sh: Allow an ASCII ESC in PS

[otis]

Summary

Introduce new prompt format character, '\e' that emits an ASCII ESC
character. This might come handy to use ANSI sequences in PS1, for
example.

Example in ~/.shrc:

PS1="\e[7m\u@\h\e[0m:\w \\$ "

This is my naive approach, as this solves my use case.

[jilles]

Adding support for escape sequences might make it important to increase PROMPTLEN.

bin/sh/parser.c

2067–2069

The \001 codes for EL_PROMPT_ESC need to be around the whole escape sequence, not just the character with code 27, so that libedit's idea of the cursor position remains correct. This is like bash's \[ and \].

Consideration: patch libedit instead so it knows how long escape sequences are (\033[ "CSI" sequences end with certain characters, \033] "OSC" sequences end with \033\\ or \007, perhaps a bit more). Terminals should be sufficiently consistent now, and this relieves the user from adding special sequences to the prompt.

2069

The above check i < PROMPTLEN - 1 is not sufficient to prevent overflow here.

[otis]

Try with '\[' and '\]'

[jilles]

bin/sh/parser.c

2070

Adding the ESC character implicitly from \[ makes some sense in a vacuum, but it's not compatible with bash (in neither direction), so I don't like it. A simple solution would be to make \[ and \] generate \001 and add \e as before.

[otis]

Add '\a', '\r', '\n'