Desktop-files are part of a standard by freedesktop.org for defining the behaviour of applications running on Xorg (an X-server implementation). In an GNOME or KDE environment most applications conveniently include a desktop-file. I mostly write desktop files for GUI applications not coming with one and as a way to connect GUI-users with shell scripts.
For an simple example let’s assume that on a certain distribution Thunderbird doesn’t come with an desktop-file. Pretty much the only way to start Thunderbird in such a situtation is by typing thunderbird in the command-line. As desktop files define where in the menus/applications a program can be found, without one you won’t find the application in your menus/applications. Especially for inexperienced users this might cause a lot of confusion and ultimately frustration because they downloaded a program they can’t find anywhere.
The simple solution
A simple and fast solution would be to place an executable shell-script (thunderbird.sh) which calls thunderbird on the desktop, this way they can double-click the script to open thunderbird. The script would look like this:
#!/bin/sh thunderbird
Use chmod +x ~/Desktop/thunderbird.sh to make the script executable. This helps but is no elegant solution. The script has no icon and the user still can’t find the program in menus/applications.
The appropriate solution
A better solution would be to write a simple desktop file like the one below and store it in ‘/usr/share/applications’, we’ll explain the details later.
#!/usr/bin/env xdg-open [Desktop Entry] Comment="Mail Client" Terminal=false Name=thunderbird Exec=thunderbird Type=Application Icon=thunderbird.png Categories=Application;Email;
The first line is the shebang which defines how to execute the script, for desktop files #!/usr/bin/env xdg-open is the way to go.
The second line is the group header which should be [Desktop Entry].
The following entries are key value pairs defining the behaviour of the application, for a list of valid keys take a look at this specification by freedesktop.org.
- Comment is some additional information for the program
- Terminal is a boolean defining if a visible terminal should be opened or not
- Name is the application name
- Exec is the command which should be executed
- Type defines the desktop-entry type, in most cases this will be Application
- Icon is the name of the icon which should be used to represent the application
- Categories are the categories the application is connected with, in a hierarchical menu this is where the application can be found
Placing this simple desktop-file in an appropriate location like /usr/share/applications tells the system how to show call and represent Thunderbird (shown in image below). This way an inexperienced user can find the application in the menus/applications as well as link it to the desktop/launcher or wherever he wants.
For adding icons to self-written scripts I google the most appropriate buzz-word and add icon like vnc icon. This usually results instantly in proper icons for my script which I save to /usr/share/pixmaps/ and reference in the desktop-file with Icon="application-name.png".
cogito overdose 
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