Question

In R, I like to use reverse search (ctrl+r) to redo infrequent but complex commands without a script. Frequently, I will do so many other commands in between that the command history discards the old command. How can I change the default length of the command history?

Answer 1

This is platform and console specific. From the help for ?savehistory:

There are several history mechanisms available for the different R
consoles, which work in similar but not identical ways…

…

The history mechanism is controlled by two environment variables:
R_HISTSIZE controls the number of lines that are saved (default 512),
and R_HISTFILE sets the filename used for the loading/saving of
history if requested at the beginning/end of a session (but not the
default for these functions). There is no limit on the number of lines
of history retained during a session, so setting R_HISTSIZE to a large
value has no penalty unless a large file is actually generated.

So, in theory, you can read and set R_HISTSIZE with:

Sys.getenv("R_HISTSIZE")
Sys.setenv(R_HISTSIZE = new_number)

But, in practise, this may or may not have any effect.

See also ?Sys.setenv and ?EnvVar