The command to get a fixed string into the clipboard is very easy, it's simply
xsel -ib <<< 'Your string goes here'
or if you want to read the string from a file
xsel -ib < your-file.txt
or from a command output
your-command | xsel -ib
Directly writing a fixed string by emulating keypresses is not much more complex though
xvkbd -file - <<< 'Your string goes here'
or if you want to read the string from a file
xvkbd -file your-file.txt
or from a command output
your-command | xvkbd -file -
You can simply create a custom shortcut in the System Settings → Keyboard → Shortcuts configuration and assign a command to the key combination you wish.
But take care that the shortcut interpreter is not Bash or a similar shell, so our <<< ("here string" syntax) or | pipes will not work. To solve this anyway, we simply enclose our shell command with bash -c "INSERT COMMAND HERE". Just pay attention that you don't use double quotes inside the command then.
Here are the commands how you would have to enter them into the shortcut settings:
Copy "my string" to clipboard:
bash -c "xsel -ib <<< 'my string'"
Copy content of my-file.txt to clipboard:
bash -c "xsel -ib < my-file.txt"
Copy output of my-command to clipboard:
bash -c "my-command | xsel -ib"
Directly paste/write "my string":
bash -c "xvkbd -file - <<< 'my string'"
Directly paste/write content of my-file.txt:
bash -c "xvkbd -file my-file.txt"
Directly paste/write output of my-command:
bash -c "my-command | xvkbd -file -"
Please note that neither xselnor xvkbd are installed by default, so you probably need to install them first using this command:
sudo apt-get install xsel xvkbd
Best Answer
To paste a single string into a textfield
install both
xdotoolandxclip:Add the following command to a shortcut key:
Choose: System Settings > "Keyboard" > "Shortcuts" > "Custom Shortcuts". Click the "+" and add the command above to a key combination of your choice.
Now when enter the cursor in a textfield and choose your shortcut, it will type your email address.