If you have a Mac, and you use Terminal.app to run UNIX commands, try executing this for a cool shell prompt

 export PS1="\360\237\220\232 $ "

See what I did there?

If you are using a UTF-8 encoding for your terminal, which you probably are, and if you're using a recent OS X, and have the right fonts installed, which you probably do, you should have a little sea-shell graphic for your prompt. Literally a cool shell prompt.

In a recent revision to Unicode , code points were assigned for many emoji. Emoji-what-now? These are little emoticon glyphs that rose to popularity in Japan . Apple have included a nice typeface with full colour icons for a subset of these in the last couple of releases of both iOS and OS X, so you can use them in most applications that use the system type rendering library, like Messages. On OS X, this includes the bundled Terminal.app terminal emulator. So you can print little icons in your shell, if you know an encoding for a particular glyph.

Here's the ever popular 'pile of poo' (  U+1F4A9 ) 

 

Not sure what that is supposed to be used for, but it's terribly popular on the internet. "But how", I hear you ask, "do you find out the encoding sequences for these appealing novelties?"

Well, you can search for unicode code tables on the internet. On the Mac though, the easiest thing to do is probably to enable the Character Viewer tool via the Language and Text System preference pane. 

This gets you a panel like this, where you can browse all the characters your computer knows how to render, including all the emoji sets, and find out their Unicode code points, and more importantly, a way to encode that code point in UTF-8.

 

So, as you can see in my fecal example, the UTF-8 byte sequence for 'pile of poo' ( U+1F4A9 ) is F0 9F 92 A9, and we can print that in a bash shell, using echo  with the -e flag to enable interpreting of escape sequences, using the \x escape prefix to indicate bytes in hex. 

Going back to the original shell trick, the shell emoji (  U+1F41A ) has the UTF-8 encoding F0 9F 90 9A. The bash shell doesn't seem to have an escape sequence for hex encoded bytes in it's prompt string, but it does interpret 3 digit codes prefixed with a plain \ as octal encoded literal bytes, so if we convert this hex string to four octal numbers, using bc or od, or emacs or just Calulator.app, we get the escape sequence from my initial shell example - "\360\237\220\232"

So far so cute. But is there anything vaguely useful you can do with this sort of thing? Sort of. A picture's worth a thousand words. So we could perhaps encode mnemonic information in icons, and somehow dynamically update the prompt to reflect this.

Bash will execute the contents of an environment variable PROMPT_COMMAND as a shell command immediately before the shell prompt is printed. Typically this is used to update terminal colours or title strings with escape sequences, or update PS1 to add some content that can't be printed using the built-in prompt escape functions. I decided to make my prompt respond to the result of my most recent command.

Here's the relevant shell glue I just stuck in my .bashrc 

 
 emoji () 

 export PROMPT_COMMAND='PS1=$(emoji $?)'

This runs a shell function called emoji in a subshell, which returns a string based on the input argument. The input argument I'm using is the exit status of the last shell command. This gets me a smiley face in my shell prompt, unless the last command I ran returned a non-zero exit state, which in UNIX, indicates a problem happened. This makes my prompt draw as a 'confused smiley', if something has gone wrong.

Still cute, and almost useful!

I think I'll keep it for a while.

 

posted by cms on 2013-04-09 19:47:14