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Terminal Capabilities
There are a few problems in the world of terminal capability detection today; the terminal is typically local to a machine running very up to date software, but applications may be running on a remote system with relatively stale system software, or even different operating systems.
There are two well-specified standard approaches to querying terminal
capabilities: termcap and terminfo. The databases for the
capabilities are typically deployed with the operating system
software and suffers from splay and freshness issues: they may
be different on different systems and out of date.
A further complication is that the terminal may be connected via a series of intermediaries or multiplexers (mosh, tmux, screen) which hides or perturbs the true capabilities of the terminal.
terminfo and termcap both allow for the user to supply a local
override. terminfo needs a locally available compiled database,
and that database may have different binary representations on disk
depending on the operating system software, making it difficult for
users that need to login to many remote machines; the burden is on
the user to configure a local profile on many machines or solve the
challenge of having a $HOME/.terminfo directory that is NFS mountable
and readable to all potential machines.
termcap defines a TERMCAP environment variable that can contain
overrides and be passed via SSH to the remote systems much more simply
than the terminfo database, but termcap is the old obsolete database
and lacks a way to express support for the newer, more interesting
features.
It’s a bit of a mess.
There’s more: slang defined the concept of COLORTERM to workaround
some of these concerns so that it was possible to influence the
availability of relatively recent higher color palette extensions.
This allows the user the ability to “know better” about their terminal
than the local configuration allows. COLORTERM has the advantage of
being able to be passed on to remote systems via SSH.
Regarding environment variables: on macOS the two main terminal
emulators export TERM_PROGRAM and TERM_PROGRAM_VERSION into the
environment, and if this were transported via SSH and adopted by
more terminal emulators then newer software could potentially also
look at this information too.
Is there or will there ever be an ideal solution to this stuff? Probably not.
With all this in mind, this module presents a Capabilities struct
that holds information about a terminal. The new_from_env method
implements some heuristics (a fancy word for guessing) to compute
the terminal capabilities, but also offers a ProbeHints
that can be used by the embedding application to override those choices.
Structs
Capabilities holds information about the capabilities of a terminal.
On POSIX systems this is largely derived from an available terminfo
database, but there are some newish capabilities that are not yet
described by the majority of terminfo installations and thus have some
additional handling in this struct.
Use the ProbeHints to configure an instance of
the ProbeHints struct. ProbeHints are passed to the Capabilities
constructor to influence the effective set of terminal capabilities.
Enums
Describes the level of color support available