Shell – Fetching user input while running in the background

inputkeyboard shortcutsshellterminalx11

I am writing a script that prompts the user for input and does some processing afterwards. I was using read to get the data, but then I noticed that when the script is ran with a keyboard shortcut (I added an entry in Ubuntu's Keyboard Shortcut list) it ignores the read and continues with the rest of the code.

How can I read user input with a script running in the background like this?

Best Answer

If I understand correctly, you're running the script directly from a GUI environment, not from inside a terminal.

The terminal is what provides the script a way to receive input. If you run the script from the GUI via a menu entry or a keyboard shortcut, the script's input is connected to nothing (this nothing is called /dev/null), so when you ask to read a line, the read command says that there is nothing to read (end of file).

You have two solutions: change the way you call your script, or change your script. The former is simpler and allows your script to be executed inside a terminal sometimes. The latter is more complex but more flexible.

You can easily change your script to run in a terminal. Instead of setting a keyboard shortcut for /path/to/script, set a keyboard shortcut for xterm -e /path/to/script or gnome-terminal -e /path/to/script or whatever terminal emulator you prefer. The terminal window will appear as soon as you press the key and will disappear when your script terminates.

Alternatively, you can change your script to read input through a GUI window instead of from its standard input. Zenity is a common choice for this (there's an Ubuntu package Install zenity http://bit.ly/software-small). Inside your script, you can use [ -t 0 ] to test whether it's running in a terminal.

if [ -t 0 ]; then
  echo "What is your favorite color?"
  read -r color
else
  color=$(zenity --entry --text "What is your favorite color?")
fi

Related Solutions

Bash – Reading and Processing Strings Character by Character

If you read a character at a time with read -n, you'll have to implement a key sequence parser. You can build a slow-and-dirty solution that works on most terminals with this: consider that a function key escape sequence begins with an escape character and continues with any number of characters amongst 0-9;[]O followed by one final character not in this set.

A better way to read input is to use a proper input library. Bash uses one for its own purposes (readline). You get a limited interface to it by declaring your own keybindings with the bind built-in; specifically bind -x to run a shell command on a key press. Because of this limited interface, implementing what you want is likely to be possible but difficult.

Zsh has its own input library, zle. Its interface is a lot richer than bash's. With zle, you can define arbitrary keymaps, and you get more access to the internals of zle from shell code. Use zle to assign shell functions to zle user-defined commands (called widgets), bindkey to create and populate your own keymap, and finally vared to read a line of input using the keymap of your choice.

Bash – output while reading keyboard input

#! /bin/bash

echo "Starting tail in the background..." 
tail -f input &
pid=$!
trap "kill $pid" EXIT
while true; do
        read kb
        test n = "$kb" && break
done

Best Answer

Related Solutions

Bash – Reading and Processing Strings Character by Character

Bash – output while reading keyboard input

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