Top posting intentionally as I don't think any (or much) context is required.
I am always impressed with Greg's knowledge of the Bash shell (maybe all
shells), but, wow, Bash seems like a convoluted mess to keep track of all the
little details / gotchas.
Is there a shell that is better in that respect? Maybe the c-shell? I don't
like c (because I have a hard time programming in it), but if the c-shell has
less intricacies (and is quite consistent with the c language), then maybe
(for me) it is worth switching to (someday ;-)
Nothing new below this line.
On Tuesday, October 20, 2020 07:36:39 AM Greg Wooledge wrote:
> The weird backslashing tells me you're defining PS1 with double quotes
> instead of single quotes, which is weird, and probably indicates that
> you're doing even worse things, like dynamically generating pieces of
> your PS1... I've seen things... things that no man should see
>
> Anyway, in addition to the weird quoting and possible abominations
> unrevealed, your command substitution forks. Every time the prompt
> is drawn, a new process is forked, just to do that simple "if" check.
>
> The standard workaround for that is to use an array variable with the
> two possible outcomes, and a bit of arithmetic.
>
> unicorn:~$ green=$(tput setaf 2) red=$(tput setaf 1) normal=$(tput sgr0)
> unicorn:~$ status=(+ -) statcolor=("$green" "$red")
> unicorn:~$ PS1='\[${statcolor[!!$?]}\]${status[!!$?]}\[$normal\] \h:\w\$ '
> + unicorn:~$ true
> + unicorn:~$ false
> - unicorn:~$ true
> + unicorn:~$
>
> (You can't see the colors in the mailing list, but they're there.)
>
> Just remember the basics:
>
> 1) Use single quotes around your definition of PS1, because you want all
>of the parts of it to be taken literally, and quoting is already hard
>enough without making it worse.
>
> 2) Variable expansions are "free" (do not cost a process fork). This
>includes variables that are set by PRMOPT_COMMAND, which is the more
>efficient way to set variables dynamically if you *really* need that,
>which you probably don't.
>
> 3) Color and other terminal escape codes can and should be stored in
>variables, which you can expand in PS1, as long as the PS1 definition
>is single-quoted. Don't store raw terminal sequences, because they
>are not the same for all terminals. This is what tput(1) is for.
>
> 4) Terminal escape sequences that don't change the cursor position should
>be enclosed in \[ \] so the shell knows where the cursor is at all
>times.
>
> 5) When expanding the index of an indexed array variable, the square
>brackets are a math context. In a math context, !!$? means "negate
>the previous command's exit status twice". So, 0 -> 1 -> 0, or
>nonzero -> 0 -> 1. Thus, !!$? always expands to either 0 or 1. And
>because it's in single quotes, it shouldn't trip history expansion,
>if you still have history expansion turned on. (I don't.)
