Linux User Cheatsheet

This is a cheatsheet for people who use Linux, specifically Ubuntu + Gnome, for their day-to-day OS: you write code, play games, or even draw on a Linux computer. This means, besides terminal stuff, we will also focus on better desktop usage.

File Compression / Decompression

We can compress file into either tar.gz or zip. tar.gz works better for Linux/Unix b/c it retains perrmissions, older version of zip doesn’t preserve it.

• .tar.gz

  The files are first archived into a single .tar file using tar, then gzip is used to compress the single file into .gz format.

• .zip

  zip compresses each file separately, then archives them together.

Compress multiple files / folders into one .targ.gz

# [c]reate an archive and write it to a [f]ile:
tar czf output.tar.gz file1 folder2 file3 folder4 ...

Extract a .tar.gz into the current directory

tar xvf output.tar.gz

Extract a .tar.gz into a target directory

tar xvf output.tar.gz --directory target/

List the content of a tar / tar.gz file

# Lis[t] the contents of a tar [f]ile [v]erbosely:
tar tvf output.tar.gz

Linter for Shell Scripts

See ShellCheck.

Do something when a program fail

for file in "$@"; do
    grep foobar "$file" > /dev/null 2> /dev/null
    # When pattern is not found, grep has exit status 1
    # We redirect STDOUT and STDERR to a null register since we do not care about them
    if [[ $? -ne 0 ]]; then
        echo "File $file does not have any foobar, adding one"
        echo "# foobar" >> "$file"
    fi
done

Chain of commands

To chain multiple commands, we can

• use ; to chain commands, and execute commands in sequence. ; only separate commands.

• use the AND operator && to chain commands. Shortcircuit. Execute next if cur returns nonzero.

  >>> true && echo "foo"
  foo
  >>> echo "shortcircuit!" && false && echo "foo"
  shortcircuit!
  

• use the OR operator || to chain commands. Shortcircuit. Execute next if cur returns zero.

  >>> false && echo "foo"
  foo
  >>> echo "shortcircuit!" && true && echo "foo"
  shortcircuit!
  

Get output of a command as a variable

>>> export ans=$(echo "1+1=$(python -c "print(1+1)")")
>>> echo $ans
1+1=2

Generate similar argumnets (file1, file2, file3, …)

upload_image.py {mountain,sea}.{png,jpg}

Managing an AppImage on Ubuntu + GNOME

What is AppImage

An AppImage to Linux is what exe is to Windows. It allows you to run an application on all common Linux distributions by simply executing an myapp.AppImage file. All you needs are

chmod a+x myapp.AppImage
./myapp.AppImage

AppImage features

• no installation: just download and run

• self-contained: includes all dependencies

• Doesn’t modify the system: everything needed is in the AppImage, and it can be runned without sudo

• Portable: just one file

However, there are some tradeoff

• no shared dependencies: this is not necessarily a bad thing. but it is a tradeoff.

• no automatic update: you must manually download the new versions and replace it. Some AppImage supports appimageupdate though.

• need to manually integrate into the system: this is just unavoidable for a portable format like AppImage.

Install an AppImage

Sure, you can just execute the AppImage like any other program, but what if I want to use it as if it was installed by a package manager?

We use Krita as an example. Krita is a professional free and open source painting program, free alternative to Photoshop, SAI, or CSP.

1. Download the AppImage

   # download to a file named krita.AppImage
   wget --output-document krita.AppImage https://download.kde.org/stable/krita/5.2.9/krita-5.2.9-x86_64.AppImage
   

2. Place the AppImage in ~/Application/krita/, and make it executable

   mkdir ~/Application/krita
   mv krita.AppImage ~/Application/krita/
   cd ~/Application/krita
   chmod +x krita.AppImage
   

3. Extract the app icon from the AppImage, so that we can create a desktop entry for it later.

   This can be done in several ways. One way I find convenient is to first mount the AppImage somewhere in /tmp folder, then copy the icon.

   # this will mount krita to /tmp/.mount_kritaXXXXX
   ./krita.AppImage --appimage-mount
   
   # the command above will block the current bash shell
   # so open another shell to run this
   cp /tmp/.mount_kritaXXXXX/krita.png .
   

   The exact path to icon in the AppImage may differ for diff apps, but the logic is the same.

4. Create a desktop file, so that it’s accessible in the application menu

   Create a desktop file

   touch ~/.local/share/applications/krita.desktop
   

   Add the following to the file

   [Desktop Entry]
   Name=MyApp
   Exec=/home/<username>/Applications/krita/krita.AppImage
   Icon=/home/<username>/Applications/krita/krita.png
   Type=Application
   Categories=Utility;
   

   Update the GNOME’s application database

   update-desktop-database ~/.local/share/applications/
   

Now, you should be able to see the Krita icon in the GNOME application menu!

Update and Delete an AppImage

To update the AppImage, simply replace the AppImage file with the newer version one

cd ~/Applications/krita
rm krita.AppImage
wget --outut-document krita.AppImage "<link to AppImage of newer version>"
# don't forget to make the new AppImage exectuable
chmod +x krita.AppImage

To delete an AppImage, simply remove the entire app folder and the desktop entry

rm -rf ~/Applicatioins/krita
rm ~/.local/share/applications/krita.desktop
update-desktop-database ~/.local/share/applications/