

The setuid bit is ignored unless the file is executable. If that’s the case, the file (presumably an app or a script) runs as if it were launched by the user who owns the file. A good example of setuid is the /bin/passwd utility, which allows a user to set or change passwords. This utility must be able to write to files no user should be allowed to change. Therefore it is carefully written, owned by the root user, and has a setuid bit so it can alter the password related files.

The setgid bit works similarly for executable files. The file will run with the permissions of the group that owns it. However, setgid also has an additional use for directories. If a file is created in a directory with setgid permission, the group owner for the file will be set to the group owner of the directory.

Finally, the sticky bit, while ignored for files, is useful for directories. The sticky bit set on a directory will prevent a user from deleting files in that directory owned by other users.

The way to set these bits with chmod in octal mode is to add a value prefix, such as 4755 to add setuid to an executable file. In symbolic mode, the u and g can be used to set or remove setuid and setgid, such as u+s,g+s. The sticky bit is set using o+t. (Other combinations, like o+s or u+t, are meaningless and ignored.)

Sharing and special permissions

Recall the example from the previous article concerning a finance team that needs to share files. As you can imagine, the special permission bits help to solve their problem even more effectively. The original solution simply made a directory the whole group could write to:

drwxrwx---. 2 root finance 4096 Jul 6 15:35 finance

One problem with this directory is that users dwayne and jill, who are both members of the finance group, can delete each other’s files. That’s not optimal for a shared space. It might be useful in some situations, but probably not when dealing with financial records!

Another problem is that files in this directory may not be truly shared, because they will be owned by the default groups of dwayne and jill — most likely the user private groups also named dwayne and jill.

A better way to solve this is to set both setgid and the sticky bit on the folder. This will do two things — cause files created in the folder to be owned by the finance group automatically, and prevent dwayne and jill from deleting each other’s files. Either of these commands will work:

sudo chmod 3770 finance
sudo chmod u+rwx,g+rwxs,o+t finance

The long listing for the file now shows the new special permissions applied. The sticky bit appears as T and not t because the folder is not searchable for users outside the finance group.

drwxrws--T. 2 root finance 4096 Jul 6 15:35 finance