bash - Detect empty command

Question

Consider this PS1

PS1='
${_:+$? }$ '

Here is the result of a few commands

$ [ 2 = 2 ]

0 $ [ 2 = 3 ]

1 $

1 $

The first line shows no status as expected, and the next two lines show the correct exit code. However on line 3 only Enter was pressed, so I would like the status to go away, like line 1. How can I do this?

Answer 1

Here's a funny, very simple possibility: it uses the # escape sequence of PS1 together with parameter expansions (and the way Bash expands its prompt).

The escape sequence # expands to the command number of the command to be executed. This is incremented each time a command has actually been executed. Try it:

$ PS1='# $ '
2 $ echo hello
hello
3 $ # this is a comment
3 $
3 $    echo hello
hello
4 $

Now, each time a prompt is to be displayed, Bash first expands the escape sequences found in PS1, then (provided the shell option promptvars is set, which is the default), this string is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal.

The trick is then to have an array that will have the k-th field set (to the empty string) whenever the (k-1)-th command is executed. Then, using appropriate parameter expansions, we'll be able to detect when these fields are set and to display the return code of the previous command if the field isn't set. If you want to call this array __cmdnbary, just do:

PS1='
${__cmdnbary[#]-$? }${__cmdnbary[#]=}$ '

Look:

$ PS1='
${__cmdnbary[#]-$? }${__cmdnbary[#]=}$ '

0 $ [ 2 = 3 ]

1 $ 

$ # it seems that it works

$     echo "it works"
it works

0 $

---

To qualify for the shortest answer challenge:

PS1='
${a[#]-$? }${a[#]=}$ '

that's 31 characters.

Don't use this, of course, as a is a too trivial name; also, $ might be better than $.

---

Seems you don't like that the initial prompt is 0 $; you can very easily modify this by initializing the array __cmdnbary appropriately: you'll put this somewhere in your configuration file:

__cmdnbary=( '' '' ) # Initialize the field 1!
PS1='
${__cmdnbary[#]-$? }${__cmdnbary[#]=}$ '