Mfcttrf

I show my full working directory plus other information (git, etc) in my bash prompt and sometimes it gets very long.

I want to add a newline to the end of my prompt so I can type the command on the next line, but only if the prompt is long e.g. more than 50 characters.

| ~ $ Typing a command here is nice |

| ~/foo/longDirectoryName/longsubirectory/src/package (master +*) $ Typing here s|
| ucks. I want to just start on a new line |

Obviously, If I wanted to always type my command on the next line, I could just add a newline to PS1 (as in this post). But I haven't found a way to do that conditionally because PS1 is just a format string.

P.S. I'm actually using ZSH trying to customize the Agnoster theme but I imagine any solution for bash in general would help.

In zsh, that's what the %<number>(l:<yes>:<no>) prompt expansion is for. When the number is negative, like -30, if there are at least 30 characters left until the right edge of the screen, then the yes text is output, otherwise no, so:

PS1=$'%~%-30(l::n)$ '

Would insert a newline if fewer than 28 characters (30 minus the "$ ") are left for you to use on the line.

You can do your 50 or more with:

PS1=$'%~%50(l:n:)$ '

But IMO, it's more useful to guarantee a minimum available space, than a maximum unusable space.

See the manual for details. You'll find other directives to truncate long prompts and replace with ellipsis for instance which you may also find useful.

Note that zsh prompt expansion is completely different from that of bash. It's actually closer to that of tcsh, so solutions for bash are unlikely to be of much use for zsh, though it's generally more true the other way round.

In newer versions of bash, you can combine PROMPT_COMMAND with the $var@P form of variable substitution ("expand as if it were a prompt").

Here is a way to do that. You simply set RPS1 instead of PS1 to the prompt you want. The \- escape inside RPS1 will act like a soft hyphen: it will expand to a newline when the prompt would be longer than COLUMNS / 2; otherwise it will be removed:

generate_ps1()
 local ps1=$RPS1@P
 if [ "$#ps1" -gt "$((COLUMNS / 2))" ]; then PS1=$RPS1//\-/$'n'
 else PS1=$RPS1//\-/
 fi

RPS1=$PS1
PROMPT_COMMAND=generate_ps1

# after this, set RPS1 instead of PS1 to the prompt you want

$ RPS1='$long_var-$ '
$ long_var=$(printf 'foobar %.0s' 0..21)
foobar foobar foobar foobar foobar foobar foobar foobar foobar foobar foobar foo
bar foobar foobar foobar foobar foobar foobar foobar foobar foobar foobar
$ _
$ long_var=$(printf 'foobar %.0s' 0..2)
foobar foobar foobar $ _

This could be improved to act like a real soft-hyphen, take into account the automatic margins of the terminal and already existing newlines inside the prompt, etc; but that will make more complex than it's worth.